path: root/src/priv-csrs.adoc

[[priv-csrs]]
== Control and Status Registers (CSRs)

The SYSTEM major opcode is used to encode all privileged instructions in
the RISC-V ISA. These can be divided into two main classes: those that
atomically read-modify-write control and status registers (CSRs), which
are defined in the Zicsr extension, and all other privileged
instructions. The privileged architecture requires the Zicsr extension;
which other privileged instructions are required depends on the
privileged-architecture feature set.

In addition to the unprivileged state described in Volume I of this
manual, an implementation may contain additional CSRs, accessible by
some subset of the privilege levels using the CSR instructions described
in Volume I. In this chapter, we map out the CSR address space. The
following chapters describe the function of each of the CSRs according
to privilege level, as well as the other privileged instructions which
are generally closely associated with a particular privilege level. Note
that although CSRs and instructions are associated with one privilege
level, they are also accessible at all higher privilege levels.

Standard CSRs do not have side effects on reads but may have side
effects on writes.

=== CSR Address Mapping Conventions

The standard RISC-V ISA sets aside a 12-bit encoding space (csr[11:0])
for up to 4,096 CSRs. By convention, the upper 4 bits of the CSR address
(csr[11:8]) are used to encode the read and write accessibility of the
CSRs according to privilege level as shown in <<csrrwpriv>>. The top two bits (csr[11:10]) indicate whether the register is read/write (`00`,`01`, or `10`) or read-only (`11`). The next two bits (csr[9:8]) encode the lowest privilege level that can access the CSR.

[NOTE]
====
The CSR address convention uses the upper bits of the CSR address to
encode default access privileges. This simplifies error checking in the
hardware and provides a larger CSR space, but does constrain the mapping
of CSRs into the address space.

Implementations might allow a more-privileged level to trap otherwise
permitted CSR accesses by a less-privileged level to allow these
accesses to be intercepted. This change should be transparent to the
less-privileged software.
====

Instructions that access a non-existent CSR are reserved.
Attempts to access a CSR without appropriate privilege level
raise illegal-instruction exceptions or, as described in
<<sec:hcauses>>, virtual-instruction exceptions.
Attempts to write a read-only register raise illegal-instruction exceptions.
A read/write register might also contain some bits that are
read-only, in which case writes to the read-only bits are ignored.

<<csrrwpriv>> also indicates the convention to
allocate CSR addresses between standard and custom uses. The CSR
addresses designated for custom uses will not be redefined by future
standard extensions.

Machine-mode standard read-write CSRs `0x7A0`-`0x7BF` are reserved for
use by the debug system. Of these CSRs, `0x7A0`-`0x7AF` are accessible
to machine mode, whereas `0x7B0`-`0x7BF` are only visible to debug mode.
Implementations should raise illegal-instruction exceptions on
machine-mode access to the latter set of registers.

[NOTE]
====
Effective virtualization requires that as many instructions run natively
as possible inside a virtualized environment, while any privileged
accesses trap to the virtual machine monitor. cite:[goldbergvm] CSRs that are read-only
at some lower privilege level are shadowed into separate CSR addresses
if they are made read-write at a higher privilege level. This avoids
trapping permitted lower-privilege accesses while still causing traps on
illegal accesses. Currently, the counters are the only shadowed CSRs.
====

=== CSR Listing

<<ucsrnames>>-<<mcsrnames1>> list the CSRs that
have currently been allocated CSR addresses. The timers, counters, and
floating-point CSRs are standard unprivileged CSRs. The other registers
are used by privileged code, as described in the following chapters.
Note that not all registers are required on all implementations.

[[csrrwpriv]]
.Allocation of RISC-V CSR address ranges.
[%autowidth,float="center",align="center",cols="^,^,^,^,<,<,<,<"]
[.monofont]
|===
3+^|CSR Address 2.2+|Hex 3.2+|Use and Accessibility
|[11:10] |[9:8] |[7:4] 
8+|Unprivileged and User-Level CSRs 
m|00 m|00 m|XXXX 2+m| 0x000-0x0FF 3+|Standard read/write
|`01` |`00` |`XXXX` 2+| `0x400-0x4FF` 3+|Standard read/write
|`10` |`00` |`XXXX` 2+| `0x800-0x8FF` 3+|Custom read/write
|`11` |`00` |`0XXX` 2+| `0xC00-0xC7F` 3+|Standard read-only
|`11` |`00` |`10XX` 2+| `0xC80-0xCBF` 3+|Standard read-only
|`11` |`00` |`11XX` 2+| `0xCC0-0xCFF` 3+|Custom read-only
8+|Supervisor-Level CSRs
|`00` |`01` |`XXXX` 2+| `0x100-0x1FF` 3+|Standard read/write
|`01` |`01` |`0XXX` 2+| `0x500-0x57F` 3+|Standard read/write
|`01` |`01` |`10XX` 2+| `0x580-0x5BF` 3+|Standard read/write
|`01` |`01` |`11XX` 2+| `0x5C0-0x5FF` 3+|Custom read/write
|`10` |`01` |`0XXX` 2+| `0x900-0x97F` 3+|Standard read/write
|`10` |`01` |`10XX` 2+| `0x980-0x9BF` 3+|Standard read/write
|`10` |`01` |`11XX` 2+| `0x9C0-0x9FF` 3+|Custom read/write
|`11` |`01` |`0XXX` 2+| `0xD00-0xD7F` 3+|Standard read-only
|`11` |`01` |`10XX` 2+| `0xD80-0xDBF` 3+|Standard read-only
|`11` |`01` |`11XX` 2+| `0xDC0-0xDFF` 3+|Custom read-only
8+|Hypervisor and VS CSRs
|`00` |`10` |`XXXX` 2+| `0x200-0x2FF` 3+|Standard read/write
|`01` |`10` |`0XXX` 2+| `0x600-0x67F` 3+|Standard read/write
|`01` |`10` |`10XX` 2+| `0x680-0x6BF` 3+|Standard read/write
|`01` |`10` |`11XX` 2+| `0x6C0-0x6FF` 3+|Custom read/write
|`10` |`10` |`0XXX` 2+| `0xA00-0xA7F` 3+|Standard read/write
|`10` |`10` |`10XX` 2+| `0xA80-0xABF` 3+|Standard read/write
|`10` |`10` |`11XX` 2+| `0xAC0-0xAFF` 3+|Custom read/write
|`11` |`10` |`0XXX` 2+| `0xE00-0xE7F` 3+|Standard read-only
|`11` |`10` |`10XX` 2+| `0xE80-0xEBF` 3+|Standard read-only
|`11` |`10` |`11XX` 2+| `0xEC0-0xEFF` 3+|Custom read-only
8+|Machine-Level CSRs
|`00` |`11` |`XXXX` 2+|`0x300-0x3FF` 3+|Standard read/write
|`01` |`11` |`0XXX` 2+|`0x700-0x77F` 3+|Standard read/write
|`01` |`11` |`100X` 2+|`0x780-0x79F` 3+|Standard read/write
|`01` |`11` |`1010` 2+|`0x7A0-0x7AF` 3+|Standard read/write debug CSRs
|`01` |`11` |`1011` 2+|`0x7B0-0x7BF` 3+|Debug-mode-only CSRs
|`01` |`11` |`11XX` 2+|`0x7C0-0x7FF` 3+|Custom read/write
|`10` |`11` |`0XXX` 2+|`0xB00-0xB7F` 3+|Standard read/write
|`10` |`11` |`10XX` 2+|`0xB80-0xBBF` 3+|Standard read/write
|`10` |`11` |`11XX` 2+|`0xBC0-0xBFF` 3+|Custom read/write
|`11` |`11` |`0XXX` 2+|`0xF00-0xF7F` 3+|Standard read-only
|`11` |`11` |`10XX` 2+|`0xF80-0xFBF` 3+|Standard read-only
|`11` |`11` |`11XX` 2+|`0xFC0-0xFFF` 3+|Custom read-only
|===

<<<

[[ucsrnames]]
.Currently allocated RISC-V unprivileged CSR addresses.
[float="center",align="center",cols="<10%,<10%,<20%,<60%",options="header"]
|===
|Number |Privilege |Name |Description
4+^|Unprivileged Floating-Point CSRs

|`0x001` +
`0x002` +
`0x003`
|URW +
URW +
URW 
|`fflags` +
`frm` +
`fcsr`
|Floating-Point Accrued Exceptions. +
Floating-Point Dynamic Rounding Mode. +
Floating-Point Control and Status Register (`frm` +`fflags`).

4+^|Unprivileged Zicfiss extension CSR
|`0x011` +
|URW +
|`ssp` +
|Shadow Stack Pointer. +

4+^|Unprivileged Counter/Timers

|`0xC00` +
`0xC01` + 
`0xC02` +
`0xC03` +
`0xC04` +
 &#160; +
`0xC1F` +
`0xC80` +
`0xC81` +
`0xC82` +
`0xC83` +
`0xC84` +
&#160; +
`0xC9F` 
|URO +
URO +
URO +
URO +
URO +
&#160; +
URO +
URO +
URO +
URO +
URO +
URO +
&#160; +
URO  
|`cycle` +
`time` +      
`instret` +       
`hpmcounter3` +   
`hpmcounter4` + 
&#8942; +
`hpmcounter31` + 
`cycleh` +
`timeh` +       
`instreth` +      
`hpmcounter3h` + 
`hpmcounter4h` +
&#8942; + 
`hpmcounter31h`
|Cycle counter for RDCYCLE instruction. +
Timer for RDTIME instruction. +
Instructions-retired counter for RDINSTRET instruction. +
Performance-monitoring counter. +
Performance-monitoring counter. +
&#160; +
Performance-monitoring counter. +
Upper 32 bits of `cycle`, RV32 only. +
Upper 32 bits of `time`, RV32 only. +
Upper 32 bits of `instret`, RV32 only. +
Upper 32 bits of `hpmcounter3`, RV32 only. +
Upper 32 bits of `hpmcounter4`, RV32 only. +
&#160; +
Upper 32 bits of `hpmcounter31`, RV32 only.
|===

<<<

[[scsrnames]]
.Currently allocated RISC-V supervisor-level CSR addresses.
[%autowidth,float="center",align="center",cols="<,<,<,<",options="header"]
|===
|Number |Privilege |Name |Description
4+^|Supervisor Trap Setup

|`0x100` +
`0x104` +
`0x105` +
`0x106`
|SRW +
SRW +
SRW +
SRW
|`sstatus` +
`sie` +
`stvec` +      
`scounteren`
|Supervisor status register. +
Supervisor interrupt-enable register. +
Supervisor trap handler base address. +
Supervisor counter enable.

4+^|Supervisor Configuration

|`0x10A` |SRW |`senvcfg` |Supervisor environment configuration register.

4+^|Supervisor Counter Setup

|`0x120` |SRW |`scountinhibit` |Supervisor counter-inhibit register.

4+^|Supervisor Trap Handling

|`0x140` +
`0x141` +
`0x142` +
`0x143` +
`0x144` +
`0xDA0`
|SRW +
SRW +
SRW +
SRW +
SRW +
SRO
|`sscratch` +
`sepc` +
`scause` +
`stval` +
`sip` +
`scountovf`
|Scratch register for supervisor trap handlers. +
Supervisor exception program counter. +
Supervisor trap cause. +
Supervisor bad address or instruction. +
Supervisor interrupt pending. +
Supervisor count overflow.

4+^|Supervisor Protection and Translation

|`0x180` |SRW |`satp` |Supervisor address translation and protection.

4+^|Debug/Trace Registers

|`0x5A8` |SRW |`scontext` |Supervisor-mode context register.

//4+^|Supervisor Resource Management Configuration
//|`0x181` |SRW |`srmcfg` |Supervisor Resource Management Configuration.

4+^|Supervisor State Enable Registers
|`0x10C` +
 `0x10D` +
 `0x10E` +
 `0x10F`
|SRW +
 SRW +
 SRW +
 SRW
|`sstateen0` +
 `sstateen1` +
 `sstateen2` +
 `sstateen3`
|Supervisor State Enable 0 Register. +
 Supervisor State Enable 1 Register. +
 Supervisor State Enable 2 Register. +
 Supervisor State Enable 3 Register.

|===

<<<

[[hcsrnames]]
.Currently allocated RISC-V hypervisor and VS CSR addresses.
[%autowidth,float="center",align="center",cols="<,<,<,<",options="header"]
|===
|Number |Privilege |Name |Description
4+^|Hypervisor Trap Setup

|`0x600` +
`0x602` +
`0x603` +
`0x604` +
`0x606` +
`0x607` +
`0x612`
|HRW + 
HRW +
HRW +
HRW +
HRW +
HRW +
HRW 
|`hstatus` +
`hedeleg` +
`hideleg` +  
`hie` +  
`hcounteren` + 
`hgeie` +
`hedelegh`
|Hypervisor status register. +
Hypervisor exception delegation register. +
Hypervisor interrupt delegation register. +
Hypervisor interrupt-enable register. +
Hypervisor counter enable. +
Hypervisor guest external interrupt-enable register. +
Upper 32 bits of `hedeleg`, RV32 only.

4+^|Hypervisor Trap Handling

|`0x643` +
`0x644` +
`0x645` +
`0x64A` +
`0xE12` 
|HRW +
HRW +
HRW +
HRW +
HRO
|`htval` +
`hip` +
`hvip` + 
`htinst` +
`hgeip`
|Hypervisor bad guest physical address. +
Hypervisor interrupt pending. +
Hypervisor virtual interrupt pending. +
Hypervisor trap instruction (transformed). +
Hypervisor guest external interrupt pending.

4+^|Hypervisor Configuration

|`0x60A` +
`0x61A`
|HRW +
HRM
|`henvcfg` +
`henvcfgh` 
|Hypervisor environment configuration register. +
Upper 32 bits of `henvcfg`, RV32 only.

4+^|Hypervisor Protection and Translation

|`0x680` |HRW |`hgatp` |Hypervisor guest address translation and protection.

4+^|Debug/Trace Registers

|`0x6A8` |HRW |`hcontext` |Hypervisor-mode context register.

4+^|Hypervisor Counter/Timer Virtualization Registers

|`0x605` +
`0x615`
|HRW +
HRW
|`htimedelta` +
`htimedeltah`
|Delta for VS/VU-mode timer. +
Upper 32 bits of `htimedelta`, RV32 only.

4+^|Hypervisor State Enable Registers
|`0x60C` +
 `0x60D` +
 `0x60E` +
 `0x60F` +
 `0x61C` +
 `0x61D` +
 `0x61E` +
 `0x61F`
|HRW +
 HRW +
 HRW +
 HRW +
 HRW +
 HRW +
 HRW +
 HRW
|`hstateen0`  +
 `hstateen1`  +
 `hstateen2`  +
 `hstateen3`  +
 `hstateen0h` +
 `hstateen1h` +
 `hstateen2h` +
 `hstateen3h`
|Hypervisor State Enable 0 Register. +
 Hypervisor State Enable 1 Register. +
 Hypervisor State Enable 2 Register. +
 Hypervisor State Enable 3 Register. +
 Upper 32 bits of Hypervisor State Enable 0 Register, RV32 only. +
 Upper 32 bits of Hypervisor State Enable 1 Register, RV32 only. +
 Upper 32 bits of Hypervisor State Enable 2 Register, RV32 only. +
 Upper 32 bits of Hypervisor State Enable 3 Register, RV32 only.

4+^|Virtual Supervisor Registers

|`0x200` +
`0x204` +
`0x205` +
`0x240` +
`0x241` +
`0x242` +
`0x243` +
`0x244` +
`0x280`  
|HRW +
HRW +
HRW +
HRW +
HRW +
HRW +
HRW +
HRW +
HRW 
|`vsstatus` +
`vsie` +
`vstvec` +   
`vsscratch` +
`vsepc` +
`vscause` +  
`vstval` +
`vsip` + 
`vsatp`
|Virtual supervisor status register. +
Virtual supervisor interrupt-enable register. +
Virtual supervisor trap handler base address. +
Virtual supervisor scratch register. +
Virtual supervisor exception program counter. +
Virtual supervisor trap cause. +
Virtual supervisor bad address or instruction. +
Virtual supervisor interrupt pending. +
Virtual supervisor address translation and protection.

|===

<<<

[[mcsrnames0]]
[.monocell]
.Currently allocated RISC-V machine-level CSR addresses.
[%autowidth,float="center",align="center",cols="<,<,<,<",options="header"]
|===
|Number |Privilege |Name |Description
4+^|Machine Information Registers

|`0xF11` +
`0xF12` +
`0xF13` +
`0xF14` +
`0xF15`
|MRO +
MRO +
MRO +
MRO +
MRO
|`mvendorid` +
`marchid` +
`mimpid` +
`mhartid` +
`mconfigptr`
|Vendor ID. +
Architecture ID. +
Implementation ID. +
Hardware thread ID. +
Pointer to configuration data structure.

4+^|Machine Trap Setup

|`0x300` +
`0x301` +
`0x302` +
`0x303` +
`0x304` +
`0x305` +
`0x306` +
`0x310` +
`0x312`
|MRW +
MRW +
MRW +
MRW +
MRW +
MRW +
MRW +
MRW +
MRW  
|`mstatus` +
`misa` + 
`medeleg` +
`mideleg` +
`mie` +
`mtvec` +
`mcounteren` +
`mstatush` +
`medelegh`
|Machine status register. +
ISA and extensions +
Machine exception delegation register. +
Machine interrupt delegation register. +
Machine interrupt-enable register. +
Machine trap-handler base address. +
Machine counter enable. +
Additional machine status register, RV32 only. +
Upper 32 bits of `medeleg`, RV32 only.

4+^|Machine Trap Handling

|`0x340` +
`0x341` +
`0x342` +
`0x343` +
`0x344` +
`0x34A` +
`0x34B` 
|MRW +
MRW +
MRW +
MRW +
MRW +
MRW +
MRW
|`mscratch` +
`mepc` +
`mcause` +
`mtval` +
`mip` +
`mtinst` +
`mtval2` 
|Scratch register for machine trap handlers. +
Machine exception program counter. +
Machine trap cause. +
Machine bad address or instruction. +
Machine interrupt pending. +
Machine trap instruction (transformed). +
Machine bad guest physical address.

4+^|Machine Configuration

|`0x30A` +
`0x31A` +
`0x747` +
`0x757` 
|MRW +
MRW +
MRW +
MRW  
|`menvcfg` +
`menvcfgh` + 
`mseccfg` +
`mseccfgh`
|Machine environment configuration register. +
Upper 32 bits of `menvcfg`, RV32 only. +
Machine security configuration register. +
Upper 32 bits of `mseccfg`, RV32 only.

4+^|Machine Memory Protection

|`0x3A0` +
`0x3A1` +
`0x3A2` +
`0x3A3` +
&#160; +
`0x3AE` +
`0x3AF` +
`0x3B0` +
`0x3B1` +
&#160; +
`0x3EF`
|MRW +
MRW +
MRW +
MRW +
&#160; +
MRW +
MRW +
MRW +
MRW +
&#160; +
MRW
|`pmpcfg0` +
`pmpcfg1` +
`pmpcfg2` +
`pmpcfg3` +
&#8943; +
`pmpcfg14` + 
`pmpcfg15` +
`pmpaddr0` +
`pmpaddr1` +
&#8943; +
`pmpaddr63`
|Physical memory protection configuration. +
Physical memory protection configuration, RV32 only. +
Physical memory protection configuration. +
Physical memory protection configuration, RV32 only. +
&#160; +
Physical memory protection configuration. +
Physical memory protection configuration, RV32 only. +
Physical memory protection address register. +
Physical memory protection address register. +
&#160; +
Physical memory protection address register.

4+^|Machine State Enable Registers
|`0x30C` +
 `0x30D` +
 `0x30E` +
 `0x30F` +
 `0x31C` +
 `0x31D` +
 `0x31E` +
 `0x31F`
|MRW +
 MRW +
 MRW +
 MRW +
 MRW +
 MRW +
 MRW +
 MRW
|`mstateen0`  +
 `mstateen1`  +
 `mstateen2`  +
 `mstateen3`  +
 `mstateen0h` +
 `mstateen1h` +
 `mstateen2h` +
 `mstateen3h`
|Machine State Enable 0 Register. +
 Machine State Enable 1 Register. +
 Machine State Enable 2 Register. +
 Machine State Enable 3 Register. +
 Upper 32 bits of Machine State Enable 0 Register, RV32 only. +
 Upper 32 bits of Machine State Enable 1 Register, RV32 only. +
 Upper 32 bits of Machine State Enable 2 Register, RV32 only. +
 Upper 32 bits of Machine State Enable 3 Register, RV32 only.
|===

<<<

[[mcsrnames1]]
.Currently allocated RISC-V machine-level CSR addresses.
[%autowidth,float="center",align="center",cols="<,<,<,<",options="header"]
|===
|Number |Privilege |Name |Description
4+^|Machine Non-Maskable Interrupt Handling

|`0x740` +
`0x741` +
`0x742` +
`0x744` 
|MRW +
MRW +
MRW +
MRW
|`mnscratch` +
`mnepc` +
`mncause` +
`mnstatus`
|Resumable NMI scratch register. +
Resumable NMI program counter. +
Resumable NMI cause. +
Resumable NMI status.

4+^|Machine Counter/Timers

|`0xB00` +
`0xB02` +
`0xB03` +
`0xB04` +
&#160; +
`0xB1F` +
`0xB80` +
`0xB82` +
`0xB83` +
`0xB84` +
&#160; +
`0xB9F`
|MRW +
MRW +
MRW +
MRW +
&#160; +
MRW +
MRW +
MRW +
MRW +
MRW +
&#160; +
MRW +
|`mcycle` +
`minstret` +
`mhpmcounter3` +
`mhpmcounter4` +
&#8942; +
`mhpmcounter31` +
`mcycleh` +
`minstreth` +
`mhpmcounter3h` +
`mhpmcounter4h` +
&#8942;
`mhpmcounter31h`
|Machine cycle counter. +
Machine instructions-retired counter. +
Machine performance-monitoring counter. +
Machine performance-monitoring counter. +
&#160; +
Machine performance-monitoring counter. +
Upper 32 bits of `mcycle`, RV32 only. +
Upper 32 bits of `minstret`, RV32 only. +
Upper 32 bits of `mhpmcounter3`, RV32 only. +
Upper 32 bits of `mhpmcounter4`, RV32 only. +
&#160; +
Upper 32 bits of `mhpmcounter31`, RV32 only.

4+^|Machine Counter Setup

|`0x320` +
`0x323` +
`0x324` +
&#160; +
`0x33F` +
`0x723` +
`0x724` +
&#160; +
`0x73F`
|MRW + 
MRW +
MRW +
&#160; +
MRW +
MRW +
MRW +
&#160; +
MRW
|`mcountinhibit` +
`mhpmevent3` +
`mhpmevent4` +
&#8942; +
`mhpmevent31` +
`mhpmevent3h` +
`mhpmevent4h` +
&#8942; +
`mhpmevent31h`
|Machine counter-inhibit register. +
Machine performance-monitoring event selector. +
Machine performance-monitoring event selector. +
&#160; +
Machine performance-monitoring event selector. +
Upper 32 bits of `mhpmevent3`, RV32 only. +
Upper 32 bits of `mhpmevent4`, RV32 only. +
&#160; +
Upper 32 bits of `mhpmevent31`, RV32 only.


4+^|Debug/Trace Registers (shared with Debug Mode)

|`0x7A0` +
`0x7A1`  +
`0x7A2`  +
`0x7A3` +
`0x7A8`
|MRW +
MRW +
MRW +
MRW +
MRW
|`tselect` +
`tdata1` +
`tdata2` +
`tdata3` +
`mcontext`

|Debug/Trace trigger register select. +
First Debug/Trace trigger data register. +
Second Debug/Trace trigger data register. +
Third Debug/Trace trigger data register. +
Machine-mode context register.

4+^|Debug Mode Registers

|`0x7B0` +
`0x7B1` +
`0x7B2` +
`0x7B3`
|DRW +
DRW +
DRW +
DRW +
|`dcsr` +
`dpc` +
`dscratch0` +
`dscratch1`
|Debug control and status register. +
Debug program counter. +
Debug scratch register 0. +
Debug scratch register 1.
|===

=== CSR Field Specifications

The following definitions and abbreviations are used in specifying the
behavior of fields within the CSRs.

==== Reserved Writes Preserve Values, Reads Ignore Values (WPRI)

Some whole read/write fields are reserved for future use. Software
should ignore the values read from these fields, and should preserve the
values held in these fields when writing values to other fields of the
same register. For forward compatibility, implementations that do not
furnish these fields must make them read-only zero. These fields are
labeled *WPRI* in the register descriptions.

[NOTE]
====
To simplify the software model, any backward-compatible future
definition of previously reserved fields within a CSR must cope with the
possibility that a non-atomic read/modify/write sequence is used to
update other fields in the CSR. Alternatively, the original CSR
definition must specify that subfields can only be updated atomically,
which may require a two-instruction clear bit/set bit sequence in
general that can be problematic if intermediate values are not legal.
====

==== Write/Read Only Legal Values (WLRL)

Some read/write CSR fields specify behavior for only a subset of
possible bit encodings, with other bit encodings reserved. Software
should not write anything other than legal values to such a field, and
should not assume a read will return a legal value unless the last write
was of a legal value, or the register has not been written since another
operation (e.g., reset) set the register to a legal value. These fields
are labeled *WLRL* in the register descriptions.

[NOTE]
====
Hardware implementations need only implement enough state bits to
differentiate between the supported values, but must always return the
complete specified bit-encoding of any supported value when read.
====

Implementations are permitted but not required to raise an
illegal-instruction exception if an instruction attempts to write a
non-supported value to a *WLRL* field. Implementations can return arbitrary
bit patterns on the read of a *WLRL* field when the last write was of an
illegal value, but the value returned should deterministically depend on
the illegal written value and the value of the field prior to the write.

==== Write Any Values, Reads Legal Values (WARL)

Some read/write CSR fields are only defined for a subset of bit
encodings, but allow any value to be written while guaranteeing to
return a legal value whenever read. Assuming that writing the CSR has no
other side effects, the range of supported values can be determined by
attempting to write a desired setting then reading to see if the value
was retained. These fields are labeled *WARL* in the register descriptions.

Implementations will not raise an exception on writes of unsupported
values to a *WARL* field. Implementations can return any legal value on the
read of a *WARL* field when the last write was of an illegal value, but the
legal value returned should deterministically depend on the illegal
written value and the architectural state of the hart.

=== CSR Field Modulation

If a write to one CSR changes the set of legal values allowed for a
field of a second CSR, then unless specified otherwise, the second CSR's
field immediately gets an `UNSPECIFIED` value from among its new legal values. This
is true even if the field's value before the write remains legal after
the write; the value of the field may be changed in consequence of the
write to the controlling CSR.

[NOTE]
====
As a special case of this rule, the value written to one CSR may control
whether a field of a second CSR is writable (with multiple legal values)
or is read-only. When a write to the controlling CSR causes the second
CSR's field to change from previously read-only to now writable, that
field immediately gets an `UNSPECIFIED` but legal value, unless specified otherwise.

***
Some CSR fields are, when writable, defined as aliases of other CSR
fields. Let _x_ be such a CSR field, and let _y_ be the CSR field it aliases when writable. If a write to a controlling CSR causes field _x_ to change from previously read-only to now writable, the new value of _x_ is not `UNSPECIFIED` but instead immediately reflects the existing value of its alias _y_, as required.
====

A change to the value of a CSR for this reason is not a write to the
affected CSR and thus does not trigger any side effects specified for
that CSR.

=== Implicit Reads of CSRs

Implementations sometimes perform _implicit_ reads of CSRs. (For
example, all S-mode instruction fetches implicitly read the `satp` CSR.)
Unless otherwise specified, the value returned by an implicit read of a
CSR is the same value that would have been returned by an explicit read
of the CSR, using a CSR-access instruction in a sufficient privilege
mode.

[[csrwidthmodulation]]
=== CSR Width Modulation

If the width of a CSR is changed (for example, by changing SXLEN or
UXLEN, as described in <<xlen-control>>), the
values of the _writable_ fields and bits of the new-width CSR are,
unless specified otherwise, determined from the previous-width CSR as
though by this algorithm:

. The value of the previous-width CSR is copied to a temporary register
of the same width.
. For the read-only bits of the previous-width CSR, the bits at the same
positions in the temporary register are set to zeros.
. The width of the temporary register is changed to the new width. If
the new width _W_ is narrower than the previous width, the
least-significant _W_ bits of the temporary register are
retained and the more-significant bits are discarded. If the new width
is wider than the previous width, the temporary register is
zero-extended to the wider width.
. Each writable field of the new-width CSR takes the value of the bits
at the same positions in the temporary register.

Changing the width of a CSR is not a read or write of the CSR and thus
does not trigger any side effects.

=== Explicit Accesses to CSRs Wider than XLEN

If a standard CSR is wider than XLEN bits, then an explicit read
of the CSR returns the register's least-significant XLEN bits,
and an explicit write to the CSR modifies only the register's
least-significant XLEN bits, leaving the upper bits unchanged.

Some standard CSRs, such as the counter CSRs of extension
Zicntr, are always 64 bits, even when XLEN=32 (RV32).
For each such 64-bit CSR (for example, counter `time`),
a corresponding 32-bit _high-half CSR_ is usually defined with
the same name but with the letter '`h`' appended at the end (`timeh`).
The high-half CSR aliases bits 63:32 of its namesake
64-bit CSR, thus providing a way for RV32 software
to read and modify the otherwise-unreachable 32 bits.

Standard high-half CSRs are accessible only when
the base RISC-V instruction set is RV32 (XLEN=32).
For RV64 (when XLEN=64), the addresses of all standard high-half CSRs
are reserved, so an attempt to access a high-half CSR
typically raises an illegal-instruction exception.