= RISC-V Debug Specification +
Version 1.0- +
Editors: +
Paul Donahue <pdonahue@ventanamicro.com>, Ventana Micro Systems +
Tim Newsome <tim@sifive.com>, SiFive, Inc.

RISC-V Debug Specification Version 1.0-

== Preface

Contributors to all versions of the spec in alphabetical order (please
contact editors to suggest corrections): Bruce Ableidinger, Krste
Asanović, Peter Ashenden, Allen Baum, Mark Beal, Alex Bradbury, Chuanhua
Chang, Yen Hao Chen, Zhong-Ho Chen, Monte Dalrymple, Paul Donahue,
Vyacheslav Dyachenko, Ernie Edgar, Peter Egold, Marc Gauthier, Markus
Goehrle, Robert Golla, John Hauser, Richard Herveille, Yung-ching Hsiao,
Po-wei Huang, Scott Johnson, L. J. Madar, Grigorios Magklis, Daniel
Mangum, Alexis Marquet, Jan Matyas, Kai Meinhard, Jean-Luc Nagel, Aram
Nahidipour, Rishiyur Nikhil, Gajinder Panesar, Deepak Panwar, Antony
Pavlov, Klaus Kruse Pedersen, Ken Pettit, Darius Rad, Joe Rahmeh, Josh
Scheid, Vedvyas Shanbhogue, Gavin Stark, Ben Staveley, Wesley Terpstra,
Tommy Thorn, Megan Wachs, Jan-Willem van de Waerdt, Philipp Wagner,
Stefan Wallentowitz, Ray Van De Walker, Andrew Waterman, Thomas Wicki,
Andy Wright, Bryan Wyatt, and Florian Zaruba.

[[sec:intro]]
== Introduction

When a design progresses from simulation to hardware implementation, a
user’s control and understanding of the system’s current state drops
dramatically. To help bring up and debug low level software and
hardware, it is critical to have good debugging support built into the
hardware. When a robust OS is running on a core, software can handle
many debugging tasks. However, in many scenarios, hardware support is
essential.

This document outlines a standard architecture for debug support on
RISC-V hardware platforms. This architecture allows a variety of
implementations and tradeoffs, which is complementary to the wide range
of RISC-V implementations. At the same time, this specification defines
common interfaces to allow debugging tools and components to target a
variety of hardware platforms based on the RISC-V ISA.

System designers may choose to add additional hardware debug support,
but this specification defines a standard interface for common
functionality.

=== Terminology

An advanced feature for advanced users. Most users will not be able to
take advantage of it.

Atomic Memory Operation.

JTAG instruction that selects a single bit data register, also called
BYPASS.

A RISC-V core, or other part of a hardware platform. Typically all
components will be connected to a single system bus.

Control and Status Register.

Debug Module (see Chapter #chap:dm[4]).

Debug Module Interface (see Section link:#dmi[4.1]).

JTAG Data Register.

Debug Transport Module (see Section link:#dtm[7]).

Debug XLEN, which is the widest XLEN a hart supports, ignoring the
current value of latexmath:[$|MXL|$] in `misa`.

An essential feature must be present in order for debug to work
correctly.

General Purpose Register.

A single system consisting of one or more _components_.

A hardware thread in a RISC-V core.

32-bit Identification CODE, and a JTAG instruction that returns the
IDCODE value.

JTAG Instruction Register.

Refers to work done by IEEE’s Joint Test Action Group, described in IEEE
1149.1.

A legacy feature should only be implemented to support legacy hardware
that is present in a system.

A subset of the full Debug Specification that allows for very small
implementations. See Chapter #chap:dm[4].

Naturally Aligned Power-Of-Two.

Non-Maskable Interrupt.

An address that is directly usable on the system bus.

A recommended feature is not required for debug to work correctly, but
it is so useful that it should not be omitted without good reason.

System Bus Access (see Section link:#systembusaccess[4.10]).

A specialized feature, that only makes sense in the context of some
specific hardware.

Test Access Port, defined in IEEE 1149.1.

Trigger Module (see Section #sec:trigger[6]).

An address as a hart sees it. If the hart is using address translation
this may be different from the physical address. If there is no
translation then it will be the same.

The exception program counter CSR (e.g. `mepc`) that is appropriate for
the mode being trapped to.

=== Context

This specification attempts to support all RISC-V ISA extensions that
have, roughly, been ratified through the first half of 2023. In
particular, though, this specification specifically addresses features
in the following extensions:

. A
. C
. D
. F
. H
. Sm1p13
. Ss1p13
. Smstateen
. V
. Zawrs
. Zcmp
. Zicbom
. Zicboz
. Zicbop

==== Versions

Version 0.13 of this document was ratified by the RISC-V Foundation’s
board. Versions 0.13.latexmath:[$x$] are bug fix releases to that
ratified specification.

Version 0.14 was a working version that was never officially ratified.

Version 1.0 is almost entirely forwards and backwards compatible with
Version 0.13.

===== Bugfixes from 0.13 to 1.0

Changes that fix a bug in the spec:

. Fix order of operations described in .
https://github.com/riscv/riscv-debug-spec/pull/392[#392]
. Resume ack is set after resume, in Section link:#runcontrol[4.5].
https://github.com/riscv/riscv-debug-spec/pull/400[#400]
. applies to . https://github.com/riscv/riscv-debug-spec/pull/402[#402]
. only applies when action=0.
https://github.com/riscv/riscv-debug-spec/pull/411[#411]
. does not affect Argument Width.
https://github.com/riscv/riscv-debug-spec/pull/420[#420]
. Clarify that harts halt out of reset if .
https://github.com/riscv/riscv-debug-spec/pull/419[#419]

===== Incompatible Changes from 0.13 to 1.0

Changes that are not backwards-compatible. Debuggers or hardware
implementations that implement 0.13 will have to change something in
order to implement 1.0:

. Make haltsum0 optional if there is only one hart.
https://github.com/riscv/riscv-debug-spec/pull/505[#505]
. System bus autoincrement only happens if an access actually takes
place. () https://github.com/riscv/riscv-debug-spec/pull/507[#507]
. Bump to 3. https://github.com/riscv/riscv-debug-spec/pull/512[#512]
. Require debugger to poll after lowering it.
https://github.com/riscv/riscv-debug-spec/pull/566[#566]
. Add to . https://github.com/riscv/riscv-debug-spec/pull/574[#574]
. When a selected trigger is disabled, and can be written with any value
supported by any of the types this trigger supports.
https://github.com/riscv/riscv-debug-spec/pull/721[#721]
. fields only apply to breakpoint traps, not any trap.
https://github.com/riscv/riscv-debug-spec/pull/723[#723]
. If is greater than 0, then (previously called .latexmath:[$|hit|$])
now contains 0 when a trigger fires more than one instruction after the
instruction that matched. (This information is now reflected in .)
https://github.com/riscv/riscv-debug-spec/pull/795[#795]
. If is greater than 0, then bit 20 of is no longer used for timing
information. (Previously the bit was called .latexmath:[$|timing|$].)
https://github.com/riscv/riscv-debug-spec/pull/807[#807]
. If is greater than 0, then the encodings of for sizes greater than 64
bit have changed.
https://github.com/riscv/riscv-debug-spec/pull/807[#807]

===== Minor Changes from 0.13 to 1.0

Changes that slightly modify defined behavior. Technically backwards
incompatible, but unlikely to be noticeable:

. only applies to hart-local counters.
https://github.com/riscv/riscv-debug-spec/pull/405[#405]
. may be invalid when .
https://github.com/riscv/riscv-debug-spec/pull/414[#414]
. Address triggers () may fire on any accessed address.
https://github.com/riscv/riscv-debug-spec/pull/421[#421]
. All trigger registers (Section link:#csrTrigger[[csrTrigger]]) are
optional. https://github.com/riscv/riscv-debug-spec/pull/431[#431]
. When extending IR, still is all ones.
https://github.com/riscv/riscv-debug-spec/pull/437[#437]
. and are WARL. https://github.com/riscv/riscv-debug-spec/pull/458[#458]
. NMIs are disabled by .
https://github.com/riscv/riscv-debug-spec/pull/465[#465]
. R/W1C fields should be cleared by writing every bit high.
https://github.com/riscv/riscv-debug-spec/pull/472[#472]
. Specify trigger priorities in Table #tab:priority[[tab:priority]]
relative to exceptions.
https://github.com/riscv/riscv-debug-spec/pull/478[#478]
. Time may pass before becomes high.
https://github.com/riscv/riscv-debug-spec/pull/500[#500]
. Clear MPRV when resuming into lower privilege mode.
https://github.com/riscv/riscv-debug-spec/pull/503[#503]
. Halt state may not be preserved across reset.
https://github.com/riscv/riscv-debug-spec/pull/504[#504]
. Hardware should clear trigger action when is cleared and action is 1.
https://github.com/riscv/riscv-debug-spec/pull/501[#501]
. Change quick access exceptions to halt the target in
Section link:#acQuickaccess[[acQuickaccess]].
https://github.com/riscv/riscv-debug-spec/pull/585[#585]
. Writing 0 to forces a state where and are writable.
https://github.com/riscv/riscv-debug-spec/pull/598[#598]
. Solutions to deal with reentrancy in Section #sec:nativetrigger[6.4]
prevent triggers from _matching_, not merely _firing_. This primarily
affects behavior.
https://github.com/riscv/riscv-debug-spec/pull/722[#722]
. Attempts to access an unimplemented CSR raise an illegal instruction
exception. https://github.com/riscv/riscv-debug-spec/pull/791[#791]

===== New Features from 0.13 to 1.0

New backwards-compatible feature that did not exist before:

. Add halt groups and external triggers in Section link:#hrgroups[4.6].
https://github.com/riscv/riscv-debug-spec/pull/404[#404]
. Reserve some DMI space for non-standard use. See , and through .
https://github.com/riscv/riscv-debug-spec/pull/406[#406]
. Reserve trigger values for non-standard use.
https://github.com/riscv/riscv-debug-spec/pull/417[#417]
. Add bit to . https://github.com/riscv/riscv-debug-spec/pull/408[#408]
and https://github.com/riscv/riscv-debug-spec/pull/709[#709]
. Recommend matching on every accessed address.
https://github.com/riscv/riscv-debug-spec/pull/449[#449]
. Add resume groups in Section link:#hrgroups[4.6].
https://github.com/riscv/riscv-debug-spec/pull/506[#506]
. Add . https://github.com/riscv/riscv-debug-spec/pull/536[#536]
. Move , renaming original to , and create .
https://github.com/riscv/riscv-debug-spec/pull/535[#535]
. Add , deprecating .
https://github.com/riscv/riscv-debug-spec/pull/538[#538]
. Add hypervisor support: , , , , , , and .
https://github.com/riscv/riscv-debug-spec/pull/549[#549]
. Optionally make and sticky, controlled by .
https://github.com/riscv/riscv-debug-spec/pull/520[#520]
. Add to support trigger module external trigger inputs.
https://github.com/riscv/riscv-debug-spec/pull/543[#543]
. Describe and behavior with atomic instructions.
https://github.com/riscv/riscv-debug-spec/pull/561[#561]
. Trigger hit bits must be set on fire, may be set on match.
https://github.com/riscv/riscv-debug-spec/pull/593[#593]
. Add and to and .
https://github.com/riscv/riscv-debug-spec/pull/588[#588]
. Allow debugger to request harts stay alive with keepalive bit in
Section link:#keepalive[[keepalive]].
https://github.com/riscv/riscv-debug-spec/pull/592[#592]
. Add to allow a debugger to determine when ndmreset is complete.
https://github.com/riscv/riscv-debug-spec/pull/594[#594]
. Add to support triggers from an interrupt controller.
https://github.com/riscv/riscv-debug-spec/pull/599[#599]

===== Incompatible Changes During 1.0 Stable

Backwards-incompatible changes between two versions that are both called
1.0 stable.

. was moved from to , and is now subject to the mode bits in that
trigger.
. https://github.com/riscv/riscv-debug-spec/pull/728[#728] introduced
Message Registers, which were later removed in
https://github.com/riscv/riscv-debug-spec/pull/878[#878].
. It may not be possible to read the contents of the Program Buffer
using the `progbuf` registers.
https://github.com/riscv/riscv-debug-spec/pull/731[#731]
. fields apply to all traps, not just breakpoint traps. This reverts
https://github.com/riscv/riscv-debug-spec/pull/723[#723].
https://github.com/riscv/riscv-debug-spec/pull/880[#880]

=== About This Document

==== Structure

This document contains two parts. The main part of the document is the
specification, which is given in the numbered chapters. The second part
of the document is a set of appendices. The information in the
appendices is intended to clarify and provide examples, but is not part
of the actual specification.

==== ISA vs. non-ISA

This specification contains both ISA and non-ISA parts. The ISA parts
define self-contained ISA extensions. The other parts of the document
describe the non-ISA external debug extension. Chapters whose contents
are solely one or the other are labeled as such in their title. Chapters
without such a label apply to both ISA and non-ISA.

==== Register Definition Format

All register definitions in this document follow the format shown below.
A simple graphic shows which fields are in the register. The upper and
lower bit indices are shown to the top left and top right of each field.
The total number of bits in the field are shown below it.

After the graphic follows a table which for each field lists its name,
description, allowed accesses, and reset value. The allowed accesses are
listed in Table #tab:access[[tab:access]]. The reset value is either a
constant or ``Preset.'' The latter means it is an
implementation-specific legal value.

Parts of the register which are currently unused are labeled with the
number 0. Software must only write 0 to those fields, and ignore their
value while reading. Hardware must return 0 when those fields are read,
and ignore the value written to them.

____
__

'''''

 +
This behavior enables us to use those fields later without having to
increase the values in the version fields.
____

Names of registers and their fields are hyperlinks to their definition,
and are also listed in the index on page .

|l|L| R & Read-only. +
R/W & Read/Write. +
R/W1C & Read/Write Ones to Clear. Writing 0 to every bit has no effect.
Writing 1 to every bit clears the field. The result of other writes is
undefined. +
WARZ & Write any, read zero. A debugger may write any value. When read
this field returns 0. +
W1 & Write-only. Only writing 1 has an effect. When read the returned
value should be 0. +
WARL & Write any, read legal. A debugger may write any value. If a value
is unsupported, the implementation converts the value to one that is
supported. +

=== Background

There are several use cases for dedicated debugging hardware, both in
native debug and external debug. Native debug (sometimes called
self-hosted debug) refers to debug software running on a RISC-V platform
which debugs the same platform. The optional Trigger Module provides
features that are useful for native debug. External debug refers to
debug software running somewhere else, debugging the RISC-V platform via
a debug transport like JTAG. The entire document provides features that
are useful for external debug.

This specification addresses the use cases listed below. Implementations
can choose not to implement every feature, which means some use cases
might not be supported.

* Accessing hardware on a hardware platform without a working CPU.
(External debug.)
* Bootstrapping a hardware platform to test, configure, and program
components before there is any executable code path in the hardware
platform. (External debug.)
* Debugging low-level software in the absence of an OS or other
software. (External debug.)
* Debugging issues in the OS itself. (External or native debug.)
* Debugging processes running on an OS. (Native or external debug.)

=== Supported Features

The debug interface described in this specification supports the
following features:

. All hart registers (including CSRs) can be read/written.
. Memory can be accessed either from the hart’s point of view, through
the system bus directly, or both.
. RV32, RV64, and future RV128 are all supported.
. Any hart in the hardware platform can be independently debugged.
. A debugger can discover almostfootnote:[Notable exceptions include
information about the memory map and peripherals.] everything it needs
to know itself, without user configuration.
. Each hart can be debugged from the very first instruction executed.
. A RISC-V hart can be halted when a software breakpoint instruction is
executed.
. Hardware single-step can execute one instruction at a time.
. Debug functionality is independent of the debug transport used.
. The debugger does not need to know anything about the
microarchitecture of the harts it is debugging.
. Arbitrary subsets of harts can be halted and resumed simultaneously.
(Optional)
. Arbitrary instructions can be executed on a halted hart. That means no
new debug functionality is needed when a core has additional or custom
instructions or state, as long as there exist programs that can move
that state into GPRs. (Optional)
. Registers can be accessed without halting. (Optional)
. A running hart can be directed to execute a short sequence of
instructions, with little overhead. (Optional)
. A system bus manager allows memory access without involving any hart.
(Optional)
. A RISC-V hart can be halted when a trigger matches the PC, read/write
address/data, or an instruction opcode. (Optional)
. Harts can be grouped, and harts in the same group will all halt when
any of them halts. These groups can also react to or notify external
triggers. (Optional)

This document does not suggest a strategy or implementation for hardware
test, debugging or error detection techniques. Scan, built-in self test
(BIST), etc. are out of scope of this specification, but this
specification does not intend to limit their use in RISC-V systems.

It is possible to debug code that uses software threads, but there is no
special debug support for it.

[[overview]]
== System Overview

Figure #fig:overview[3.1] shows the main components of Debug Support.
Blocks shown in dotted lines are optional.

.RISC-V Debug System Overview
[#fig:overview]
image::fig/overview-eps-converted-to.pdf[fig/overview-eps-converted-to]

The user interacts with the Debug Host (e.g. laptop), which is running a
debugger (e.g. gdb). The debugger communicates with a Debug Translator
(e.g. OpenOCD, which may include a hardware driver) to communicate with
Debug Transport Hardware (e.g. Olimex USB-JTAG adapter). The Debug
Transport Hardware connects the Debug Host to the hardware platform’s
Debug Transport Module (DTM). The DTM provides access to one or more
Debug Modules (DMs) using the Debug Module Interface (DMI).

Each hart in the hardware platform is controlled by exactly one DM.
Harts may be heterogeneous. There is no further limit on the hart-DM
mapping, but usually all harts in a single core are controlled by the
same DM. In most hardware platforms there will only be one DM that
controls all the harts in the hardware platform.

DMs provide run control of their harts in the hardware platform.
Abstract commands provide access to GPRs. Additional registers are
accessible through abstract commands or by writing programs to the
optional Program Buffer.

The Program Buffer allows the debugger to execute arbitrary instructions
on a hart. This mechanism can also be used to access memory. An optional
system bus access block allows memory accesses without using a RISC-V
hart to perform the access.

Each RISC-V hart may implement a Trigger Module. When trigger conditions
are met, harts will halt and inform the debug module that they have
halted.

[[chap:dm]]
== Debug Module (DM) (non-ISA extension)

The Debug Module implements a translation interface between abstract
debug operations and their specific implementation. It might support the
following operations:

. Give the debugger necessary information about the implementation.
(Required)
. Allow any individual hart to be halted and resumed. (Required)
. Provide status on which harts are halted. (Required)
. Provide abstract read and write access to a halted hart’s GPRs.
(Required)
. Provide access to a reset signal that allows debugging from the very
first instruction after reset. (Required)
. Provide a mechanism to allow debugging harts immediately out of reset
(regardless of the reset cause). (Optional)
. Provide abstract access to non-GPR hart registers. (Optional)
. Provide a Program Buffer to force the hart to execute arbitrary
instructions. (Optional)
. Allow multiple harts to be halted, resumed, and/or reset at the same
time. (Optional)
. Allow memory access from a hart’s point of view. (Optional)
. Allow direct System Bus Access. (Optional)
. Group harts. When any hart in the group halts, they all halt.
(Optional)
. Respond to external triggers by halting each hart in a configured
group. (Optional)
. Signal an external trigger when a hart in a group halts. (Optional)

In order to be compatible with this specification an implementation
must:

. Implement all the required features listed above.
. Implement at least one of Program Buffer, System Bus Access, or
Abstract Access Memory command mechanisms.
. Do at least one of:
.. Implement the Program Buffer.
.. Implement abstract access to all registers that are visible to
software running on the hart including all the registers that are
present on the hart and listed in Table #tab:regno[[tab:regno]].
.. Implement abstract access to at least all GPRs, , and , and advertise
the implementation as conforming to the ``Minimal RISC-V Debug
Specification 1.0-'', instead of the ``RISC-V Debug Specification
1.0-''.

A single DM can debug up to latexmath:[$2^{20}$] harts.

[[dmi]]
=== Debug Module Interface (DMI)

Debug Modules are subordinates on a bus called the Debug Module
Interface (DMI). The bus manager is the Debug Transport Module(s). The
Debug Module Interface can be a trivial bus with one manager and one
subordinate (see link:#dmi_signals[8.3]), or use a more full-featured
bus like TileLink or the AMBA Advanced Peripheral Bus. The details are
left to the system designer.

The DMI uses between 7 and 32 address bits. Each address points at a
single 32-bit register that can be read or written. The bottom of the
address space is used for the first (and usually only) DM. Extra space
can be used for custom debug devices, other cores, additional DMs, etc.
If there are additional DMs on this DMI, the base address of the next DM
in the DMI address space is given in .

The Debug Module is controlled via register accesses to its DMI address
space.

[[reset]]
=== Reset Control

There are two methods that allow a debugger to reset harts. resets all
the harts in the hardware platform, as well as all other parts of the
hardware platform except for the Debug Modules, Debug Transport Modules,
and Debug Module Interface. Exactly what is affected by this reset is
implementation dependent, but it must be possible to debug programs from
the first instruction executed. resets all the currently selected harts.
In this case an implementation may reset more harts than just the ones
that are selected. The debugger can discover which other harts are reset
(if any) by selecting them and checking and .

To perform either of these resets, the debugger first asserts the bit,
and then clears it. The actual reset may start as soon as the bit is
asserted, but may start an arbitrarily long time after the bit is
deasserted. The reset itself may also take an arbitrarily long time.
While the reset is on-going, harts are either in the running state,
indicating it’s possible to perform some abstract commands during this
time, or in the unavailable state, indicating it’s not possible to
perform any abstract commands during this time. Once a hart’s reset is
complete, `havereset` becomes set. When a hart comes out of reset and or
latexmath:[$|\hyperref[resethaltreq]{resethaltreq}|$] are set, the hart
will immediately enter Debug Mode (halted state). Otherwise, if the hart
was initially running it will execute normally (running state) and if
the hart was initially halted it should now be running but may be
halted.

____
__

'''''

 +
There is no general, reliable way for the debugger to know when reset
has actually begun.
____

The Debug Module’s own state and registers should only be reset at
power-up and while in is 0. If there is another mechanism to reset the
DM, this mechanism must also reset all the harts accessible to the DM.

Due to clock and power domain crossing issues, it might not be possible
to perform arbitrary DMI accesses across hardware platform reset. While
or any external reset is asserted, the only supported DM operations are
reading and writing . The behavior of other accesses is undefined.

When harts have been reset, they must set a sticky `havereset` state
bit. The conceptual `havereset` state bits can be read for selected
harts in and in . These bits must be set regardless of the cause of the
reset. The `havereset` bits for the selected harts can be cleared by
writing 1 to in . The `havereset` bits might or might not be cleared
when is low.

[[selectingharts]]
=== Selecting Harts

Up to latexmath:[$2^{20}$] harts can be connected to a single DM.
Commands issued to the DM only apply to the currently selected harts.

To enumerate all the harts, a debugger must first determine `HARTSELLEN`
by writing all ones to
latexmath:[$|\hyperref[hartsel]{hartsel}|$](assuming the maximum size)
and reading back the value to see which bits were actually set. Then it
selects each hart starting from 0 until either in is 1, or the highest
index (depending on `HARTSELLEN`) is reached.

The debugger can discover the mapping between hart indices and `mhartid`
by using the interface to read `mhartid`, or by reading the hardware
platform’s configuration structure.

==== Selecting a Single Hart

All debug modules must support selecting a single hart. The debugger can
select a hart by writing its index to
latexmath:[$|\hyperref[hartsel]{hartsel}|$]. Hart indexes start at 0 and
are contiguous until the final index.

[[hartarraymask]]
==== Selecting Multiple Harts

Debug Modules may implement a Hart Array Mask register to allow
selecting multiple harts at once. The latexmath:[$n$]th bit in the Hart
Array Mask register applies to the hart with index latexmath:[$n$]. If
the bit is 1 then the hart is selected. Usually a DM will have a Hart
Array Mask register exactly wide enough to select all the harts it
supports, but it’s allowed to tie any of these bits to 0.

The debugger can set bits in the hart array mask register using and ,
then apply actions to all selected harts by setting . If this feature is
supported, multiple harts can be halted, resumed, and reset
simultaneously. The state of the hart array mask register is not
affected by setting or clearing .

Execution of Abstract Commands ignores this mechanism and only applies
to the hart selected by latexmath:[$|\hyperref[hartsel]{hartsel}|$].

=== Hart DM States

Every hart that can be selected is in exactly one of the following four
DM states: non-existent, unavailable, running, or halted. Which state
the selected harts are in is reflected by , , , , , , , and .

Harts are nonexistent if they will never be part of this hardware
platform, no matter how long a user waits. E.g. in a simple single-hart
hardware platform only one hart exists, and all others are nonexistent.
Debuggers may assume that a hardware platform has no harts with indexes
higher than the first nonexistent one.

Harts are unavailable if they might exist/become available at a later
time, or if there are other harts with higher indexes than this one.
Harts may be unavailable for a variety of reasons including being reset,
temporarily powered down, and not being plugged into the hardware
platform. That means harts might become available or unavailable at any
time, although these events should be rare in hardware platforms built
to be easily debugged. There are no guarantees about the state of the
hart when it becomes available.

Hardware platforms with very large number of harts may permanently
disable some during manufacturing, leaving holes in the otherwise
continuous hart index space. In order to let the debugger discover all
harts, they must show up as unavailable even if there is no chance of
them ever becoming available.

Harts are running when they are executing normally, as if no debugger
was attached. This includes being in a low power mode or waiting for an
interrupt, as long as a halt request will result in the hart being
halted.

Harts are halted when they are in Debug Mode, only performing tasks on
behalf of the debugger.

Which states a hart that is reset goes through is implementation
dependent. Harts may be unavailable while reset is asserted, and some
time after reset is deasserted. They might transition to running for
some time after reset is deasserted. Finally they end up either running
or halted, depending on and
latexmath:[$|\hyperref[resethaltreq]{resethaltreq}|$].

[[runcontrol]]
=== Run Control

For every hart, the Debug Module tracks 4 conceptual bits of state: halt
request, resume ack, halt-on-reset request, and hart reset. (The hart
reset and halt-on-reset request bits are optional.) These 4 bits reset
to 0, except for resume ack, which may reset to either 0 or 1. The DM
receives halted, running, and havereset signals from each hart. The
debugger can observe the state of resume ack in and , and the state of
halted, running, and havereset signals in , , , , , and . The state of
the other bits cannot be observed directly.

When a debugger writes 1 to , each selected hart’s halt request bit is
set. When a running hart, or a hart just coming out of reset, sees its
halt request bit high, it responds by halting, deasserting its running
signal, and asserting its halted signal. Halted harts ignore their halt
request bit.

When a debugger writes 1 to , each selected hart’s resume ack bit is
cleared and each selected, halted hart is sent a resume request. Harts
respond by resuming, clearing their halted signal, and asserting their
running signal. At the end of this process the resume ack bit is set.
These status signals of all selected harts are reflected in , , , and .
Resume requests are ignored by running harts.

When halt or resume is requested, a hart must respond in less than one
second, unless it is unavailable. (How this is implemented is not
further specified. A few clock cycles will be a more typical latency).

The DM can implement optional halt-on-reset bits for each hart, which it
indicates by setting to 1. This means the DM implements the and bits.
Writing 1 to sets the halt-on-reset request bit for each selected hart.
When a hart’s halt-on-reset request bit is set, the hart will
immediately enter debug mode on the next deassertion of its reset. This
is true regardless of the reset’s cause. The hart’s halt-on-reset
request bit remains set until cleared by the debugger writing 1 to while
the hart is selected, or by DM reset.

If the DM is reset while a hart is halted, it is unspecified whether
that hart resumes. Debuggers should use to explicitly resume harts
before clearing and disconnecting.

[[hrgroups]]
=== Halt Groups, Resume Groups, and External Triggers

An optional feature allows a debugger to place harts into two kinds of
groups: halt groups and resume groups. It is also possible to add
external triggers to a halt and resume groups. At any given time, each
hart and each trigger is a member of exactly one halt group and exactly
one resume group.

In both halt and resume groups, group 0 is special. Harts in group 0
halt/resume as if groups aren’t implemented at all.

When any hart in a halt group halts:

. That hart halts normally, with reflecting the original cause of the
halt.
. All the other harts in the halt group that are running will quickly
halt. for those harts should be set to 6, but may be set to 3. Other
harts in the halt group that are halted but have started the process of
resuming must also quickly become halted, even if they do resume
briefly.
. Any external triggers in that group are notified.

Adding a hart to a halt group does not automatically halt that hart,
even if other harts in the group are already halted.

When an external trigger that’s a member of the halt group fires:

. All the harts in the halt group that are running will quickly halt.
for those harts should be set to 6, but may be set to 3. Other harts in
the halt group that are halted but have started the process of resuming
must also quickly become halted, even if they do resume briefly.

When any hart in a resume group resumes:

. All the other harts in that group that are halted will quickly resume
as soon as any currently executing abstract commands have completed.
Each hart in the group sets its resume ack bit as soon as it has
resumed. Harts that are in the process of halting should complete that
process and stay halted.
. Any external triggers in that group are notified.

Adding a hart to a resume group does not automatically resume that hart,
even if other harts in the group are currently running.

When an external trigger that’s a member of the resume group fires:

. All the harts in that group that are halted will quickly resume as
soon as any currently executing abstract commands have completed. Each
hart in the group sets its resume ack bit as soon as it has resumed.
Harts that are in the process of halting should complete that process
and stay halted.

External triggers are abstract concepts that can signal the DM and/or
receive signals from the DM. This configuration is done through , where
external triggers are referred to by a number. Commonly, external
triggers are capable of sending a signal from the hardware platform into
the DM, as well as receiving a signal from the DM to take their own
action on. It is also allowable for an external trigger to be input-only
or output-only. By convention external triggers 0–7 are bidirectional,
triggers 8–11 are input-only, and triggers 12–15 are output-only but
this is not required.

____
__

'''''

 +
External triggers could be used to implement near simultaneous
halting/resuming of all cores in a hardware platform, when not all cores
are RISC-V cores.
____

When the DM is reset, all harts must be placed in the lowest-numbered
halt and resume groups that they can be in. (This will usually be group
0.)

Some designs may choose to hardcode hart groups to a group other than
group 0, meaning it is never possible to halt or resume just a single
hart. This is explicitly allowed. In that case it must be possible to
discover the groups by using even if it’s not possible to change the
configuration.

[[abstractcommands]]
=== Abstract Commands

The DM supports a set of abstract commands, most of which are optional.
Depending on the implementation, the debugger may be able to perform
some abstract commands even when the selected hart is not halted.
Debuggers can only determine which abstract commands are supported by a
given hart in a given state (running, halted, or held in reset) by
attempting them and then looking at in to see if they were successful.
Commands may be supported with some options set, but not with other
options set. If a command has unsupported options set or if bits that
are defined as 0 aren’t 0, then the DM must set to 2 (not supported).

____
__

'''''

 +
Example: Every DM must support the Access Register command, but might
not support accessing CSRs. If the debugger requests to read a CSR in
that case, the command will return ``not supported.''
____

Debuggers execute abstract commands by writing them to . They can
determine whether an abstract command is complete by reading in . If the
debugger starts a new command while is set, becomes 1 (busy), the
currently executing command still gets to run to completion, but any
error generated by the currently executing command is lost. After
completion, indicates whether the command was successful or not.
Commands may fail because a hart is not halted, not running,
unavailable, or because they encounter an error during execution.

If the command takes arguments, the debugger must write them to the
`data` registers before writing to . If a command returns results, the
Debug Module must ensure they are placed in the `data` registers before
is cleared. Which `data` registers are used for the arguments is
described in Table #tab:datareg[[tab:datareg]]. In all cases the
least-significant word is placed in the lowest-numbered `data` register.
The argument width depends on the command being executed, and is DXLEN
where not explicitly specified.

|r|l|l|l| Argument Width & arg0/return value & arg1 & arg2 +
& & `data1` & `data2` +
& , `data1` & `data2`, `data3` & `data4`, `data5` +
& –`data3` & `data4`–`data7` & `data8`–`data11` +

____
__

'''''

 +
The Abstract Command interface is designed to allow a debugger to write
commands as fast as possible, and then later check whether they
completed without error. In the common case the debugger will be much
slower than the target and commands succeed, which allows for maximum
throughput. If there is a failure, the interface ensures that no
commands execute after the failing one. To discover which command
failed, the debugger has to look at the state of the DM (e.g. contents
of ) or hart (e.g. contents of a register modified by a Program Buffer
program) to determine which one failed.
____

Before starting an abstract command, a debugger must ensure that , , and
are all 0.

While an abstract command is executing (in is high), a debugger must not
change latexmath:[$|\hyperref[hartsel]{hartsel}|$], and must not write 1
to , , , , or .

If an abstract command does not complete in the expected time and
appears to be hung, the debugger can try to reset the hart (using or ).
If that doesn’t clear , then it can try resetting the Debug Module
(using ).

If an abstract command is started while the selected hart is unavailable
or if a hart becomes unavailable while executing an abstract command,
then the Debug Module may terminate the abstract command, setting low,
and to 4 (halt/resume). Alternatively, the command could just appear to
be hung (never goes low).

==== Abstract Command Listing

This section describes each of the different abstract commands and how
their fields should be interpreted when they are written to .

Each abstract command is a 32-bit value. The top 8 bits contain which
determines the kind of command. Table #tab:cmdtype[[tab:cmdtype]] lists
all commands.

|r|l|l|l| & Command & Page +
& Access Register Command & +
& Quick Access & +
& Access Memory Command & +

[[programbuffer]]
=== Program Buffer

To support executing arbitrary instructions on a halted hart, a Debug
Module can include a Program Buffer that a debugger can write small
programs to. DMs that support all necessary functionality using abstract
commands only may choose to omit the Program Buffer.

A debugger can write a small program to the Program Buffer, and then
execute it exactly once with the Access Register Abstract Command,
setting the bit in . The debugger can write whatever program it likes
(including jumps out of the Program Buffer), but the program must end
with `ebreak` or `c.ebreak`. An implementation may support an implicit
`ebreak` that is executed when a hart runs off the end of the Program
Buffer. This is indicated by . With this feature, a Program Buffer of
just 2 32-bit words can offer efficient debugging.

While these programs are executed, the hart does not leave Debug Mode
(see Section link:#debugmode[5.1]). If an exception is encountered
during execution of the Program Buffer, no more instructions are
executed, the hart remains in Debug Mode, and is set to 3
(`exception error`). If the debugger executes a program that doesn’t
terminate with an `ebreak` instruction, the hart will remain in Debug
Mode and the debugger will lose control of the hart.

If is 1 then the following apply:

. must be 1.
. If the debugger writes a compressed instruction into the Program
Buffer, it must be placed into the lower 16 bits and accompanied by a
compressed `nop` in the upper 16 bits.

____
__

'''''

 +
This requirement on the debugger for the case of equal to 1 is to
accommodate hardware designs that prefer to stuff instructions directly
into the pipeline when halted, instead of having the Program Buffer
exist in the address space somewhere.
____

The Program Buffer may be implemented as RAM which is accessible to the
hart. A debugger can determine if this is the case by executing small
programs that attempt to write and read back relative to `pc` while
executing from the Program Buffer. If so, the debugger has more
flexibility in what it can do with the program buffer.

=== Overview of Hart Debug States

Figure #fig:abstract_sm[4.1] shows a conceptual view of the states
passed through by a hart during run/halt debugging as influenced by the
different fields of , , , and .

.Run/Halt Debug State Machine for single-hart hardware platforms. As only a small amount of state is visible to the debugger, the states and transitions are conceptual.
[#fig:abstract_sm]
image::fig/abstract_commands.pdf[fig/abstract_commands]

[[systembusaccess]]
=== System Bus Access

A debugger can access memory from a hart’s point of view using a Program
Buffer or the Abstract Access Memory command. (Both these features are
optional.) A Debug Module may also include a System Bus Access block to
provide memory access without involving a hart, regardless of whether
Program Buffer is implemented. The System Bus Access block uses physical
addresses.

The System Bus Access block may support 8-, 16-, 32-, 64-, and 128-bit
accesses. Table #tab:sbdatabits[[tab:sbdatabits]] shows which bits in
`sbdata` are used for each access size.

|r|l| Access Size & Data Bits +
& bits 7:0 +
& bits 15:0 +
& +
& , +
& , , , +

Depending on the microarchitecture, data accessed through System Bus
Access might not always be coherent with that observed by each hart. It
is up to the debugger to enforce coherency if the implementation does
not. This specification does not define a standard way to do this.
Possibilities may include writing to special memory-mapped locations, or
executing special instructions via the Program Buffer.

____
__

'''''

 +
Implementing a System Bus Access block has several benefits even when a
Debug Module also implements a Program Buffer. First, it is possible to
access memory in a running system with minimal impact. Second, it may
improve performance when accessing memory. Third, it may provide access
to devices that a hart does not have access to.
____

=== Minimally Intrusive Debugging

Depending on the task it is performing, some harts can only be halted
very briefly. There are several mechanisms that allow accessing
resources in such a running system with a minimal impact on the running
hart.

First, an implementation may allow some abstract commands to execute
without halting the hart.

Second, the Quick Access abstract command can be used to halt a hart,
quickly execute the contents of the Program Buffer, and let the hart run
again. Combined with instructions that allow Program Buffer code to
access the `data` registers, as described in , this can be used to
quickly perform a memory or register access. For some hardware platforms
this will be too intrusive, but many hardware platforms that can’t be
halted can bear an occasional hiccup of a hundred or less cycles.

Third, if the System Bus Access block is implemented, it can be used
while a hart is running to access system memory.

=== Security

To protect intellectual property it may be desirable to lock access to
the Debug Module. To allow access during a manufacturing process and not
afterwards, a reasonable solution could be to add a fuse bit to the
Debug Module that can be used to be permanently disable it. Since this
is technology specific, it is not further addressed in this spec.

Another option is to allow the DM to be unlocked only by users who have
an access key. Between , , and arbitrarily complex authentication
mechanism can be supported. When is clear, the DM must not interact with
the rest of the hardware platform, nor expose details about the harts
connected to the DM. All DM registers should read 0, while writes should
be ignored, with the following mandatory exceptions:

. in is readable.
. in is readable.
. in is readable.
. in is readable and writable.
. is readable and writable.

Implementations where it’s not possible to unlock the DM by using should
not implement that register.

=== Version Detection

To detect the version of the Debug Module with a minimum of side
effects, use the following procedure:

. Read .
. If is 0 or is 1:
.. Write , preserving , , , and from the value that was read, setting ,
and clearing all the other bits.
.. Read until is high.
. Read , which contains .

If it was necessary to clear , this might have the following unavoidable
side effects:

. is cleared, potentially preventing a halt request made by a previous
debugger from taking effect.
. is cleared, potentially preventing a resume request made by a previous
debugger from taking effect.
. is deasserted, releasing the hardware platform from reset if a
previous debugger had set it.
. is asserted, releasing the DM from reset. This in itself is not
observable by any harts.

This procedure is guaranteed to work in future versions of this spec.
The meaning of the bits where , , , and currently reside might change,
but preserving them will have no side effects. Clearing the bits of not
explicitly mentioned here will have no side effects beyond the ones
mentioned above.

[[dmdebbus]]
=== Debug Module Registers

The registers described in this section are accessed over the DMI bus.
Each DM has a base address (which is 0 for the first DM). The register
addresses below are offsets from this base address.

Debug Module DMI Registers that are unimplemented or not mentioned in
the table below return 0 when read. Writing them has no effect.

[[dm]]
.Debug Module Debug Bus Registers
[cols=">,<,<",options="header",]
|===
|Address |Name |Page
|_Continued on next page_ | |
|0x04 |Abstract Data 0 (`data0`) |
|0x05 |Abstract Data 1 (`data1`) |
|0x06 |Abstract Data 2 (`data2`) |
|0x07 |Abstract Data 3 (`data3`) |
|0x08 |Abstract Data 4 (`data4`) |
|0x09 |Abstract Data 5 (`data5`) |
|0x0a |Abstract Data 6 (`data6`) |
|0x0b |Abstract Data 7 (`data7`) |
|0x0c |Abstract Data 8 (`data8`) |
|0x0d |Abstract Data 9 (`data9`) |
|0x0e |Abstract Data 10 (`data10`) |
|0x0f |Abstract Data 11 (`data11`) |
|0x10 |Debug Module Control (`dmcontrol`) |
|0x11 |Debug Module Status (`dmstatus`) |
|0x12 |Hart Info (`hartinfo`) |
|0x13 |Halt Summary 1 (`haltsum1`) |
|0x14 |Hart Array Window Select (`hawindowsel`) |
|0x15 |Hart Array Window (`hawindow`) |
|0x16 |Abstract Control and Status (`abstractcs`) |
|0x17 |Abstract Command (`command`) |
|0x18 |Abstract Command Autoexec (`abstractauto`) |
|0x19 |Configuration Structure Pointer 0 (`confstrptr0`) |
|0x1a |Configuration Structure Pointer 1 (`confstrptr1`) |
|0x1b |Configuration Structure Pointer 2 (`confstrptr2`) |
|0x1c |Configuration Structure Pointer 3 (`confstrptr3`) |
|0x1d |Next Debug Module (`nextdm`) |
|0x1f |Custom Features (`custom`) |
|0x20 |Program Buffer 0 (`progbuf0`) |
|0x21 |Program Buffer 1 (`progbuf1`) |
|0x22 |Program Buffer 2 (`progbuf2`) |
|0x23 |Program Buffer 3 (`progbuf3`) |
|0x24 |Program Buffer 4 (`progbuf4`) |
|0x25 |Program Buffer 5 (`progbuf5`) |
|0x26 |Program Buffer 6 (`progbuf6`) |
|0x27 |Program Buffer 7 (`progbuf7`) |
|0x28 |Program Buffer 8 (`progbuf8`) |
|0x29 |Program Buffer 9 (`progbuf9`) |
|0x2a |Program Buffer 10 (`progbuf10`) |
|0x2b |Program Buffer 11 (`progbuf11`) |
|0x2c |Program Buffer 12 (`progbuf12`) |
|0x2d |Program Buffer 13 (`progbuf13`) |
|0x2e |Program Buffer 14 (`progbuf14`) |
|0x2f |Program Buffer 15 (`progbuf15`) |
|0x30 |Authentication Data (`authdata`) |
|0x32 |Debug Module Control and Status 2 (`dmcs2`) |
|0x34 |Halt Summary 2 (`haltsum2`) |
|0x35 |Halt Summary 3 (`haltsum3`) |
|0x37 |System Bus Address 127:96 (`sbaddress3`) |
|0x38 |System Bus Access Control and Status (`sbcs`) |
|0x39 |System Bus Address 31:0 (`sbaddress0`) |
|0x3a |System Bus Address 63:32 (`sbaddress1`) |
|0x3b |System Bus Address 95:64 (`sbaddress2`) |
|0x3c |System Bus Data 31:0 (`sbdata0`) |
|0x3d |System Bus Data 63:32 (`sbdata1`) |
|0x3e |System Bus Data 95:64 (`sbdata2`) |
|0x3f |System Bus Data 127:96 (`sbdata3`) |
|0x40 |Halt Summary 0 (`haltsum0`) |
|0x70 |Custom Features 0 (`custom0`) |
|0x71 |Custom Features 1 (`custom1`) |
|0x72 |Custom Features 2 (`custom2`) |
|0x73 |Custom Features 3 (`custom3`) |
|0x74 |Custom Features 4 (`custom4`) |
|0x75 |Custom Features 5 (`custom5`) |
|0x76 |Custom Features 6 (`custom6`) |
|0x77 |Custom Features 7 (`custom7`) |
|0x78 |Custom Features 8 (`custom8`) |
|0x79 |Custom Features 9 (`custom9`) |
|0x7a |Custom Features 10 (`custom10`) |
|0x7b |Custom Features 11 (`custom11`) |
|0x7c |Custom Features 12 (`custom12`) |
|0x7d |Custom Features 13 (`custom13`) |
|0x7e |Custom Features 14 (`custom14`) |
|0x7f |Custom Features 15 (`custom15`) |
|===

==== Debug Module Status (`dmstatus`, at 0x11)

[#dmDmstatus]## This register reports status for the overall Debug
Module as well as the currently selected harts, as defined in . Its
address will not change in the future, because it contains .

This entire register is read-only.

p3.5 exp3.5 exp7.5 exp7.5 exp6.5 exp6.5 exp4.5 exp4.5 exp2.4 exp2.4
exp6.0 exp6.0 ex 31 & & & & & 21 & & +
& & & & & +
& & & & & +

p6.0 exp6.0 exp6.0 exp6.0 exp6.0 exp6.0 exp7.0 exp7.0 exp7.0 exp7.0
exp5.0 exp5.0 ex & & & & & +
& & & & & +
& & & & & +

p5.0 exp5.0 exp5.0 exp5.0 exp5.0 exp5.0 exp4.5 exp4.5 exp4.5 exp4.5
exp6.5 exp6.5 ex & & & & & +
& & & & & +
& & & & & +

p4.0 exp4.0 exp7.5 exp7.5 exp7.5 exp7.5 exp3.5 exp3.5 ex & & & 3 & +
& & & +
& & & +

|l|p0.5|c|l| Field & Description & Access & Reset +
 +
[#dmDmstatusNdmresetpending]## |ndmresetpending| &

0 (false): Unimplemented, or is zero and no ndmreset is currently in
progress.

1 (true): is currently nonzero, or there is an ndmreset in progress. & R
& - +
[#dmDmstatusStickyunavail]## |stickyunavail| &

0 (current): The per-hart `unavail` bits reflect the current state of
the hart.

1 (sticky): The per-hart `unavail` bits are sticky. Once they are set,
they will not clear until the debugger acknowledges them using . & R &
Preset +
[#dmDmstatusImpebreak]## |impebreak| & If 1, then there is an implicit
`ebreak` instruction at the non-existent word immediately after the
Program Buffer. This saves the debugger from having to write the
`ebreak` itself, and allows the Program Buffer to be one word smaller.

This must be 1 when is 1. & R & Preset +
[#dmDmstatusAllhavereset]## |allhavereset| & This field is 1 when all
currently selected harts have been reset and reset has not been
acknowledged for any of them. & R & - +
[#dmDmstatusAnyhavereset]## |anyhavereset| & This field is 1 when at
least one currently selected hart has been reset and reset has not been
acknowledged for that hart. & R & - +
[#dmDmstatusAllresumeack]## |allresumeack| & This field is 1 when all
currently selected harts have their resume ack bit set. & R & - +
[#dmDmstatusAnyresumeack]## |anyresumeack| & This field is 1 when any
currently selected hart has its resume ack bit set. & R & - +
[#dmDmstatusAllnonexistent]## |allnonexistent| & This field is 1 when
all currently selected harts do not exist in this hardware platform. & R
& - +
[#dmDmstatusAnynonexistent]## |anynonexistent| & This field is 1 when
any currently selected hart does not exist in this hardware platform. &
R & - +
[#dmDmstatusAllunavail]## |allunavail| & This field is 1 when all
currently selected harts are unavailable, or (if is 1) were unavailable
without that being acknowledged. & R & - +
[#dmDmstatusAnyunavail]## |anyunavail| & This field is 1 when any
currently selected hart is unavailable, or (if is 1) was unavailable
without that being acknowledged. & R & - +
[#dmDmstatusAllrunning]## |allrunning| & This field is 1 when all
currently selected harts are running. & R & - +
[#dmDmstatusAnyrunning]## |anyrunning| & This field is 1 when any
currently selected hart is running. & R & - +
[#dmDmstatusAllhalted]## |allhalted| & This field is 1 when all
currently selected harts are halted. & R & - +
[#dmDmstatusAnyhalted]## |anyhalted| & This field is 1 when any
currently selected hart is halted. & R & - +
[#dmDmstatusAuthenticated]## |authenticated| &

0 (false): Authentication is required before using the DM.

1 (true): The authentication check has passed.

On components that don’t implement authentication, this bit must be
preset as 1. & R & Preset +
[#dmDmstatusAuthbusy]## |authbusy| &

0 (ready): The authentication module is ready to process the next
read/write to .

1 (busy): The authentication module is busy. Accessing results in
unspecified behavior.

only becomes set in immediate response to an access to . & R & 0 +
[#dmDmstatusHasresethaltreq]## |hasresethaltreq| & 1 if this Debug
Module supports halt-on-reset functionality controllable by the and
bits. 0 otherwise. & R & Preset +
[#dmDmstatusConfstrptrvalid]## |confstrptrvalid| &

0 (invalid): –hold information which is not relevant to the
configuration structure.

1 (valid): –hold the address of the configuration structure. & R &
Preset +
[#dmDmstatusVersion]## |version| &

0 (none): There is no Debug Module present.

1 (0.11): There is a Debug Module and it conforms to version 0.11 of
this specification.

2 (0.13): There is a Debug Module and it conforms to version 0.13 of
this specification.

3 (1.0): There is a Debug Module and it conforms to version 1.0 of this
specification.

15 (custom): There is a Debug Module but it does not conform to any
available version of this spec. & R & 3 +

==== Debug Module Control (`dmcontrol`, at 0x10)

[#dmDmcontrol]## This register controls the overall Debug Module as well
as the currently selected harts, as defined in .

[#hartsel]## Throughout this document we refer to
latexmath:[$|\hyperref[hartsel]{hartsel}|$], which is combined with .
While the spec allows for 20 latexmath:[$|\hyperref[hartsel]{hartsel}|$]
bits, an implementation may choose to implement fewer than that. The
actual width of latexmath:[$|\hyperref[hartsel]{hartsel}|$] is called
`HARTSELLEN`. It must be at least 0 and at most 20. A debugger should
discover `HARTSELLEN` by writing all ones to
latexmath:[$|\hyperref[hartsel]{hartsel}|$](assuming the maximum size)
and reading back the value to see which bits were actually set.
Debuggers must not change latexmath:[$|\hyperref[hartsel]{hartsel}|$]
while an abstract command is executing.

____
__

'''''

 +
There are separate and bits so that it is possible to write without
changing the halt-on-reset request bit for each selected hart, when not
all selected harts have the same configuration.
____

On any given write, a debugger may only write 1 to at most one of the
following bits: , , , , and . The others must be written 0.

[#resethaltreq]## latexmath:[$|\hyperref[resethaltreq]{resethaltreq}|$]
is an optional internal bit of per-hart state that cannot be read, but
can be written with and .

[#keepalive]## latexmath:[$|\hyperref[keepalive]{keepalive}|$] is an
optional internal bit of per-hart state. When it is set, it suggests
that the hardware should attempt to keep the hart available for the
debugger, e.g. by keeping it from entering a low-power state once
powered on. Even if the bit is implemented, hardware might not be able
to keep a hart available. The bit is written through and .

For forward compatibility, will always be readable when bit 1 () is 0
and bit 0 () is 1.

p3.5 exp3.5 exp4.5 exp4.5 exp4.5 exp4.5 exp6.0 exp6.0 exp5.0 exp5.0 ex &
& & & +
& & & & +
& & & & +

p2.5 exp2.5 exp5.0 exp5.0 exp6.7 exp3.3 exp6.0 exp6.0 exp6.0 exp6.0 ex &
25 & & 15 & & & +
& & & & +
& & & & +

p7.5 exp7.5 exp7.5 exp7.5 exp4.0 exp4.0 exp4.0 exp4.0 ex & & & +
& & & +
& & & +

|l|p0.5|c|l| Field & Description & Access & Reset +
 +
[#dmDmcontrolHaltreq]## |haltreq| & Writing 0 clears the halt request
bit for all currently selected harts. This may cancel outstanding halt
requests for those harts.

Writing 1 sets the halt request bit for all currently selected harts.
Running harts will halt whenever their halt request bit is set.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & WARZ & - +
[#dmDmcontrolResumereq]## |resumereq| & Writing 1 causes the currently
selected harts to resume once, if they are halted when the write occurs.
It also clears the resume ack bit for those harts.

is ignored if is set.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & W1 & - +
[#dmDmcontrolHartreset]## |hartreset| & This optional field writes the
reset bit for all the currently selected harts. To perform a reset the
debugger writes 1, and then writes 0 to deassert the reset signal.

While this bit is 1, the debugger must not change which harts are
selected.

If this feature is not implemented, the bit always stays 0, so after
writing 1 the debugger can read the register back to see if the feature
is supported.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & WARL & 0 +
[#dmDmcontrolAckhavereset]## |ackhavereset| &

0 (nop): No effect.

1 (ack): Clears `havereset` for any selected harts.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & W1 & - +
[#dmDmcontrolAckunavail]## |ackunavail| &

0 (nop): No effect.

1 (ack): Clears `unavail` for any selected harts that are currently
available.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & W1 & - +
[#dmDmcontrolHasel]## |hasel| & Selects the definition of currently
selected harts.

0 (single): There is a single currently selected hart, that is selected
by latexmath:[$|\hyperref[hartsel]{hartsel}|$].

1 (multiple): There may be multiple currently selected harts – the hart
selected by latexmath:[$|\hyperref[hartsel]{hartsel}|$], plus those
selected by the hart array mask register.

An implementation which does not implement the hart array mask register
must tie this field to 0. A debugger which wishes to use the hart array
mask register feature should set this bit and read back to see if the
functionality is supported. & WARL & 0 +
[#dmDmcontrolHartsello]## |hartsello| & The low 10 bits of
latexmath:[$|\hyperref[hartsel]{hartsel}|$]: the DM-specific index of
the hart to select. This hart is always part of the currently selected
harts. & WARL & 0 +
[#dmDmcontrolHartselhi]## |hartselhi| & The high 10 bits of
latexmath:[$|\hyperref[hartsel]{hartsel}|$]: the DM-specific index of
the hart to select. This hart is always part of the currently selected
harts. & WARL & 0 +
[#dmDmcontrolSetkeepalive]## |setkeepalive| & This optional field sets
latexmath:[$|\hyperref[keepalive]{keepalive}|$] for all currently
selected harts, unless is simultaneously set to 1.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & W1 & - +
[#dmDmcontrolClrkeepalive]## |clrkeepalive| & This optional field clears
latexmath:[$|\hyperref[keepalive]{keepalive}|$] for all currently
selected harts.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & W1 & - +
[#dmDmcontrolSetresethaltreq]## |setresethaltreq| & This optional field
writes the halt-on-reset request bit for all currently selected harts,
unless is simultaneously set to 1. When set to 1, each selected hart
will halt upon the next deassertion of its reset. The halt-on-reset
request bit is not automatically cleared. The debugger must write to to
clear it.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and .

If is 0, this field is not implemented. & W1 & - +
[#dmDmcontrolClrresethaltreq]## |clrresethaltreq| & This optional field
clears the halt-on-reset request bit for all currently selected harts.

Writes apply to the new value of
latexmath:[$|\hyperref[hartsel]{hartsel}|$] and . & W1 & - +
[#dmDmcontrolNdmreset]## |ndmreset| & This bit controls the reset signal
from the DM to the rest of the hardware platform. The signal should
reset every part of the hardware platform, including every hart, except
for the DM and any logic required to access the DM. To perform a
hardware platform reset the debugger writes 1, and then writes 0 to
deassert the reset. & R/W & 0 +
[#dmDmcontrolDmactive]## |dmactive| & This bit serves as a reset signal
for the Debug Module itself. After changing the value of this bit, the
debugger must poll until has taken the requested value before performing
any action that assumes the requested state change has completed.
Hardware may take an arbitrarily long time to complete activation or
deactivation and will indicate completion by setting to the requested
value.

0 (inactive): The module’s state, including authentication mechanism,
takes its reset values (the bit is the only bit which can be written to
something other than its reset value). Any accesses to the module may
fail. Specifically, might not return correct data.

1 (active): The module functions normally.

No other mechanism should exist that may result in resetting the Debug
Module after power up.

To place the Debug Module into a known state, a debugger may write 0 to
, poll until is observed 0, write 1 to , and poll until is observed 1.

Implementations may pay attention to this bit to further aid debugging,
for example by preventing the Debug Module from being power gated while
debugging is active. & R/W & 0 +

==== Hart Info (`hartinfo`, at 0x12)

[#dmHartinfo]## This register gives information about the hart currently
selected by latexmath:[$|\hyperref[hartsel]{hartsel}|$].

This register is optional. If it is not present it should read all-zero.

If this register is included, the debugger can do more with the Program
Buffer by writing programs which explicitly access the `data` and/or
`dscratch` registers.

This entire register is read-only.

p4.0 exp4.0 exp4.0 exp4.0 exp2.4 exp2.4 exp5.0 exp5.0 exp4.0 exp4.0
exp8.0 exp4.0 ex 31 & & 23 & & 19 & & & 15 & & 11 & +
& & & & & +
& & & & & +

|l|p0.5|c|l| Field & Description & Access & Reset +
 +
[#dmHartinfoNscratch]## |nscratch| & Number of `dscratch` registers
available for the debugger to use during program buffer execution,
starting from . The debugger can make no assumptions about the contents
of these registers between commands. & R & Preset +
[#dmHartinfoDataaccess]## |dataaccess| &

0 (csr): The `data` registers are shadowed in the hart by CSRs. Each CSR
is DXLEN bits in size, and corresponds to a single argument, per
Table #tab:datareg[[tab:datareg]].

1 (memory): The `data` registers are shadowed in the hart’s memory map.
Each register takes up 4 bytes in the memory map. & R & Preset +
[#dmHartinfoDatasize]## |datasize| & If is 0: Number of CSRs dedicated
to shadowing the `data` registers.

If is 1: Number of 32-bit words in the memory map dedicated to shadowing
the `data` registers.

If this value is non-zero, then the tt data registers must be
traditional registers and not MRs.

Since there are at most 12 `data` registers, the value in this register
must be 12 or smaller. & R & Preset +
[#dmHartinfoDataaddr]## |dataaddr| & If is 0: The number of the first
CSR dedicated to shadowing the `data` registers.

If is 1: Address of RAM where the data registers are shadowed. This
address is sign extended giving a range of -2048 to 2047, easily
addressed with a load or store using `x0` as the address register. & R &
Preset +

==== Hart Array Window Select (`hawindowsel`, at 0x14)

[#dmHawindowsel]## This register selects which of the 32-bit portion of
the hart array mask register (see Section link:#hartarraymask[4.3.2]) is
accessible in .

p8.5 exp8.5 exp10.0 exp5.0 ex 31 & & 14 & +
& +
& +

|l|p0.5|c|l| Field & Description & Access & Reset +
 +
[#dmHawindowselHawindowsel]## |hawindowsel| & The high bits of this
field may be tied to 0, depending on how large the array mask register
is. E.g. on a hardware platform with 48 harts only bit 0 of this field
may actually be writable. & WARL & 0 +

==== Hart Array Window (`hawindow`, at 0x15)

[#dmHawindow]## This register provides R/W access to a 32-bit portion of
the hart array mask register (see Section link:#hartarraymask[4.3.2]).
The position of the window is determined by . I.e. bit 0 refers to hart
latexmath:[${dm-hawindowsel} * 32$], while bit 31 refers to hart
latexmath:[${dm-hawindowsel} * 32 + 31$].

Since some bits in the hart array mask register may be constant 0, some
bits in this register may be constant 0, depending on the current value
of .

p21.3 exp10.7 ex 31 & +
 +
 +

==== Abstract Control and Status (`abstractcs`, at 0x16)

[#dmAbstractcs]## Writing this register while an abstract command is
executing causes to become 1 (busy) once the command completes (busy
becomes 0).

____
__

'''''

 +
must be at least 1 to support RV32 harts, 2 to support RV64 harts, or 4
to support RV128 harts.
____

p2.4 exp2.4 exp5.5 exp5.5 exp5.5 exp5.5 exp2.4 exp2.4 ex 31 & & 28 & &
23 & & +
& & & +
& & & +

p5.5 exp5.5 exp4.0 exp2.0 exp2.0 exp2.0 exp4.5 exp4.5 ex & 10 & & 7 & &
3 & +
& & & +
& & & +

|l|p0.5|c|l| Field & Description & Access & Reset +
 +
[#dmAbstractcsProgbufsize]## |progbufsize| & Size of the Program Buffer,
in 32-bit words. Valid sizes are 0 - 16. & R & Preset +
[#dmAbstractcsBusy]## |busy| &

0 (ready): There is no abstract command currently being executed.

1 (busy): An abstract command is currently being executed.

This bit is set as soon as is written, and is not cleared until that
command has completed. & R & 0 +
[#dmAbstractcsRelaxedpriv]## |relaxedpriv| & This optional bit controls
whether program buffer and abstract memory accesses are performed with
the exact and full set of permission checks that apply based on the
current architectural state of the hart performing the access, or with a
relaxed set of permission checks (e.g. PMP restrictions are ignored).
The details of the latter are implementation-specific.

0 (full checks): Full permission checks apply.

1 (relaxed checks): Relaxed permission checks apply. & WARL & Preset +
[#dmAbstractcsCmderr]## |cmderr| & Gets set if an abstract command
fails. The bits in this field remain set until they are cleared by
writing 1 to them. No abstract command is started until the value is
reset to 0.

This field only contains a valid value if is 0.

0 (none): No error.

1 (busy): An abstract command was executing while , , or was written, or
when one of the `data` or `progbuf` registers was read or written. This
status is only written if contains 0.

2 (not supported): The command in is not supported. It may be supported
with different options set, but it will not be supported at a later time
when the hart or system state are different.

3 (exception): An exception occurred while executing the command
(e.g. while executing the Program Buffer).

4 (halt/resume): The abstract command couldn’t execute because the hart
wasn’t in the required state (running/halted), or unavailable.

5 (bus): The abstract command failed due to a bus error (e.g. alignment,
access size, or timeout).

6 (reserved): Reserved for future use.

7 (other): The command failed for another reason. & R/W1C & 0 +
[#dmAbstractcsDatacount]## |datacount| & Number of `data` registers that
are implemented as part of the abstract command interface. Valid sizes
are 1 – 12. & R & Preset +

==== Abstract Command (`command`, at 0x17)

[#dmCommand]## Writes to this register cause the corresponding abstract
command to be executed.

Writing this register while an abstract command is executing causes to
become 1 (busy) once the command completes (busy becomes 0).

If is non-zero, writes to this register are ignored.

____
__

'''''

 +
inhibits starting a new command to accommodate debuggers that, for
performance reasons, send several commands to be executed in a row
without checking in between. They can safely do so and check at the end
without worrying that one command failed but then a later command (which
might have depended on the previous one succeeding) passed.
____

[cols="<,<,<,<",options="header",]
|===
|31 |24 |23 |0
|latexmath:[$|cmdtype|$] | |latexmath:[$|control|$] |
|8 | |24 |
|===

[[dmCommandControl]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|[#dmCommandCmdtype]## \|cmdtype\| |The type determines the overall
functionality of this abstract command. |WARZ |0

|\|control\| |This field is interpreted in a command-specific manner,
described for each abstract command. |WARZ |0
|===

==== Abstract Command Autoexec (`abstractauto`, at 0x18)

[#dmAbstractauto]## This register is optional. Including it allows more
efficient burst accesses. A debugger can detect whether it is supported
by setting bits and reading them back.

If this register is implemented then bits corresponding to implemented
progbuf and data registers must be writable. Other bits must be
hard-wired to 0.

If this register is written while an abstract command is executing then
the write is ignored and becomes 1 (busy) once the command completes
(busy becomes 0).

[cols="<,<,<,<,<,<",options="header",]
|===
|31 |16 |15 |12 |11 |0
|latexmath:[$|autoexecprogbuf|$] | |latexmath:[$|0|$] |
|latexmath:[$|autoexecdata|$] |

|16 | |4 | |12 |
|===

[[dmAbstractautoAutoexecdata]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|[#dmAbstractautoAutoexecprogbuf]## \|autoexecprogbuf\| |When a bit in
this field is 1, read or write accesses to the corresponding `progbuf`
word cause the DM to act as if the current value in was written there
again after the access to `progbuf` completes. |WARL |0

|\|autoexecdata\| |When a bit in this field is 1, read or write accesses
to the corresponding `data` word cause the DM to act as if the current
value in was written there again after the access to `data` completes.
|WARL |0
|===

==== Configuration Structure Pointer 0 (`confstrptr0`, at 0x19)

[#dmConfstrptrZero]## When is set, reading this register returns bits
31:0 of the configuration structure pointer. Reading the other
`confstrptr` registers returns the upper bits of the address.

When system bus access is implemented, this must be an address that can
be used with the System Bus Access module. Otherwise, this must be an
address that can be used to access the configuration structure from the
hart with ID 0.

If is 0, then the `confstrptr` registers hold identifier information
which is not further specified in this document.

The configuration structure itself is a data structure of the same
format as the data structure pointed to by mconfigptr as described in
the Privileged Spec.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|addr|$] |
|32 |
|===

==== Configuration Structure Pointer 1 (`confstrptr1`, at 0x1a)

[#dmConfstrptrOne]## When is set, reading this register returns bits
63:32 of the configuration structure pointer. See for more details.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|addr|$] |
|32 |
|===

==== Configuration Structure Pointer 2 (`confstrptr2`, at 0x1b)

[#dmConfstrptrTwo]## When is set, reading this register returns bits
95:64 of the configuration structure pointer. See for more details.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|addr|$] |
|32 |
|===

==== Configuration Structure Pointer 3 (`confstrptr3`, at 0x1c)

[#dmConfstrptrThree]## When is set, reading this register returns bits
127:96 of the configuration structure pointer. See for more details.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|addr|$] |
|32 |
|===

==== Next Debug Module (`nextdm`, at 0x1d)

[#dmNextdm]## If there is more than one DM accessible on this DMI, this
register contains the base address of the next one in the chain, or 0 if
this is the last one in the chain.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|addr|$] |
|32 |
|===

==== Abstract Data 0 (`data0`, at 0x04)

[#dmDataZero]## through may be Message Registers, whose behavior is
described in Section #sec:mr[[sec:mr]]. These registers may be read or
changed by abstract commands. indicates how many of them are
implemented, starting at , counting up.
Table #tab:datareg[[tab:datareg]] shows how abstract commands use these
registers.

Accessing these registers while an abstract command is executing causes
to be set to 1 (busy) if it is 0.

Attempts to write them while is set does not change their value.

The values in these registers might not be preserved after an abstract
command is executed. The only guarantees on their contents are the ones
offered by the command in question. If the command fails, no assumptions
can be made about the contents of these registers.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|data|$] |
|32 |
|===

==== Program Buffer 0 (`progbuf0`, at 0x20)

[#dmProgbufZero]## through must provide write access to the optional
program buffer. It may also be possible for the debugger to read from
the program buffer through these registers. If reading is not supported,
then all reads return 0.

indicates how many `progbuf` registers are implemented starting at ,
counting up.

Accessing these registers while an abstract command is executing causes
to be set to 1 (busy) if it is 0.

Attempts to write them while is set does not change their value.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|data|$] |
|32 |
|===

==== Authentication Data (`authdata`, at 0x30)

[#dmAuthdata]## This register serves as a 32-bit serial port to/from the
authentication module.

When is clear, the debugger can communicate with the authentication
module by reading or writing this register. There is no separate
mechanism to signal overflow/underflow.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|data|$] |
|32 |
|===

==== Debug Module Control and Status 2 (`dmcs2`, at 0x32)

[#dmDmcsTwo]## This register contains DM control and status bits that
didn’t easily fit in and . All are optional.

If halt groups are not implemented, then will always be 0 when is 0.

If resume groups are not implemented, then will remain 0 even after 1 is
written there.

The DM external triggers available to add to halt groups may be the same
as or distinct from the DM external triggers available to add to resume
groups.

[cols="<,<,<,<,<,<,<,<,<,<,<,<",options="header",]
|===
|31 |12 |11 | |10 |7 |6 |2 |1 | |0 |
|latexmath:[$|0|$] | |latexmath:[$|grouptype|$] |
|latexmath:[$|dmexttrigger|$] | |latexmath:[$|group|$] |
|latexmath:[$|hgwrite|$] | |latexmath:[$|hgselect|$] |

|20 | |1 | |4 | |5 | |1 | |1 |
|===

[[dmDmcsTwoHgselect]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|[#dmDmcsTwoGrouptype]## \|grouptype\| a|
0 (halt): The remaining fields in this register configure halt groups.

1 (resume): The remaining fields in this register configure resume
groups.

|WARL |0

|[#dmDmcsTwoDmexttrigger]## \|dmexttrigger\| a|
This field contains the currently selected DM external trigger.

If a non-existent trigger value is written here, the hardware will
change it to a valid one or 0 if no DM external triggers exist.

|WARL |0

|[#dmDmcsTwoGroup]## \|group\| a|
When is 0, contains the group of the hart specified by
latexmath:[$|\hyperref[hartsel]{hartsel}|$].

When is 1, contains the group of the DM external trigger selected by .

The value written to this field is ignored unless is also written 1.

Group numbers are contiguous starting at 0, with the highest number
being implementation-dependent, and possibly different between different
group types. Debuggers should read back this field after writing to
confirm they are using a hart group that is supported.

If groups aren’t implemented, then this entire field is 0.

|WARL |preset

|[#dmDmcsTwoHgwrite]## \|hgwrite\| a|
When 1 is written and is 0, for every selected hart the DM will change
its group to the value written to , if the hardware supports that group
for that hart. Implementations may also change the group of a minimal
set of unselected harts in the same way, if that is necessary due to a
hardware limitation.

When 1 is written and is 1, the DM will change the group of the DM
external trigger selected by to the value written to , if the hardware
supports that group for that trigger.

Writing 0 has no effect.

|W1 |-

|\|hgselect\| a|
0 (harts): Operate on harts.

1 (triggers): Operate on DM external triggers.

If there are no DM external triggers, this field must be tied to 0.

|WARL |0
|===

==== Halt Summary 0 (`haltsum0`, at 0x40)

[#dmHaltsumZero]## Each bit in this read-only register indicates whether
one specific hart is halted or not. Unavailable/nonexistent harts are
not considered to be halted.

This register might not be present if fewer than 2 harts are connected
to this DM.

The LSB reflects the halt status of hart \{hartsel[19:5],5’h0}, and the
MSB reflects halt status of hart \{hartsel[19:5],5’h1f}.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|haltsum0|$] |
|32 |
|===

==== Halt Summary 1 (`haltsum1`, at 0x13)

[#dmHaltsumOne]## Each bit in this read-only register indicates whether
any of a group of harts is halted or not. Unavailable/nonexistent harts
are not considered to be halted.

This register might not be present if fewer than 33 harts are connected
to this DM.

The LSB reflects the halt status of harts \{hartsel[19:10],10’h0}
through \{hartsel[19:10],10’h1f}. The MSB reflects the halt status of
harts \{hartsel[19:10],10’h3e0} through \{hartsel[19:10],10’h3ff}.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|haltsum1|$] |
|32 |
|===

==== Halt Summary 2 (`haltsum2`, at 0x34)

[#dmHaltsumTwo]## Each bit in this read-only register indicates whether
any of a group of harts is halted or not. Unavailable/nonexistent harts
are not considered to be halted.

This register might not be present if fewer than 1025 harts are
connected to this DM.

The LSB reflects the halt status of harts \{hartsel[19:15],15’h0}
through \{hartsel[19:15],15’h3ff}. The MSB reflects the halt status of
harts \{hartsel[19:15],15’h7c00} through \{hartsel[19:15],15’h7fff}.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|haltsum2|$] |
|32 |
|===

==== Halt Summary 3 (`haltsum3`, at 0x35)

[#dmHaltsumThree]## Each bit in this read-only register indicates
whether any of a group of harts is halted or not.
Unavailable/nonexistent harts are not considered to be halted.

This register might not be present if fewer than 32769 harts are
connected to this DM.

The LSB reflects the halt status of harts 20’h0 through 20’h7fff. The
MSB reflects the halt status of harts 20’hf8000 through 20’hfffff.

This entire register is read-only.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|haltsum3|$] |
|32 |
|===

==== System Bus Access Control and Status (`sbcs`, at 0x38)

[#dmSbcs]##

[cols="<,<,<,<,<,<,<,<,<,<",options="header",]
|===
|31 |29 |28 |23 |22 | |21 | |20 |
|latexmath:[$|sbversion|$] | |latexmath:[$|0|$] |
|latexmath:[$|sbbusyerror|$] | |latexmath:[$|sbbusy|$] |
|latexmath:[$|sbreadonaddr|$] |

|3 | |6 | |1 | |1 | |1 |
|===

[cols="<,<,<,<,<,<,<,<,<,<",options="header",]
|===
|19 |17 |16 | |15 | |14 |12 |11 |5
|latexmath:[$|sbaccess|$] | |latexmath:[$|sbautoincrement|$] |
|latexmath:[$|sbreadondata|$] | |latexmath:[$|sberror|$] |
|latexmath:[$|sbasize|$] |

|3 | |1 | |1 | |3 | |7 |
|===

[cols="<,<,<,<,<,<,<,<,<,<",options="header",]
|===
|4 | |3 | |2 | |1 | |0 |
|latexmath:[$|sbaccess128|$] | |latexmath:[$|sbaccess64|$] |
|latexmath:[$|sbaccess32|$] | |latexmath:[$|sbaccess16|$] |
|latexmath:[$|sbaccess8|$] |

|1 | |1 | |1 | |1 | |1 |
|===

[[dmSbcsSbaccessEight]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|[#dmSbcsSbversion]## \|sbversion\| a|
0 (legacy): The System Bus interface conforms to mainline drafts of this
spec older than 1 January, 2018.

1 (1.0): The System Bus interface conforms to this version of the spec.

Other values are reserved for future versions.

|R |1

|[#dmSbcsSbbusyerror]## \|sbbusyerror\| a|
Set when the debugger attempts to read data while a read is in progress,
or when the debugger initiates a new access while one is already in
progress (while is set). It remains set until it’s explicitly cleared by
the debugger.

While this field is set, no more system bus accesses can be initiated by
the Debug Module.

|R/W1C |0

|[#dmSbcsSbbusy]## \|sbbusy\| a|
When 1, indicates the system bus manager is busy. (Whether the system
bus itself is busy is related, but not the same thing.) This bit goes
high immediately when a read or write is requested for any reason, and
does not go low until the access is fully completed.

Writes to while is high result in undefined behavior. A debugger must
not write to until it reads as 0.

|R |0

|[#dmSbcsSbreadonaddr]## \|sbreadonaddr\| |When 1, every write to
automatically triggers a system bus read at the new address. |R/W |0

|[#dmSbcsSbaccess]## \|sbaccess\| a|
Select the access size to use for system bus accesses.

0 (8bit): 8-bit

1 (16bit): 16-bit

2 (32bit): 32-bit

3 (64bit): 64-bit

4 (128bit): 128-bit

If has an unsupported value when the DM starts a bus access, the access
is not performed and is set to 4.

|R/W |2

|[#dmSbcsSbautoincrement]## \|sbautoincrement\| |When 1, `sbaddress` is
incremented by the access size (in bytes) selected in after every system
bus access. |R/W |0

|[#dmSbcsSbreadondata]## \|sbreadondata\| |When 1, every read from
automatically triggers a system bus read at the (possibly
auto-incremented) address. |R/W |0

|[#dmSbcsSberror]## \|sberror\| a|
When the Debug Module’s system bus manager encounters an error, this
field gets set. The bits in this field remain set until they are cleared
by writing 1 to them. While this field is non-zero, no more system bus
accesses can be initiated by the Debug Module.

An implementation may report ``Other'' (7) for any error condition.

0 (none): There was no bus error.

1 (timeout): There was a timeout.

2 (address): A bad address was accessed.

3 (alignment): There was an alignment error.

4 (size): An access of unsupported size was requested.

7 (other): Other.

|R/W1C |0

|[#dmSbcsSbasize]## \|sbasize\| |Width of system bus addresses in bits.
(0 indicates there is no bus access support.) |R |Preset

|[#dmSbcsSbaccessOneTwentyeight]## \|sbaccess128\| |1 when 128-bit
system bus accesses are supported. |R |Preset

|[#dmSbcsSbaccessSixtyfour]## \|sbaccess64\| |1 when 64-bit system bus
accesses are supported. |R |Preset

|[#dmSbcsSbaccessThirtytwo]## \|sbaccess32\| |1 when 32-bit system bus
accesses are supported. |R |Preset

|[#dmSbcsSbaccessSixteen]## \|sbaccess16\| |1 when 16-bit system bus
accesses are supported. |R |Preset

|\|sbaccess8\| |1 when 8-bit system bus accesses are supported. |R
|Preset
|===

==== System Bus Address 31:0 (`sbaddress0`, at 0x39)

[#dmSbaddressZero]## If is 0, then this register is not present.

When the system bus manager is busy, writes to this register will set
and don’t do anything else.

If is 0, is 0, and is set then writes to this register start the
following:

. Set .
. Perform a bus read from the new value of `sbaddress`.
. If the read succeeded and is set, increment `sbaddress`.
. Clear .

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|address|$] |
|32 |
|===

[[dmSbaddressZeroAddress]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|\|address\| |Accesses bits 31:0 of the physical address in `sbaddress`.
|R/W |0
|===

==== System Bus Address 63:32 (`sbaddress1`, at 0x3a)

[#dmSbaddressOne]## If is less than 33, then this register is not
present.

When the system bus manager is busy, writes to this register will set
and don’t do anything else.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|address|$] |
|32 |
|===

[[dmSbaddressOneAddress]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|\|address\| |Accesses bits 63:32 of the physical address in `sbaddress`
(if the system address bus is that wide). |R/W |0
|===

==== System Bus Address 95:64 (`sbaddress2`, at 0x3b)

[#dmSbaddressTwo]## If is less than 65, then this register is not
present.

When the system bus manager is busy, writes to this register will set
and don’t do anything else.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|address|$] |
|32 |
|===

[[dmSbaddressTwoAddress]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|\|address\| |Accesses bits 95:64 of the physical address in `sbaddress`
(if the system address bus is that wide). |R/W |0
|===

==== System Bus Address 127:96 (`sbaddress3`, at 0x37)

[#dmSbaddressThree]## If is less than 97, then this register is not
present.

When the system bus manager is busy, writes to this register will set
and don’t do anything else.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|address|$] |
|32 |
|===

[[dmSbaddressThreeAddress]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|\|address\| |Accesses bits 127:96 of the physical address in
`sbaddress` (if the system address bus is that wide). |R/W |0
|===

==== System Bus Data 31:0 (`sbdata0`, at 0x3c)

[#dmSbdataZero]## If all of the `sbaccess` bits in are 0, then this
register is not present.

Any successful system bus read updates `sbdata`. If the width of the
read access is less than the width of `sbdata`, the contents of the
remaining high bits may take on any value.

If either or isn’t 0 then accesses do nothing.

If the bus manager is busy then accesses set , and don’t do anything
else.

Writes to this register start the following:

. Set .
. Perform a bus write of the new value of `sbdata` to `sbaddress`.
. If the write succeeded and is set, increment `sbaddress`.
. Clear .

Reads from this register start the following:

. ``Return'' the data.
. Set .
. If is set:
.. Perform a system bus read from the address contained in `sbaddress`,
placing the result in `sbdata`.
.. If is set and the read was successful, increment `sbaddress`.
. Clear .

Only has this behavior. The other `sbdata` registers have no side
effects. On systems that have buses wider than 32 bits, a debugger
should access after accessing the other ` sbdata` registers.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|data|$] |
|32 |
|===

[[dmSbdataZeroData]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |
|\|data\| |Accesses bits 31:0 of `sbdata`. |R/W |0
|===

==== System Bus Data 63:32 (`sbdata1`, at 0x3d)

[#dmSbdataOne]## If and are 0, then this register is not present.

If the bus manager is busy then accesses set , and don’t do anything
else.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|data|$] |
|32 |
|===

[[dmSbdataOneData]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|\|data\| |Accesses bits 63:32 of `sbdata` (if the system bus is that
wide). |R/W |0
|===

==== System Bus Data 95:64 (`sbdata2`, at 0x3e)

[#dmSbdataTwo]## This register only exists if is 1.

If the bus manager is busy then accesses set , and don’t do anything
else.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|data|$] |
|32 |
|===

[[dmSbdataTwoData]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|\|data\| |Accesses bits 95:64 of `sbdata` (if the system bus is that
wide). |R/W |0
|===

==== System Bus Data 127:96 (`sbdata3`, at 0x3f)

[#dmSbdataThree]## This register only exists if is 1.

If the bus manager is busy then accesses set , and don’t do anything
else.

[cols="<,<",options="header",]
|===
|31 |0
|latexmath:[$|data|$] |
|32 |
|===

[[dmSbdataThreeData]]
[cols="<,<,^,<",options="header",]
|===
|Field |Description |Access |Reset
|_Continued on next page_ | | |

|\|data\| |Accesses bits 127:96 of `sbdata` (if the system bus is that
wide). |R/W |0
|===

==== Custom Features (`custom`, at 0x1f)

[#dmCustom]## This optional register may be used for non-standard
features. Future version of the debug spec will not use this address.

==== Custom Features 0 (`custom0`, at 0x70)

[#dmCustomZero]## The optional through registers may be used for
non-standard features. Future versions of the debug spec will not use
these addresses.

[[sec:core_debug]]
== Sdext (ISA Extension)

This chapter describes the Sdext ISA extension. It must be implemented
to make external debug work, and is only useful in conjunction with
external debug.

Modifications to the RISC-V core to support debug are kept to a minimum.
There is a special execution mode (Debug Mode) and a few extra CSRs. The
DM takes care of the rest.

In order to be compatible with this specification an implementation must
implement everything described in this chapter that is not explicitly
listed as optional.

If Sdext is implemented and Sdtrig is not implemented, then accessing
any of the Sdtrig CSRs must raise an illegal instruction exception.

[[debugmode]]
=== Debug Mode

Debug Mode is a special processor mode used only when a hart is halted
for external debugging. Because the hart is halted, there is no forward
progress in the normal instruction stream. How Debug Mode is implemented
is not specified here.

When executing code due to an abstract command, the hart stays in Debug
Mode and the following apply:

. All implemented instructions operate just as they do in M-mode, unless
an exception is mentioned in this list.
. All operations are executed with machine mode privilege, except that
additional Debug Mode CSRs are accessible and latexmath:[$|MPRV|$] in
`mstatus` may be ignored according to . Full permission checks, or a
relaxed set of permission checks, will apply according to .
. All interrupts (including NMI) are masked.
. Traps don’t take place. Instead, they end execution of the program
buffer and the hart remains in Debug Mode. Because they do not trap to
M-mode, they do not update registers such as `mstatus`, `mepc`,
`mcause`, ` mtval`, `mtval2`, and `mtinst`. The same is true for the
equivalent privileged registers that are updated when trapping to other
modes. Registers that may be updated as part of execution before the
exception are allowed to be updated. For example, vector load/store
instructions which raise exceptions may partially update the destination
register and set `vstart` appropriately.
. Triggers don’t match or fire.
. If is 0 then counters continue. If it is 1 then counters are stopped.
. If is 0 then `time` continues to update. If it is 1 then `time` will
not update. It will resynchronize with `mtime` after leaving Debug Mode.
. Instructions that place the hart into a stalled state act as a `nop`.
This includes `wfi`, `wrs.sto`, and `wrs.nto`.
. Almost all instructions that change the privilege mode have
unspecified behavior. This includes `ecall`, `mret`, `sret`, and `uret`.
(To change the privilege mode, the debugger can write and in ). The only
exception is `ebreak`, which ends execution of the Program Buffer when
executed.
. All control transfer instructions may act as illegal instructions if
their destination is in the Program Buffer. If one such instruction acts
as an illegal instruction, all such instructions must act as illegal
instructions.
. All control transfer instructions may act as illegal instructions if
their destination is outside the Program Buffer. If one such instruction
acts as an illegal instruction, all such instructions must act as
illegal instructions.
. Instructions that depend on the value of the PC (e.g. `auipc`) may act
as illegal instructions.
. Effective XLEN is DXLEN.
. Forward progress is guaranteed.

____
__

'''''

 +
When , the external debugger can set MPRV and MPP appropriately to have
hardware perform memory accesses with the appropriate endianness,
address translation, permission checks, and PMP/PMA checks (subject to
). This is also the only way to access all of physical memory when
34-bit physical addresses are supported on a Sv32 hart. If hardware ties
to 0 then the external debugger is expected to simulate all the effects
of MPRV, including any extensions that affect memory accesses. For these
reasons it is recommended to tie to 1.
____

=== Load-Reserved/Store-Conditional Instructions

The reservation registered by an `lr` instruction on a memory address
may be lost when entering Debug Mode or while in Debug Mode. This means
that there may be no forward progress if Debug Mode is entered between
`lr` and ` sc` pairs.

____
__

'''''

 +
This is a behavior that debug users must be aware of. If they have a
breakpoint set between a `lr` and `sc` pair, or are stepping through
such code, the `sc` may never succeed. Fortunately in general use there
will be very few instructions in such a sequence, and anybody debugging
it will quickly notice that the reservation is not occurring. The
solution in that case is to set a breakpoint on the first instruction
after the `sc` and run to it. A higher level debugger may choose to
automate this.
____

=== Wait for Interrupt Instruction

If halt is requested while `wfi` is executing, then the hart must leave
the stalled state, completing this instruction’s execution, and then
enter Debug Mode.

=== Wait-on-Reservation-Set Instructions

If halt is requested while `wrs.sto` or `wrs.nto` is executing, then the
hart must leave the stalled state, completing this instruction’s
execution, and then enter Debug Mode.

=== Single Step

[[stepBit]]
==== Step Bit In Dcsr

This method is only available to external debuggers, and is the
preferred way to single step.

An external debugger can cause a halted hart to execute a single
instruction or trap and then re-enter Debug Mode by setting before
resuming. If is set when a hart resumes then it will single step,
regardless of the reason for resuming.

If control is transferred to a trap handler while executing the
instruction, then Debug Mode is re-entered immediately after the PC is
changed to the trap handler, and the appropriate `tval` and `cause`
registers are updated. In this case none of the trap handler is
executed, and if the cause was a pending interrupt no instructions might
be executed at all.

If executing or fetching the instruction causes a trigger to fire with
action=1, Debug Mode is re-entered immediately after that trigger has
fired. In that case is set to 2 (trigger) instead of 4 (single step).
Whether the instruction is executed or not depends on the specific
configuration of the trigger.

If the instruction that is executed causes the PC to change to an
address where an instruction fetch causes an exception, that exception
does not occur until the next time the hart is resumed. Similarly, a
trigger at the new address does not fire until the hart actually
attempts to execute that instruction.

If the instruction being stepped over would normally stall the hart,
then instead the instruction is treated as a `nop`. This includes `wfi`,
`wrs.sto`, and `wrs.nto`.

[[stepIcount]]
==== Icount Trigger

Native debuggers won’t have access to , but can use the trigger by
setting to 1.

This approach does have some limitations:

. Interrupts will fire as usual. Debuggers that want to disable
interrupts while stepping must disable them by changing `mstatus`, and
specially handle instructions that read `mstatus`.
. `wfi` instructions are not treated specially and might take a very
long time to complete.

This mechanism cleanly supports a system which supports multiple
privilege levels, where the OS or a debug stub runs in M-Mode while the
program being debugged runs in a less privileged mode. Systems that only
support M-Mode can use as well, but count must be able to count several
instructions (depending on the software implementation). See Section
link:#nativestep[9.3.1].

[[reset-1]]
=== Reset

If the halt signal (driven by the hart’s halt request bit in the Debug
Module) or latexmath:[$|\hyperref[resethaltreq]{resethaltreq}|$] are
asserted when a hart comes out of reset, the hart must enter Debug Mode
before executing any instructions, but after performing any
initialization that would usually happen before the first instruction is
executed.

=== Halt

When a hart halts:

. is updated.
. and are set to reflect current privilege mode.
. is set to the next instruction that should be executed.
. If the current instruction can be partially executed and should be
restarted to complete, then the relevant state for that is updated. E.g.
if a halt occurs during a partially executed vector instruction, then
`vstart` is updated, and is updated to the address of the partially
executed instruction. This is analogous to how vector instructions
behave for exceptions.
. The hart enters Debug Mode.

=== Resume

When a hart resumes:

. `pc` changes to the value stored in .
. The current privilege mode and virtualization mode are changed to that
specified by and .
. If the new privilege mode is less privileged than M-mode,
latexmath:[$|MPRV|$] in `mstatus` is cleared.
. The hart is no longer in debug mode.

=== XLEN

While in Debug Mode, XLEN is DXLEN. It is up to the debugger to
determine the XLEN during normal program execution (by looking at
`misa`) and to clearly communicate this to the user.

[[debreg]]
=== Core Debug Registers

The supported Core Debug Registers must be implemented for each hart
that can be debugged. They are CSRs, accessible using the RISC-V `csr`
opcodes and optionally also using abstract debug commands.

Attempts to access an unimplemented Core Debug Register raise an illegal
instruction exception.

[[virtreg]]
=== Virtual Debug Registers

[[sec:trigger]]
== Sdtrig (ISA Extension)

This chapter describes the Sdtrig ISA extension, which can be
implemented independently of functionality described in the other
chapters. It consists exclusively of the Trigger Module (TM).

Triggers can cause a breakpoint exception, entry into Debug Mode, or a
trace action without having to execute a special instruction. This makes
them invaluable when debugging code from ROM. They can trigger on
execution of instructions at a given memory address, or on the
address/data in loads/stores.

If Sdtrig is implemented, the Trigger Module must support at least one
trigger. Accessing trigger CSRs that are not used by any of the
implemented triggers must result in an illegal instruction exception.
M-Mode and Debug Mode accesses to trigger CSRs that are used by any of
the implemented triggers must succeed, regardless of the current type of
the currently selected trigger.

A trigger matches when the conditions that it specifies (e.g. a load
from a specific address) are met. A trigger fires when a trigger that
matches performs the action configured for that trigger.

Triggers do not fire while in Debug Mode.

=== Enumeration

Each trigger may support a variety of features. A debugger can build a
list of all triggers and their features as follows:

. Write 0 to . If this results in an illegal instruction exception, then
there are no triggers implemented.
. Read back and check that it contains the written value. If not, exit
the loop.
. Read .
. If that caused an exception, the debugger must read to discover the
type. (If is 0, this trigger doesn’t exist. Exit the loop.)
. If is 1, this trigger doesn’t exist. Exit the loop.
. Otherwise, the selected trigger supports the types discovered in .
. Repeat, incrementing the value in .

____
__

'''''

 +
The above algorithm reads back so that implementations which have
latexmath:[$2^n$] triggers only need to implement latexmath:[$n$] bits
of .

The algorithm checks and in case the implementation has latexmath:[$m$]
bits of but fewer than latexmath:[$2^m$] triggers.
____

=== Actions

Triggers can be configured to take one of several actions when they
fire. Table #tab:action[[tab:action]] lists all options.

|r|L| Value & Description +
& Raise a breakpoint exception. (Used when software wants to use the
trigger module without an external debugger attached.) `xepc` must
contain the virtual address of the next instruction that must be
executed to preserve the program flow. +
& Enter Debug Mode. must contain the virtual address of the next
instruction that must be executed to preserve the program flow.

This action is only legal when the trigger’s is 1. Since the `tdata`
registers are WARL, hardware should clear the action field whenever the
action field is 1, the new value of would be 0, and the new value of the
action field would be 1.

This action can only be supported if Sdext is implemented on the hart. +
& Trace on, described in the trace specification. +
& Trace off, described in the trace specification. +
& Trace notify, described in the trace specification. +
& Reserved for use by the trace specification. +
– 9 & Send a signal to TM external trigger output 0 or 1
(respectively). +
other & Reserved for future use. +

____
__

'''''

 +
Actions 8 and 9 are intended to increment custom event counters, but
these signals could also be brought to outputs for use by external
logic.
____

=== Priority

Table #tab:priority[[tab:priority]] lists the synchronous exceptions
from the Privileged Spec, and where the various types of triggers fit
in. The first 3 columns come from the Privileged Spec, and the final
column shows where triggers fit in. Priorities in the table are
separated by horizontal lines, so e.g. etrigger and itrigger have the
same priority. If this table contradicts the table in the Privileged
Spec, then the latter takes precedence.

This table only applies if triggers are precise. Otherwise triggers will
fire some indeterminate time after the event, and the priority is
irrelevant. When triggers are chained, the priority is the lowest
priority of the triggers in the chain.

|l|p.7in|p2.3in|p2.5in| Priority & Exception Code & Description &
Trigger +
_Highest_ & 3 & & etrigger +
& 3 & & icount +
& 3 & & itrigger +
& 3 & & mcontrol/mcontrol6 after (on previous instruction) +
& 3 & Instruction address breakpoint & mcontrol/mcontrol6 execute
address before +
& 12, 20, 1 & During instruction address translation: First encountered
page fault, guest-page fault, or access fault & +
& 1 & With physical address for instruction: Instruction access fault
& +
& 3 & & mcontrol/mcontrol6 execute data before +
& 2 & Illegal instruction & +
& 22 & Virtual instruction & +
& 0 & Instruction address misaligned & +
& 8, 9, 10, 11 & Environment call & +
& 3 & Environment break & +
& 3 & Load/Store/AMO address breakpoint & mcontrol/mcontrol6 load/store
address/data before +
& 4, 6 & Optionally: Load/Store/AMO address misaligned & +
& 13, 15, 21, 23, 5, 7 & During address translation for an explicit
memory access: First encountered page fault, guest-page fault, or access
fault & +
& 5, 7 & With physical address for an explicit memory access:
Load/store/AMO access fault & +
& 4, 6 & If not higher priority: Load/store/AMO address misaligned & +
_Lowest_ & 3 & & mcontrol/mcontrol6 load data before +

When multiple triggers in the same priority fire at once, (if
implemented) is set for all of them. If more than one of these triggers
has then `tval` is updated in accordance with one of them, but which one
is unspecified. If one of these triggers has the ``enter Debug Mode''
action (1) and another trigger has the ``raise a breakpoint exception''
action (0), the preferred behavior is to have both actions take place.
It is implementation-dependent which of the two happens first. This
ensures both that the presence of an external debugger doesn’t affect
execution and that a trigger set by user code doesn’t affect the
external debugger. If this is not implemented, then the hart must enter
Debug Mode and ignore the breakpoint exception. In the latter case, of
the trigger whose action is 0 must still be set, giving a debugger an
opportunity to handle this case. What happens with trace actions when
triggers with different actions are also firing is left to the trace
specification.

[[sec:nativetrigger]]
=== Native Triggers

Triggers can be used for native debugging when . If supported by the
hart and desired by the debugger, triggers will often be programmed to
have so that when they fire they cause a breakpoint exception to trap to
a more privileged mode. That breakpoint exception can either be taken in
M-mode or it can be delegated to a less privileged mode. However, it is
possible for triggers to fire in the same mode that the resulting
exception will be handled in.

In these cases such a trigger may cause a breakpoint exception while
already in a trap handler. This might leave the hart unable to resume
normal execution because state such as `mcause` and `mepc` would be
overwritten.

____
__

'''''

 +
In particular, when :

. mcontrol and mcontrol6 triggers with can cause a breakpoint exception
that is taken from M-mode to M-mode (regardless of delegation).
. mcontrol and mcontrol6 triggers with can cause a breakpoint exception
that is taken from S-mode to S-mode if `medeleg`[3]=1.
. mcontrol6 triggers with can cause a breakpoint exception that is taken
from VS-mode to VS-mode if `medeleg`[3]=1 and `hedeleg`[3]=1.
. icount triggers with can cause a breakpoint exception that is taken
from M-mode to M-mode (regardless of delegation).
. icount triggers with can cause a breakpoint exception that is taken
from S-mode to S-mode if `medeleg`[3]=1.
. icount triggers with can cause a breakpoint exception that is taken
from VS-mode to VS-mode if `medeleg`[3]=1 and `hedeleg`[3]=1.
. etrigger and itrigger triggers will always be taken from a trap
handler before the first instruction of the handler. If
etrigger/itrigger is set to trigger on exception/interrupt X and if X is
delegated to mode Y then the trigger will cause a breakpoint exception
that is taken from mode Y to mode Y unless breakpoint exceptions are
delegated to a more privileged mode than Y.
. tmexttrigger triggers are asynchronous and may occur in any mode and
at any time.
____

Harts that support triggers with should implement one of the following
two solutions to solve the problem of reentrancy:

. The hardware prevents triggers with from matching or firing while in
M-mode and while latexmath:[$|MIE|$] in `mstatus` is 0. If
`medeleg`[3]=1 then it prevents triggers with from matching or firing
while in S-mode and while latexmath:[$|SIE|$] in `sstatus` is 0. If
`medeleg`[3]=1 and `hedeleg`[3]=1 then it prevents triggers with from
matching or firing while in VS-mode and while latexmath:[$|SIE|$] in
`vsstatus` is 0.
. and in is implemented. `medeleg`[3] is hard-wired to 0.

____
__

'''''

 +
The first option has the limitation that interrupts might be disabled at
times when a user still might want triggers to fire. It has the benefit
that breakpoints are not required to be handled in M-mode.

The second option has the benefit that it only disables triggers during
the trap handler, though it requires specific software support for this
debug feature in the M-mode trap handlers. It can only work if
breakpoints are not delegated to less privileged modes and therefore
targets primarily implementations without S-mode.

Because is not accessible to S-mode, the second option can not be
extended to accommodate delegation without adding additional S-mode and
VS-mode CSRs.

Both options prevent etrigger and itrigger from having any effect on
exceptions and interrupts that are handled in M-mode. They also prevent
triggering during some initial portion of each handler. Debuggers should
use other mechanisms to debug these cases, such as patching the handler
or setting a breakpoint on the instruction after latexmath:[$|MIE|$] is
cleared.
____

=== Memory Access Triggers

and both enable triggers on memory accesses. This section describes for
both of them how certain corner cases are treated.

==== A Extension

If the A extension is supported, then triggers on loads/stores treat
them as follows:

. `lr` instructions are loads.
. Successful `sc` instructions are stores.
. It is unspecified whether failing `sc` instructions are stores or not.
. Each AMO instruction is a load for the read portion of the operation.
The address is always available to trigger on, although the value loaded
might not be, depending on the hardware implementation.
. Each AMO instruction is a store for the write portion of the
operation. The address is always available to trigger on, although the
value stored might not be, depending on the hardware implementation.

If the destination register of any load or AMO is `zero` then it is
unspecified whether a data load trigger will match. Whether data store
triggers match on AMOs is unspecified.

==== Combined Accesses

Some instructions lead a hart to perform multiple memory accesses. This
includes vector loads and stores, as well as `cm.push` and `cm.pop`
instructions. The Trigger Module should match such accesses as if they
all happened individually. E.g. a vector load should be treated as if it
performed multiple loads of size SEW (selected element width), and
`cm.push` should be treated as if it performed multiple stores of size
XLEN.

==== Cache Operations

Cache operations are infrequently performed, and code that uses them can
have hard-to-find bugs. For the purposes of debug triggers, two classes
of cache operations must match as stores:

. Cache operations that enable software to maintain coherence between
otherwise non-coherent implicit and explicit memory accesses.
. Cache operations that perform block writes of constant data.

Only triggers with and will match. Since cache operations affect
multiple addresses, there are multiple possible values to compare
against. Implementations must implement one of the following options.
From most desirable to least desirable, they are:

. Every address from the effective address rounded down to the nearest
cache block boundary (inclusive) to the effective address rounded up to
the nearest cache block boundary (exclusive) is a compare value.
. The effective address rounded down to the nearest cache block boundary
is a compare value.
. The effective address of the instruction is a compare value.

Cache operations encoded as HINTs do not match debug triggers.

____
__

'''''

 +
The above language intends to capture the trigger behavior with respect
to the cache operations to be introduced in a forthcoming I/D
consistency extension.

For RISC-V Base Cache Management Operation ISA Extensions 1.0.1, this
means the following:

. `cbo.clean`, `cbo.flush`, and `cbo.inval` match as if they are stores
because they affect consistency.
. `cbo.zero` matches as if it is a store because it performs a block
write of constant data.
. The prefetch instructions don’t match at all.
____

[[sec:multistate]]
=== Multiple State Change Instructions

An instruction that performs multiple architectural state changes (e.g.,
register updates and/or memory accesses) might cause a trigger to fire
at an intermediate point in its execution. As a result, architectural
state changes up to that point might have been performed, while
subsequent state changes, starting from the event that activated the
trigger, might not have been. The definition of such an instruction will
specify the order in which architectural state changes take place.
Alternatively, it may state that partial execution is not allowed,
implying that a mid-execution trigger must prevent any architectural
state changes from occurring.

Debuggers won’t be aware if an instruction has been partially executed.
When they resume execution, they will execute the same instruction once
more. Therefore, it’s crucial that partially executing the instruction
and then executing it again leaves the hart in a state closely
resembling the state it would have been in if the instruction had only
been executed once.

=== Trigger Registers

These registers are CSRs, accessible using the RISC-V `csr` opcodes and
optionally also using abstract debug commands.

Almost all trigger functionality is optional. All `tdata` registers
follow write-any-read-legal semantics. If a debugger writes an
unsupported configuration, the register will read back a value that is
supported (which may simply be a disabled trigger). This means that a
debugger must always read back values it writes to `tdata` registers,
unless it already knows already what is supported. Writes to one `tdata`
register must not modify the contents of other `tdata` registers, nor
the configuration of any trigger besides the one that is currently
selected.

The combination of these rules means that a debugger cannot simply set a
trigger by writing , then , etc. The current value of might not be legal
with the new value of . To help with this situation, it is guaranteed
that writing 0 to disables the trigger, and leaves it in a state where
and can be written with any value that makes sense for any trigger type
supported by this trigger.

As a result, a debugger can write any supported trigger as follows:

. Write 0 to . (This will result in containing a non-zero value, since
the register is *WARL*.)
. Write desired values to and .
. Write desired value to .

Code that restores CSR context of triggers that might be configured to
fire in the current privilege mode must use this same sequence to
restore the triggers. This avoids the problem of a partially written
trigger firing at a different time than is expected.

Attempts to access an unimplemented Trigger Register raise an illegal
instruction exception.

[[dtm]]
== Debug Transport Module (DTM) (non-ISA extension)

Debug Transport Modules provide access to the DM over one or more
transports (e.g. JTAG or USB).

There may be multiple DTMs in a single hardware platform. Ideally every
component that communicates with the outside world includes a DTM,
allowing a hardware platform to be debugged through every transport it
supports. For instance a USB component could include a DTM. This would
trivially allow any hardware platform to be debugged over USB. All that
is required is that the USB module already in use also has access to the
Debug Module Interface.

Using multiple DTMs at the same time is not supported. It is left to the
user to ensure this does not happen.

This specification defines a JTAG DTM in Section #sec:jtagdtm[7.1].
Additional DTMs may be added in future versions of this specification.

An implementation can be compatible with this specification without
implementing any of this section. In that case it must be advertised as
conforming to ``RISC-V Debug Specification 1.0-, with custom DTM.'' If
the JTAG DTM described here is implemented, it must be advertised as
conforming to the ``RISC-V Debug Specification 1.0-, with JTAG DTM.''

[[sec:jtagdtm]]
=== JTAG Debug Transport Module

This Debug Transport Module is based around a normal JTAG Test Access
Port (TAP). The JTAG TAP allows access to arbitrary JTAG registers by
first selecting one using the JTAG instruction register (IR), and then
accessing it through the JTAG data register (DR).

==== JTAG Background

JTAG refers to IEEE Std 1149.1-2013. It is a standard that defines test
logic that can be included in an integrated circuit to test the
interconnections between integrated circuits, test the integrated
circuit itself, and observe or modify circuit activity during the
component’s normal operation. This specification uses the latter
functionality. The JTAG standard defines a Test Access Port (TAP) that
can be used to read and write a few custom registers, which can be used
to communicate with debug hardware in a component.

==== JTAG DTM Registers

JTAG TAPs used as a DTM must have an IR of at least 5 bits. When the TAP
is reset, IR must default to 00001, selecting the IDCODE instruction. A
full list of JTAG registers along with their encoding is in
Table #dtmTable:jtagregisters[[dtmTable:jtagregisters]]. If the IR
actually has more than 5 bits, then the encodings in
Table #dtmTable:jtagregisters[[dtmTable:jtagregisters]] should be
extended with 0’s in their most significant bits, except for the 0x1f
encoding of BYPASS, which must be extended with 1’s in the most
significant bits. The only regular JTAG registers a debugger might use
are BYPASS and IDCODE, but this specification leaves IR space for many
other standard JTAG instructions. Unimplemented instructions must select
the BYPASS register.

==== JTAG Connector

===== Recommended JTAG Connector

To make it easy to acquire debug hardware, this spec recommends a
connector that is compatible with the MIPI-10 .05 inch connector
specification, as described in MIPI Debug & Trace Connector
Recommendations, Version 1.20, 2 July 2021.

The connector has .05 inch spacing, gold-plated male header with .016
inch thick hardened copper or beryllium bronze square posts (SAMTEC FTSH
or equivalent). Female connectors are compatible latexmath:[$20\mu m$]
gold connectors.

Viewing the male header from above (the pins pointing at your eye), a
target’s connector looks as it does in Table #tab:mipiten[7.1]. The
function of each pin is described in Table #tab:pinout[[tab:pinout]].

[[tab:mipiten]]
.MIPI 10-pin JTAG + nRESET Connector Diagram
[cols=">,^,^,<",options="header",]
|===
|VREF DEBUG |1 |2 |TMS
|GND |3 |4 |TCK
|GND |5 |6 |TDO
|GND or KEY |7 |8 |TDI
|GND |9 |10 |nRESET
|===

|l|c|L| Essential & GND & Connected to ground. +
& TCK & JTAG TCK signal, driven by the debug adapter. +
& TDI & JTAG TDI signal, driven by the debug adapter. +
& TDO & JTAG TDO signal, driven by the target. +
& TMS & JTAG TMS signal, driven by the debug adapter. +
& VREF DEBUG & Reference voltage for logic high. +
Recommended & nRESET & Open drain active low reset signal, usually
driven by the debug adapter. The signal may be used bi-directional to
drive or sense the target reset signal.

Asserting reset should reset any RISC-V cores as well as any other
peripherals on the PCB. It should not reset the debug logic. This pin is
optional but strongly encouraged.

nRESET should never be connected to the TAP reset, otherwise the
debugger might not be able to debug through a reset to discover the
cause of a crash or to maintain execution control after the reset. +
& KEY & This pin may be cut on the male and plugged on the female header
to ensure the header is always plugged in correctly. It is, however,
recommended to use this pin as an additional ground, to allow for
fastest TCK speeds. A shrouded connector should be used to prevent the
cable from being plugged in incorrectly. +
Advanced & EXT & Reserved for custom use. Could be an input or an
output. +
& TRIGIN & Not used by this specification, to be driven by debug
adapter. (Can be used for extended functions like UART or boot mode
selection by some debug adapters). +
& TRIGOUT & Not used by this specification, driven by the target. +
Specialized & nTRST & Test reset, driven by the debug adapter. Asserting
nTRST initializes the JTAG DTM asynchronously. It is used in systems
where the JTAG DTM is not ready to be used after a normal power up. This
signal is sometimes called TRST*. +
Legacy & RTCK & Return test clock, driven by the target. A target may
relay the TCK signal here once it has processed it, allowing a debugger
to adjust its TCK frequency in response.

This signal should only be used to support legacy components that rely
on this functionality. +
& nTRST_PD & Test reset pull-down, driven by the debug adapter. Same
function as nTRST, but with pull-down resistor on target.

This signal should only be used to support legacy components that rely
on this functionality. +

If a hardware platform requires nTRST then it is permissible to reuse
the nRESET pin as the nTRST signal, resulting in a MIPI 10-pin JTAG +
nTRST connector.

===== Alternate JTAG Connector

The MIPI-10 connector should provide plenty of signals for all modern
hardware. If a design does need legacy JTAG signals, then the MIPI-20
connector should be used. Pins whose functionality isn’t needed may be
left unconnected.

Its physical connector is virtually identical to MIPI-10, except that
it’s twice as long, supporting twice as many pins. Its pinout is shown
in Table #tab:mipitwenty[7.2]. The function of each pin is described in
Table #tab:pinout[[tab:pinout]].

[[tab:mipitwenty]]
.MIPI 20-pin JTAG Connector Diagram
[cols=">,^,^,<",options="header",]
|===
|VREF DEBUG |1 |2 |TMS
|GND |3 |4 |TCK
|GND |5 |6 |TDO
|GND or KEY |7 |8 |TDI
|GND |9 |10 |nRESET
|GND |11 |12 |GND or RTCK
|GND |13 |14 |NC or nTRST_PD
|GND |15 |16 |nTRST or NC
|GND |17 |18 |TRIGIN or NC
|GND |19 |20 |TRIGOUT or GND
|===

==== cJTAG

This spec does not have specific recommendations on how to use the cJTAG
protocol.

When implementing cJTAG access to a JTAG DTM, the MIPI 10-pin Narrow
JTAG connector should be used. Pins whose functionality isn’t needed may
be left unconnected.

Viewing the male header from above (the pins pointing at your eye), a
target’s connector looks as it does in Table #tab:mipicjtag[7.3].

[[tab:mipicjtag]]
.MIPI 10-pin Narrow JTAG Connector Diagram
[cols=">,^,^,<",options="header",]
|===
|VREF DEBUG |1 |2 |TMSC
|GND |3 |4 |TCKC
|GND |5 |6 |EXT or NC
|GND or KEY |7 |8 |NC or nTRST_PD
|GND |9 |10 |nRESET
|===

[[sec:implementations]]
== Hardware Implementations

Below are two possible implementations. A designer could choose one, mix
and match, or come up with their own design.

=== Abstract Command Based

Halting happens by stalling the hart execution pipeline.

Muxes on the register file(s) allow for accessing GPRs and CSRs using
the Access Register abstract command.

Memory is accessed using the Abstract Access Memory command or through
System Bus Access.

This implementation could allow a debugger to collect information from
the hart even when that hart is unable to execute instructions.

[[execution_based]]
=== Execution Based

This implementation only implements the Access Register abstract command
for GPRs on a halted hart, and relies on the Program Buffer for all
other operations. It uses the hart’s existing pipeline and ability to
execute from arbitrary memory locations to avoid modifications to a
hart’s datapath.

When the halt request bit is set, the Debug Module raises a special
interrupt to the selected harts. This interrupt causes each hart to
enter Debug Mode and jump to a defined memory region that is serviced by
the DM and is only accessible to the harts in Debug Mode. Accesses to
this memory should be uncached to avoid side effects from debugging
operations. When taking this jump, `pc` is saved to and is updated in .
This jump is similar to a trap but it is not architecturally considered
a trap, so for instance doesn’t count as a trap for trigger behavior.

The code in the Debug Module causes the hart to execute a ``park loop.''
In the park loop the hart writes its `mhartid` to a memory location
within the Debug Module to indicate that it is halted. To allow the DM
to individually control one out of several halted harts, each hart polls
for flags in a DM-controlled memory location to determine whether the
debugger wants it to execute the Program Buffer or perform a resume.

To execute an abstract command, the DM first populates some internal
words of program buffer according to . When is set, the DM populates
these words with `lw <gpr>, 0x400(zero)` or `sw 0x400(zero), <gpr>`. 64-
and 128-bit accesses use `ld`/`sd` and `lq`/`sq` respectively. If is not
set, the DM populates these instructions as `nop`s. If is set, execution
continues to the debugger-controlled Program Buffer, otherwise the DM
causes a `ebreak` to execute immediately.

When `ebreak` is executed (indicating the end of the Program Buffer
code) the hart returns to its park loop. If an exception is encountered,
the hart jumps to an address within the Debug Module. The code there
causes the hart to write to the Debug Module indicating an exception.
Then the hart jumps back to the park loop. The DM infers from the write
that there was an exception, and sets appropriately. Typically the hart
will execute a `fence` instruction before entering the park loop, to
ensure that any effects from the abstract command, such as a write to ,
take effect before the DM returns to 0.

To resume execution, the debug module sets a flag which causes the hart
to execute a `dret`. `dret` is an instruction that only has meaning
while in Debug Mode and not executing from the Program Buffer. Its
recommended encoding is 0x7b200073. When `dret` is executed, `pc` is
restored from and normal execution resumes at the privilege set by and .

etc. are mapped into regular memory at an address relative to `zero`
with only a 12-bit `imm`. The exact address is an implementation detail
that a debugger must not rely on. For example, the `data` registers
might be mapped to `0x400`.

For additional flexibility, , etc. are mapped into regular memory
immediately preceding , in order to form a contiguous region of memory
which can be used for either program execution or data transfer.

The PMP must not disallow fetches, loads, or stores in the address range
associated with the Debug Module when the hart is in Debug Mode,
regardless of how the PMP is configured. The same is true of PMA.
Without this guarantee, the park loop would enter an infinite loop of
traps and debug would not be possible.

[[dmi_signals]]
=== Debug Module Interface Signals

As stated in section link:#dmi[4.1] the details of the DMI are left to
the system designer. It is quite often the case that only one DTM and
one DM is implemented. In this case it might be useful to comply with
the signals suggested in table #tab:dmi_signals[8.1], which is the
implementation used in the open-source
https://github.com/chipsalliance/rocket-chip/blob/375045a7db1bdc7b4f7851f1a59b3f10a2b922ff/src/main/scala/devices/debug/Debug.scala#L170[rocket-chip]
RISC-V core.

The DTM can start a request when the DM sets REQ_READY to 1. When this
is the case REQ_OP can be set to 1 for a read or 2 for a write request.
The desired address is driven with the REQ_ADDRESS signal. Finally
REQ_VALID is set high, indicating to the DM that a valid request is
pending.

The DM must respond to a request from the DTM when RSP_READY is high.
The status of the response is indicated by the RSP_OP signal (see ). The
data of the response is driven to RSP_DATA. A pending response is
signalled by setting RSP_VALID.

[[tab:dmi_signals]]
.Signals for the suggested DMI between one DTM and one DM
[cols="<,<,<,<",options="header",]
|===
|Signal |Width |Source |Description
|REQ_VALID |1 |DTM |Indicates that a valid request is pending
|REQ_READY |1 |DM |Indicates that the DM is able to process a request
|REQ_ADDRESS | |DTM |Requested address
|REQ_DATA |32 |DTM |Requested data
|REQ_OP |2 |DTM |Same meaning as the field
|RSP_VALID |1 |DM |Indicates that a valid respond is pending
|RSP_READY |1 |DTM |Indicates that the DTM is able to process a respond
|RSP_DATA |32 |DM |Response data
|RSP_OP |2 |DM |Same meaning as the field
|===

== Debugger Implementation

=== C Header File

https://github.com/riscv/riscv-debug-spec contains instructions for
generating a C header file that defines macros for every field in every
register/abstract command mentioned in this document.

=== External Debugger Implementation

This section details how an external debugger might use the described
debug interface to perform some common operations on RISC-V cores using
the JTAG DTM described in Section #sec:jtagdtm[7.1]. All these examples
assume a 32-bit core but it should be easy to adapt the examples to 64-
or 128-bit cores.

To keep the examples readable, they all assume that everything succeeds,
and that they complete faster than the debugger can perform the next
access. This will be the case in a typical JTAG setup. However, the
debugger must always check the sticky error status bits after performing
a sequence of actions. If it sees any that are set, then it should
attempt the same actions again, possibly while adding in some delay, or
explicit checks for status bits.

[[dmiaccess]]
==== Debug Module Interface Access

To read an arbitrary Debug Module register, select , and scan in a value
with set to 1, and set to the desired register address. In Update-DR the
operation will start, and in Capture-DR its results will be captured
into . If the operation didn’t complete in time, will be 3 and the value
in must be ignored. The busy condition must be cleared by writing in ,
and then the second scan scan must be performed again. This process must
be repeated until returns 0. In later operations the debugger should
allow for more time between Update-DR and Capture-DR.

To write an arbitrary Debug Bus register, select , and scan in a value
with set to 2, and and set to the desired register address and data
respectively. From then on everything happens exactly as with a read,
except that a write is performed instead of the read.

It should almost never be necessary to scan IR, avoiding a big part of
the inefficiency in typical JTAG use.

==== Checking for Halted Harts

A user will want to know as quickly as possible when a hart is halted
(e.g. due to a breakpoint). To efficiently determine which harts are
halted when there are many harts, the debugger uses the `haltsum`
registers. Assuming the maximum number of harts exist, first it checks .
For each bit set there, it writes
latexmath:[$|\hyperref[hartsel]{hartsel}|$], and checks . This process
repeats through and . Depending on how many harts exist, the process
should start at one of the lower `haltsum` registers.

[[deb:halt]]
==== Halting

To halt one or more harts, the debugger selects them, sets , and then
waits for to indicate the harts are halted. Then it can clear to 0, or
leave it high to catch a hart that resets while halted.

==== Running

First, the debugger should restore any registers that it has
overwritten. Then it can let the selected harts run by setting . Once is
set, the debugger knows the selected harts have resumed. Harts might
halt very quickly after resuming (e.g. by hitting a software breakpoint)
so the debugger cannot use /to check whether the hart resumed.

==== Single Step

Using the hardware single step feature is almost the same as regular
running. The debugger just sets in before letting the hart run. The hart
behaves exactly as in the running case, except that interrupts may be
disabled (depending on ) and it only fetches and executes a single
instruction before re-entering Debug Mode.

==== Accessing Registers

[[deb:abstractreg]]
===== Using Abstract Command

Read `s0` using abstract command:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & latexmath:[$=2$], , = 0x1008 & Read `s0` +
Read & & - & Returns value that was in `s0` +

Write `mstatus` using abstract command:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & new value & +
Write & & latexmath:[$=2$], , , = 0x300 & Write `mstatus` +

[[deb:regprogbuf]]
===== Using Program Buffer

Abstract commands are used to exchange data with GPRs. Using this
mechanism, other registers can be accessed by moving their value
into/out of GPRs.

Write `mstatus` using program buffer:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `csrw s0, MSTATUS` & +
Write & `progbuf1` & `ebreak` & +
Write & & new value & +
Write & & latexmath:[$=2$], , , , = 0x1008 & Write `s0`, then execute
program buffer +

Read `f1` using program buffer:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `fmv.x.s s0, f1` & +
Write & `progbuf1` & `ebreak` & +
Write & & & Execute program buffer +
Write & & , = 0x1008 & read `s0` +
Read & & - & Returns the value that was in `f1` +

==== Reading Memory

[[deb:mrsysbus]]
===== Using System Bus Access

With system bus access, addresses are physical system bus addresses.

Read a word from memory using system bus access:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & latexmath:[$=2$], & Setup +
Write & & address & +
Read & & - & Value read from memory +

Read block of memory using system bus access:

|r|r|p13em|l| Op & Address & Value & Comment +
Write & & latexmath:[$=2$], , , & Turn on autoread and autoincrement +
Write & & address & Writing address triggers read and increment +
Read & & - & Value read from memory +
Read & & - & Next value read from memory +
... & ... & ... & ... +
Write & & 0 & Disable autoread +
Read & & - & Get last value read from memory. +

[[deb:mrprogbuf]]
===== Using Program Buffer

Through the Program Buffer, the hart performs the memory accesses.
Addresses are physical or virtual (depending on and other system
configuration).

Read a word from memory using program buffer:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `lw s0, 0(s0)` & +
Write & `progbuf1` & `ebreak` & +
Write & & address & +
Write & & , , , = 0x1008 & Write `s0`, then execute program buffer +
Write & & = 0x1008 & Read `s0` +
Read & & - & Value read from memory +

Read block of memory using program buffer:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `lw s1, 0(s0)` & +
Write & `progbuf1` & `addi s0, s0, 4` & +
Write & `progbuf2` & `ebreak` & +
Write & & address & +
Write & & , , , = 0x1008 & Write `s0`, then execute program buffer +
Write & & , = 0x1009 & Read `s1`, then execute program buffer +
Write & & & Set +
Read & & - & Get value read from memory, then execute program buffer +
Read & & - & Get next value read from memory, then execute program
buffer +
... & ... & ... & ... +
Write & & 0 & Clear +
Read & & - & Get last value read from memory. +

[[deb:mrabstract]]
===== Using Abstract Memory Access

Abstract memory accesses act as if they are performed by the hart,
although the actual implementation may differ.

Read a word from memory using abstract memory access:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & `data1`& address & +
Write & & cmdtype=2, & +
Read & & - & Value read from memory +

Read block of memory using abstract memory access:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & 1 & Re-execute the command when is accessed +
Write & `data1`& address & +
Write & & cmdtype=2, , & +
Read & & - & Read value, and trigger reading of next address +
... & ... & ... & ... +
Write & & 0 & Disable auto-exec +
Read & & - & Get last value read from memory. +

[[writemem]]
==== Writing Memory

[[deb:mrsysbus]]
===== Using System Bus Access

With system bus access, addresses are physical system bus addresses.

Write a word to memory using system bus access:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & latexmath:[$=2$] & Configure access size +
Write & & address & +
Write & & value & +

Write a block of memory using system bus access:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & latexmath:[$=2$], & Turn on autoincrement +
Write & & address & +
Write & & value0 & +
Write & & value1 & +
... & ... & ... & ... +
Write & & valueN & +

[[deb:mrprogbuf]]
===== Using Program Buffer

Through the Program Buffer, the hart performs the memory accesses.
Addresses are physical or virtual (depending on and other system
configuration).

Write a word to memory using program buffer:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `sw s1, 0(s0)` & +
Write & `progbuf1` & `ebreak` & +
Write & & address & +
Write & & , , = 0x1008 & Write `s0` +
Write & & value & +
Write & & , , , = 0x1009 & Write `s1`, then execute program buffer +

Write block of memory using program buffer:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `sw s1, 0(s0)` & +
Write & `progbuf1` & `addi s0, s0, 4` & +
Write & `progbuf2` & `ebreak` & +
Write & & address & +
Write & & , , = 0x1008 & Write `s0` +
Write & & value0 & +
Write & & , , , = 0x1009 & Write `s1`, then execute program buffer +
Write & & & Set +
Write & & value1 & +
... & ... & ... & ... +
Write & & valueN & +
Write & & 0 & Clear +

[[deb:mwabstract]]
===== Using Abstract Memory Access

Abstract memory accesses act as if they are performed by the hart,
although the actual implementation may differ.

Write a word to memory using abstract memory access:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & `data1`& address & +
Write & & value & +
Write & & cmdtype=2, , write=1 & +

Write a block of memory using abstract memory access:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & `data1`& address & +
Write & & value0 & +
Write & & cmdtype=2, , write=1, & +
Write & & 1 & Re-execute the command when is accessed +
Write & & value1 & +
Write & & value2 & +
... & ... & ... & ... +
Write & & valueN & +
Write & & 0 & Disable auto-exec +

==== Triggers

A debugger can use hardware triggers to halt a hart when a certain event
occurs. Below are some examples, but as there is no requirement on the
number of features of the triggers implemented by a hart, these examples
might not be applicable to all implementations. When a debugger wants to
set a trigger, it writes the desired configuration, and then reads back
to see if that configuration is supported. All examples assume XLEN=32.

Enter Debug Mode when the instruction at 0x80001234 is executed, to be
used as an instruction breakpoint in ROM:

|r|r|L| & 0x6980105c & type=6, dmode=1, action=1, select=0, match=0,
m=1, s=1, u=1, vs=1, vu=1, execute=1 +
& 0x80001234 & address +

Enter Debug Mode when performing a load at address 0x80007f80 in M-mode
or S-mode or U-mode:

|r|r|L| & 0x68001059 & type=6, dmode=1, action=1, select=0, match=0,
m=1, s=1, u=1, load=1 +
& 0x80007f80 & address +

Enter Debug Mode when storing to an address between 0x80007c80 and
0x80007cef (inclusive) in VS-mode or VU-mode when hgatp.VMID=1:

|r|r|L| & 0x69801902 & type=6, dmode=1, action=1, chain=1, select=0,
match=2, vs=1, vu=1, store=1 +
& 0x80007c80 & start address (inclusive) +
& 0x03000000 & mhselect=6, mhvalue=0 +
& 0x69801182 & type=6, dmode=1, action=1, select=0, match=3, vs=1, vu=1,
store=1 +
& 0x80007cf0 & end address (exclusive) +
& 0x03000000 & mhselect=6, mhvalue=0 +

Enter Debug Mode when storing to an address between 0x81230000 and
0x8123ffff (inclusive):

|r|r|L| & 0x698010da & type=6, dmode=1, action=1, select=0, match=1,
m=1, s=1, u=1, vs=1, vu=1, store=1 +
& 0x81237fff & 16 upper bits to match exactly, then 0, then all ones. +

Enter Debug Mode when loading from an address between 0x86753090 and
0x8675309f or between 0x96753090 and 0x9675309f (inclusive):

|r|r|L| & 0x69801a59 & type=6, dmode=1, action=1, chain=1, match=4, m=1,
s=1, u=1, vs=1, vu=1, load=1 +
& 0xfff03090 & Mask for low half, then match for low half +
& 0x698012d9 & type=6, dmode=1, action=1, match=5, m=1, s=1, u=1, vs=1,
vu=1, load=1 +
& 0xefff8675 & Mask for high half, then match for high half +

==== Handling Exceptions

Generally the debugger can avoid exceptions by being careful with the
programs it writes. Sometimes they are unavoidable though, e.g. if the
user asks to access memory or a CSR that is not implemented. A typical
debugger will not know enough about the hardware platform to know what’s
going to happen, and must attempt the access to determine the outcome.

When an exception occurs while executing the Program Buffer, becomes
set. The debugger can check this field to see whether a program
encountered an exception. If there was an exception, it’s left to the
debugger to know what must have caused it.

[[quickaccess]]
==== Quick Access

There are a variety of instructions to transfer data between GPRs and
the ` data` registers. They are either loads/stores or CSR reads/writes.
The specific addresses also vary. This is all specified in . The
examples here use the pseudo-op `transfer dest, src` to represent all
these options.

Halt the hart for a minimum amount of time to perform a single memory
write:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `transfer arg2, s0` & Save `s0` +
Write & `progbuf1` & `transfer s0, arg0` & Read first argument
(address) +
Write & `progbuf2` & `transfer arg0, s1` & Save `s1` +
Write & `progbuf3` & `transfer s1, arg1` & Read second argument (data) +
Write & `progbuf4` & `sw s1, 0(s0)` & +
Write & `progbuf5` & `transfer s1, arg0` & Restore `s1` +
Write & `progbuf6` & `transfer s0, arg2` & Restore `s0` +
Write & `progbuf7` & `ebreak` & +
Write & & address & +
Write & `data1` & data & +
Write & & 0x10000000 & Perform quick access +

This shows an example of setting the bit in to enable a hardware
breakpoint in M-mode. Similar quick access instructions could have been
used previously to configure the trigger that is being enabled here:

|c|r|p0.3|p0.3| Op & Address & Value & Comment +
Write & & `transfer arg0, s0` & Save `s0` +
Write & `progbuf1` & `li s0, (1 << 6)` & Form the mask for bit +
Write & `progbuf2` & `csrrs x0, , s0` & Apply the mask to +
Write & `progbuf3` & `transfer s0, arg2` & Restore `s0` +
Write & `progbuf4` & `ebreak` & +
Write & & 0x10000000 & Perform quick access +

=== Native Debugger Implementation

The spec contains a few features to aid in writing a native debugger.
This section describes how some common tasks might be achieved.

[[nativestep]]
==== Single Step

Single step is straightforward if the OS or a debug stub runs in M-Mode
while the program being debugged runs in a less privileged mode. When a
step is required, the OS or debug stub writes , , before returning
control to the lower user program with an `mret` instruction.

Stepping code running in the same privilege mode as the debugger is more
complicated, depending on what other debug features are implemented.

If hardware implements and , then stepping through non-trap code which
doesn’t allow for nested interrupts is also straightforward.

If hardware automatically prevents triggers from matching when entering
a trap handler as described in Section #sec:nativetrigger[6.4], then a
carefully written trap handler can ensure that interrupts are disabled
whenever the icount trigger must not match.

If neither of these features exist, then single step is doable, but
tricky to get right. To single step, the debug stub would execute
something like:

....
    li    t0, {icount-count}=4, {icount-action}=0, {icount-m}=1
    csrw  tdata1, t0    /* Write the trigger. */
    lw    t0, 8(sp)     /* Restore t0, count decrements to 3 */
    lw    sp, 0(sp)     /* Restore sp, count decrements to 2 */
    mret                /* Return to program being debugged. count decrements to 1 */
....

There is an additional problem with using to single step. An instruction
may cause an exception into a more privileged mode where the trigger is
not enabled. The exception handler might address the cause of the
exception, and then restart the instruction. Examples of this include
page faults, FPU instructions when the FPU is not yet enabled, and
interrupts. When a user is single stepping through such code, they will
have to step twice to get past the restarted instruction. The first time
the exception handler runs, and the second time the instruction actually
executes. That is confusing and usually undesirable.

To help users out, debuggers should detect when a single step restarted
an instruction, and then step again. This way the users see the expected
behavior of stepping over the instruction. Ideally the debugger would
notify the user that an exception handler executed the first time.

The debugger should perform this extra step when the PC doesn’t change
during a regular step.

____
__

'''''

 +
It is safe to perform an extra step when the PC changes, because every
RISC-V instruction either changes the PC or has side effects when
repeated, but never both.
____

To avoid an infinite loop if the exception handler does not address the
cause of the exception, the debugger must execute no more than a single
extra step.

[#index]##

== Change Log
