\chapter{Debug Module (DM), non-ISA} \label{chap:dm}

\begin{steps}{The Debug Module implements a translation interface between abstract debug
    operations and their specific implementation. It might support the following
    operations:}
\item Give the debugger necessary information about the implementation. (Required)
\item Allow any individual hart to be halted and resumed. (Required)
\item Provide status on which harts are halted. (Required)
\item Provide abstract read and write access to a halted hart's GPRs. (Required)
\item Provide access to a reset signal that allows debugging from the very
    first instruction after reset. (Required)
\item Provide a mechanism to allow debugging harts immediately out of reset
      (regardless of the reset cause). (Optional)
\item Provide abstract access to non-GPR hart registers. (Optional)
\item Provide a Program Buffer to force the hart to execute arbitrary instructions. (Optional)
\item Allow multiple harts to be halted, resumed, and/or reset at the same time. (Optional)
\item Allow memory access from a hart's point of view. (Optional)
\item Allow direct System Bus Access. (Optional)
\item Group harts. When any hart in the group halts, they all halt. (Optional)
\item Respond to external triggers by halting each hart in a configured group. (Optional)
\item Signal an external trigger when a hart in a group halts. (Optional)
\end{steps}

\begin{steps}{In order to be compatible with this specification an
    implementation must:}
\item Implement all the required features listed above.
\item Implement at least one of Program Buffer, System Bus Access, or Abstract
    Access Memory command mechanisms.
\item
    \begin{steps}{Do at least one of:}
        \item Implement the Program Buffer.
        \item 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~\ref{tab:regno}.
        \item Implement abstract access to at least all GPRs, \RcsrDcsr, and
            \RcsrDpc, and advertise the implementation as conforming to the
            ``Minimal RISC-V Debug Specification \versionnum'', instead of the
            ``RISC-V Debug Specification \versionnum''.
    \end{steps}
\end{steps}

A single DM can debug up to $2^{20}$ harts.

\section{Debug Module Interface (DMI)} \label{dmi}

Debug Modules are subordinate devices on a bus called the Debug Module Interface (DMI). The
coordinator of the bus is the Debug Transport Module(s).
The Debug Module Interface can be a trivial bus with one coordinator and one subordinate device (see \ref{dmi_signals}),
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.  It supports read and write
operations.  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 \RdmNextdm.

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

\section{Reset Control} \label{reset}

There are two methods that allow a debugger to reset harts.
\FdmDmcontrolNdmreset 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.
\FdmDmcontrolHartreset 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 \FdmDmstatusAnyhavereset and \FdmDmstatusAllhavereset.

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, {\tt havereset} becomes set.  When a hart comes out
of reset and \FdmDmcontrolHaltreq or \Fresethaltreq 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.

\begin{commentary}
    There is no general, reliable way for the debugger to know when reset has
    actually begun.
\end{commentary}

The Debug Module's own state and registers should only be
reset at power-up and while
\FdmDmcontrolDmactive in \RdmDmcontrol 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 \FdmDmcontrolNdmreset or any external reset is asserted, the only supported DM
operations are reading and writing \RdmDmcontrol. The behavior of other accesses
is undefined.

When harts have been reset, they must set a sticky {\tt havereset} state bit.
The conceptual {\tt havereset} state bits can be read for selected harts in
\FdmDmstatusAnyhavereset and \FdmDmstatusAllhavereset in \RdmDmstatus.
These bits must be set regardless of the cause of the reset.
The {\tt havereset} bits for the selected harts
can be cleared by writing 1 to \FdmDmcontrolAckhavereset in \RdmDmcontrol.
The {\tt havereset} bits might or might not be cleared
when \FdmDmcontrolDmactive is low.

\section{Selecting Harts} \label{selectingharts}

Up to $2^{20}$ 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 {\tt HARTSELLEN}
by writing  all ones to \Fhartsel (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 \FdmDmstatusAnynonexistent in \RdmDmstatus is 1, or the
highest index (depending on {\tt HARTSELLEN}) is reached.

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

\subsection {Selecting a Single Hart}

All debug modules must support selecting a single hart.
The debugger can select a hart by writing its index to \Fhartsel.
Hart indexes start at 0 and are contiguous until the final index.

\subsection {Selecting Multiple Harts} \label{hartarraymask}

Debug Modules may implement a Hart Array Mask register to allow selecting
multiple harts at once. The $n$th bit in the Hart Array Mask register applies to
the hart with index $n$. 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 \RdmHawindowsel
and \RdmHawindow, then apply actions to all selected harts by setting \FdmDmcontrolHasel. 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 \FdmDmcontrolHasel.

Execution of Abstract Commands ignores this mechanism and only applies to the
hart selected by \Fhartsel.

\section{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 \FdmDmstatusAllnonexistent,
\FdmDmstatusAnynonexistent, \FdmDmstatusAllunavail, \FdmDmstatusAnyunavail,
\FdmDmstatusAllrunning, \FdmDmstatusAnyrunning, \FdmDmstatusAllhalted, and
\FdmDmstatusAnyhalted.

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
\FdmDmcontrolHaltreq and \Fresethaltreq.

\section{Run Control} \label{runcontrol}

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 \FdmDmstatusAllresumeack and
\FdmDmstatusAnyresumeack, and the state of halted, running, and havereset signals
in \FdmDmstatusAllhalted, \FdmDmstatusAnyhalted, \FdmDmstatusAllrunning, \FdmDmstatusAnyrunning, \FdmDmstatusAllhavereset,
and \FdmDmstatusAnyhavereset. The state of the other bits cannot be observed directly.

When a debugger writes 1 to \FdmDmcontrolHaltreq, 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 \FdmDmcontrolResumereq, 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\index{resume ack bit} is set.  These
status signals of all selected harts are reflected in \FdmDmstatusAllresumeack,
\FdmDmstatusAnyresumeack, \FdmDmstatusAllrunning, and \FdmDmstatusAnyrunning. 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 \FdmDmstatusHasresethaltreq to 1.
This means the DM implements the \FdmDmcontrolSetresethaltreq and \FdmDmcontrolClrresethaltreq bits.
Writing 1 to \FdmDmcontrolSetresethaltreq 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 \FdmDmcontrolClrresethaltreq
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 \FdmDmcontrolResumereq to
explicitly resume harts before clearing \FdmDmcontrolDmactive and
disconnecting.

\section{Halt Groups, Resume Groups, and External Triggers} \label{hrgroups}

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.

\begin{steps}{When any hart in a halt group halts:}
\item That hart halts normally, with \FcsrDcsrCause reflecting the original
    cause of the halt.
\item All the other harts in the halt group that are running will quickly halt.
    \FcsrDcsrCause 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.
\item Any external triggers in that group are notified.
\end{steps}
Adding a hart to a halt group does not automatically halt that hart, even if
other harts in the group are already halted.

\begin{steps}{When an external trigger that's a member of the halt group fires:}
\item All the harts in the halt group that are running will quickly halt.
    \FcsrDcsrCause 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.
\end{steps}

\begin{steps}{When any hart in a resume group resumes:}
\item 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\index{resume ack bit} as soon as it has resumed.
    Harts that are in the process of halting should complete that process and stay
    halted.
\item Any external triggers in that group are notified.
\end{steps}
Adding a hart to a resume group does not automatically resume that hart, even
if other harts in the group are currently running.

\begin{steps}{When an external trigger that's a member of the resume group fires:}
\item 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\index{resume ack bit} as soon as it has resumed.
    Harts that are in the process of halting should complete that process and stay
    halted.
\end{steps}

External triggers are abstract concepts that can signal the DM and/or receive
signals from the DM. This configuration is done through \RdmDmcsTwo, 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.

\begin{commentary}
    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.
\end{commentary}

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 \RdmDmcsTwo even if it's not possible to change the configuration.

\section{Message Registers}
\label{sec:mr}

Message Registers (MRs) are registers that are only used in a limited way,
allowing for different implementations. They exist to let two sides communicate
when the two sides already know who is the sender and who is the receiver.

An MR implements read and write operations on two sides.  When one side reads an
MR, and the last write was by the other side, then the result value of the read
is the value last written by the other side.  When one side reads an MR, and the
last write was by that same side, then the result value of the read is
\unspecified.  Thus the MR can be used to exchange data with the other side, but
not as storage to be accessed later.

\begin{commentary}
    A regular register can be used to implement an MR. In some FPGAs it is
    cheaper to trade off storage for muxes, and in that case the storage can be
    duplicated (one set of bits for sending and one for receiving) to avoid
    having to implement muxes to read/write data from/to the correct side.
\end{commentary}

\section{Abstract Commands} \label{abstractcommands}

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 \FdmAbstractcsCmderr in \RdmAbstractcs 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 \FdmAbstractcsCmderr to 2 (not supported).

\begin{commentary}
    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.''
\end{commentary}

Debuggers execute abstract commands by writing them to \RdmCommand.  They
can determine whether an abstract command is complete by reading \FdmAbstractcsBusy in
\RdmAbstractcs. If the debugger starts a new command while \FdmAbstractcsBusy is set,
\FdmAbstractcsCmderr 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, \FdmAbstractcsCmderr 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 {\tt data} MRs before writing to \RdmCommand. If a
command returns results, the Debug Module must ensure they are placed
in the {\tt data} MRs before \FdmAbstractcsBusy is cleared.
Which {\tt data} MRs are used for the arguments is
described in Table~\ref{tab:datareg}.  In all cases the least-significant word
is placed in the lowest-numbered {\tt data} MR. The argument width
depends on the command being executed, and is DXLEN where not explicitly
specified.

\begin{table}[htp]
    \centering
    \caption{Use of Data Registers}
    \label{tab:datareg}
    \begin{tabulary}{\textwidth}{|r|l|l|l|}
        \hline
        Argument Width & arg0/return value & arg1 & arg2 \\
        \hline
        32 & \RdmDataZero & {\tt data1} & {\tt data2} \\
        \hline
        64 & \RdmDataZero, {\tt data1} & {\tt data2}, {\tt data3} & {\tt data4}, {\tt data5} \\
        \hline
        128 & \RdmDataZero--{\tt data3} & {\tt data4}--{\tt data7} & {\tt data8}--{\tt data11} \\
        \hline
    \end{tabulary}
\end{table}

\begin{commentary}
    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 \RdmDataZero) or hart (e.g.\ 
    contents of a register modified by a Program Buffer program) to determine
    which one failed.
\end{commentary}

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

While an abstract command is executing (\FdmAbstractcsBusy in \RdmAbstractcs is high), a
debugger must not change \Fhartsel, and must not write 1 to \FdmDmcontrolHaltreq,
\FdmDmcontrolResumereq, \FdmDmcontrolAckhavereset, \FdmDmcontrolSetresethaltreq, or \FdmDmcontrolClrresethaltreq.

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 \FdmDmcontrolHartreset or
\FdmDmcontrolNdmreset). If that doesn't clear \FdmAbstractcsBusy, then it
can try resetting the Debug Module (using \FdmDmcontrolDmactive).

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 \FdmAbstractcsBusy low, and
\FdmAbstractcsCmderr to 4 (halt/resume). Alternatively, the command could just appear to be
hung (\FdmAbstractcsBusy never goes low).

\subsection{Abstract Command Listing}

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

Each abstract command is a 32-bit value. The top 8 bits contain \FdmCommandCmdtype which
determines the kind of command. Table~\ref{tab:cmdtype} lists all commands.

\begin{table}[htp]
    \centering
    \caption{Meaning of \FdmCommandCmdtype}
    \label{tab:cmdtype}
    \begin{tabulary}{\textwidth}{|r|l|l|l|}
        \hline
        \FdmCommandCmdtype & Command & Page \\
        \hline
        0 & Access Register Command & \pageref{acAccessregister} \\
        \hline
        1 & Quick Access & \pageref{acQuickaccess} \\
        \hline
        2 & Access Memory Command & \pageref{acAccessmemory} \\
        \hline
    \end{tabulary}
\end{table}

\input{abstract_commands.tex}

\section{Program Buffer} \label{programbuffer}

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 \FacAccessregisterPostexec bit in \RdmCommand.
The debugger can write whatever program it likes (including jumps out of the
Program Buffer), but the program must end with
{\tt ebreak} or {\tt c.ebreak}. An implementation may support
an implicit {\tt ebreak} that is executed when a hart runs off the end of the
Program Buffer. This is indicated by \FdmDmstatusImpebreak. With this feature, a Program
Buffer of just 2 32-bit words can offer efficient debugging.

If \FdmAbstractcsProgbufsize is 1, \FdmDmstatusImpebreak must be 1. It is possible that the Program
Buffer can hold only one 32- or 16-bit instruction, so the debugger must only
write a single instruction in this case, regardless of its size.
This instruction can be a 32-bit
instruction, or a compressed instruction in the lower 16 bits accompanied by a
compressed {\tt nop} in the upper 16 bits.

\begin{commentary}
    The slightly inconsistent behavior with a Program Buffer of size 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.
\end{commentary}

While these programs are executed, the hart does not leave Debug Mode (see
Section~\ref{debugmode}).  If an exception is encountered during execution of
the Program Buffer, no more instructions are executed, the hart remains in Debug
Mode, and \FdmAbstractcsCmderr is set to 3 ({\tt exception error}).  If the debugger
executes a program that doesn't terminate with an {\tt ebreak} instruction, the
hart will remain in Debug Mode and the debugger will lose control of the hart.

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 \Rpc while executing
from the Program Buffer.
If so, the debugger has more flexibility in what it can do with the program buffer.

\section{Overview of Hart Debug States}

Figure~\ref{fig:abstract_sm} shows a conceptual view of the states
passed through by a hart during run/halt debugging as influenced
by the different fields of \RdmDmcontrol, \RdmAbstractcs, \RdmAbstractauto, and
\RdmCommand.

\begin{figure}
   \centering
   \includegraphics[width=\textwidth]{fig/abstract_commands.pdf}
   \caption[Run/Halt Debug State Machine]{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.}
   \label{fig:abstract_sm}
\end{figure}

\section{System Bus Access} \label{systembusaccess}

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~\ref{tab:sbdatabits} shows which bits in {\tt sbdata} are used
for each access size.

\begin{table}[htp]
    \centering
    \caption{System Bus Data Bits}
    \label{tab:sbdatabits}
    \begin{tabulary}{\textwidth}{|r|l|}
        \hline
        Access Size & Data Bits \\
        \hline
        8 & \RdmSbdataZero bits 7:0 \\
        \hline
        16 & \RdmSbdataZero bits 15:0 \\
        \hline
        32 & \RdmSbdataZero \\
        \hline
        64 & \RdmSbdataOne, \RdmSbdataZero \\
        \hline
        128 & \RdmSbdataThree, \RdmSbdataTwo, \RdmSbdataOne, \RdmSbdataZero \\
        \hline
    \end{tabulary}
\end{table}

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.

\begin{commentary}
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.
\end{commentary}

\section{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
{\tt data} registers, as described in \RdmHartinfo, 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.

\section{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 \FdmDmstatusAuthenticated, \FdmDmstatusAuthbusy, and \RdmAuthdata arbitrarily
complex authentication mechanism can be supported.  When \FdmDmstatusAuthenticated 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:
\begin{steps}{}
    \item \FdmDmstatusAuthenticated in \RdmDmstatus is readable.
    \item \FdmDmstatusAuthbusy in \RdmDmstatus is readable.
    \item \FdmDmstatusVersion in \RdmDmstatus is readable.
    \item \FdmDmcontrolDmactive in \RdmDmcontrol is readable and writable.
    \item \RdmAuthdata is readable and writable.
\end{steps}

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

\section{Version Detection}

\begin{steps}{To detect the version of the Debug Module with a minimum of side
    effects, use the following procedure:}
    \item Read \RdmDmcontrol.
    \item
        If \FdmDmcontrolDmactive is 0 or \FdmDmcontrolNdmreset is 1:
        \begin{enumerate}[nolistsep]
            \item Write \RdmDmcontrol, preserving \FdmDmcontrolHartreset,
            \FdmDmcontrolHasel, \FdmDmcontrolHartsello, and
            \FdmDmcontrolHartselhi from the value that was read, setting
            \FdmDmcontrolDmactive, and clearing all the other bits.
            \item Read \RdmDmcontrol until \FdmDmcontrolDmactive is high.
        \end{enumerate}
    \item Read \RdmDmstatus, which contains \FdmDmstatusVersion.
\end{steps}

\begin{steps}{If it was necessary to clear \FdmDmcontrolNdmreset, this might
        have the following unavoidable side effects:}
    \item \FdmDmcontrolHaltreq is cleared, potentially preventing a halt request made by a
        previous debugger from taking effect.
    \item \FdmDmcontrolResumereq is cleared, potentially preventing a resume request made
        by a previous debugger from taking effect.
    \item \FdmDmcontrolNdmreset is deasserted, releasing the hardware platform from reset if a
        previous debugger had set it.
    \item \FdmDmcontrolDmactive is asserted, releasing the DM from reset. This in itself
        is not observable by any harts.
\end{steps}

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

\section{Debug Module Registers} \label{dmdebbus}

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.

When read, unimplemented or non-existent Debug Module DMI Registers return 0.
Writing them has no effect.

\input{dm_registers.tex}
