:sectnums:
:version-label: v1.0.2
:lifecycle-state: frozen

[#Zc]
== Zc* {version-label}

=== Change history since v0.70.1 (tagged release)

.Change history
[width="100%",options=header]
|====================================================================================
|Version  | change
|v1.0.2      | Fix Architecture Review Committee feedback on instruction formats
|v1.0.1      | Post public review fixes: Add instruction formats (issue 192). Clarify that Zcmt/Zcmp are for embedded CPUs (issue 190). Fix some typos.
|v1.0.0-RC5.7| Add Zcb description and fix some typos. PUBLIC REVIEW REVISION.
|v1.0.0-RC5.6| Remove Zcmpe which is _not_ frozen and is causing confusion
|v1.0.0-RC5.5| Following ARC review Adjust the split so we have 224 cm.jalt and 32 cm.jt
|v1.0.0-RC5.4| Change wording for dependencies to match arch manual "Zxxx requires Zyyy" changed to "Zxxx depends on Zyyy"
|v1.0.0-RC5.3| Add dependency on Zicsr for Zcmt
|v1.0.0-RC5.2| Adjust the split so we have 240 cm.jalt and 16 cm.jt
|v1.0.0-RC5.1| Make cm.jt/cm.jalt only valid if JVT.mode=0, and allow different behaviour in the future if JVT.mode>0
|v1.0.0-RC5| Revert to cm.jt and cm.jalt encodings, to avoid toolchain and trace problems
|v1.0.0-RC4.1| Resolve typographical issues with the document only, no actual changes
|v1.0.0-RC4| Release candidate
|         | Remove Zcmb as benefit is low. Remove cm.jalt, read LSB of jump table entry to determine whether to link
|v0.70.5  | Resolve https://github.com/riscv/riscv-code-size-reduction/issues/163 - jvt.base is WARL and fewer bits than the max can be implemented
|v0.70.4  | Clarified https://github.com/riscv/riscv-code-size-reduction/issues/159 - Need Zbb and Zba for RV64 and M/ZMmul to get _all_ of Zcb
|         | Resolved https://github.com/riscv/riscv-code-size-reduction/issues/161
|         | Resolved https://github.com/riscv/riscv-code-size-reduction/issues/160 - Allocated Smstateen bit 2 and added the relevant text
|v0.70.3  | Added rule that Zcf and Zcmt imply Zca (this text was missing, this is not a spec change: https://github.com/riscv/riscv-code-size-reduction/pull/151)
|         | Added that Zcf is illegal for RV64, as it contains no instructions (clarification: https://github.com/riscv/riscv-code-size-reduction/issues/149)
|         | Added push/pop examples in the push/pop section
|v0.70.2  | Stylistic changes only, removing redundant text. 
|         | Corrected field names on JVT CSR diagram, and fixed synopsis for cm.mvsa01
|====================================================================================

=== Zc* Overview

This document is in the Frozen state. Change is extremely unlikely. A high threshold will be used, and a change will only occur because of some truly critical issue being identified during the public review cycle. Any other desired or needed changes can be the subject of a follow-on new extension. See https://riscv.org/spec-state

Zc* is a group of extensions which define subsets of the existing C extension (Zca, Zcd, Zcf) and new extensions which only contain 16-bit encodings.

Zcm* all reuse the encodings for _c.fld_, _c.fsd_, _c.fldsp_, _c.fsdsp_.

.Zc* extension overview
[width="100%",options=header,cols="3,1,1,1,1,1,1"]
|====================================================================================
|Instruction    |Zca     |Zcf     |Zcd     |Zcb     |Zcmp    |Zcmt
7+|*The Zca extension is added as way to refer to instructions in the C extension that do not include the floating-point loads and stores*
|C excl. c.f*   |yes     |        |        |        |        |         
7+|*The Zcf extension is added as a way to refer to compressed single-precision floating-point load/stores*
|c.flw          |        |yes     |        |        |        |       
|c.flwsp        |        |yes     |        |        |        |       
|c.fsw          |        |yes     |        |        |        |       
|c.fswsp        |        |yes     |        |        |        |       
7+|*The Zcd extension is added as a way to refer to compressed double-precision floating-point load/stores*
|c.fld          |        |        |yes     |        |        |       
|c.fldsp        |        |        |yes     |        |        |       
|c.fsd          |        |        |yes     |        |        |       
|c.fsdsp        |        |        |yes     |        |        |       
7+|*Simple operations for use on all architectures*
|c.lbu          |        |        |        |yes     |        |    
|c.lh           |        |        |        |yes     |        |    
|c.lhu          |        |        |        |yes     |        |    
|c.sb           |        |        |        |yes     |        |    
|c.sh           |        |        |        |yes     |        |    
|c.zext.b       |        |        |        |yes     |        |    
|c.sext.b       |        |        |        |yes     |        |    
|c.zext.h       |        |        |        |yes     |        |    
|c.sext.h       |        |        |        |yes     |        |    
|c.zext.w       |        |        |        |yes     |        |    
|c.mul          |        |        |        |yes     |        |    
|c.not          |        |        |        |yes     |        |    
7+|*PUSH/POP and double move which overlap with _c.fsdsp_. Complex operations intended for embedded CPUs*
|cm.push        |        |        |        |        |yes     |
|cm.pop         |        |        |        |        |yes     |
|cm.popret      |        |        |        |        |yes     |
|cm.popretz     |        |        |        |        |yes     |
|cm.mva01s      |        |        |        |        |yes     |
|cm.mvsa01      |        |        |        |        |yes     |
7+|*Table jump which overlaps with _c.fsdsp_. Complex operations intended for embedded CPUs*
|cm.jt          |        |        |        |        |        |yes     
|cm.jalt        |        |        |        |        |        |yes     
|====================================================================================

[#Zca]
=== Zca

The Zca extension is added as way to refer to instructions in the C extension that do not include the floating-point loads and stores.

Therefore it _excluded_ all 16-bit floating point loads and stores: _c.flw_, _c.flwsp_, _c.fsw_, _c.fswsp_, _c.fld_, _c.fldsp_, _c.fsd_, _c.fsdsp_.

NOTE: the C extension only includes F/D instructions when D and F are also specified

[#Zcf]
=== Zcf (RV32 only)

Zcf is the existing set of compressed single precision floating point loads and stores: _c.flw_, _c.flwsp_, _c.fsw_, _c.fswsp_.

Zcf is only relevant to RV32, it cannot be specified for RV64.

The Zcf extension depends on the <<Zca>> extension.

[#Zcd]
=== Zcd 

Zcd is the existing set of compressed double precision floating point loads and stores: _c.fld_, _c.fldsp_, _c.fsd_, _c.fsdsp_.

The Zcd extension depends on the <<Zca>> extension.

[#Zcb]
=== Zcb

Zcb has simple code-size saving instructions which are easy to implement on all CPUs.

All proposed encodings are currently reserved for all architectures, and have no conflicts with any existing extensions.

NOTE: Zcb can be implemented on _any_ CPU as the instructions are 16-bit versions of existing 32-bit instructions from the application class profile.

The Zcb extension depends on the <<Zca>> extension.

As shown on the individual instruction pages, many of the instructions in Zcb depend upon another extension being implemented. For example, _c.mul_ is only implemented if M or Zmmul is implemented, and _c.sext.b_ is only implemented if Zbb is implemented.

The _c.mul_ encoding uses the CA register format along with other instructions such as _c.sub_, _c.xor_ etc. 

[NOTE]

  _c.sext.w_ is a pseudo-instruction for _c.addiw rd, 0_ (RV64)

[%header,cols="^1,^1,4,8"]
|===
|RV32
|RV64
|Mnemonic
|Instruction

|yes     
|yes     
|c.lbu _rd'_, uimm(_rs1'_)
|<<#insns-c_lbu>>

|yes     
|yes     
|c.lhu _rd'_, uimm(_rs1'_)
|<<#insns-c_lhu>>

|yes     
|yes     
|c.lh _rd'_, uimm(_rs1'_)
|<<#insns-c_lh>>

|yes     
|yes     
|c.sb _rs2'_, uimm(_rs1'_)
|<<#insns-c_sb>>

|yes     
|yes     
|c.sh _rs2'_, uimm(_rs1'_)
|<<#insns-c_sh>>

|yes     
|yes     
|c.zext.b _rsd'_
|<<#insns-c_zext_b>> 

|yes     
|yes     
|c.sext.b _rsd'_
|<<#insns-c_sext_b>> 

|yes     
|yes     
|c.zext.h _rsd'_
|<<#insns-c_zext_h>> 

|yes     
|yes     
|c.sext.h _rsd'_
|<<#insns-c_sext_h>> 

|
|yes     
|c.zext.w _rsd'_
|<<#insns-c_zext_w>> 

|yes     
|yes     
|c.not _rsd'_
|<<#insns-c_not>> 

|yes     
|yes     
|c.mul _rsd'_, _rs2'_
|<<#insns-c_mul>> 

|===

<<< 

[#Zcmp]
=== Zcmp

The Zcmp extension is a set of instructions which may be executed as a series of existing 32-bit RISC-V instructions.

This extension reuses some encodings from _c.fsdsp_.  Therefore it is _incompatible_ with <<Zcd>>,
 which is included when C and D extensions are both present. 

NOTE: Zcmp is primarily targeted at embedded class CPUs due to implementation complexity. Additionally, it is not compatible with architecture class profiles.

The Zcmp extension depends on the <<Zca>> extension.

The PUSH/POP assembly syntax uses several variables, the meaning of which are:

* _reg_list_ is a list containing 1 to 13 registers (ra and 0 to 12 s registers)
** valid values: {ra}, {ra, s0}, {ra, s0-s1}, {ra, s0-s2}, ..., {ra, s0-s8}, {ra, s0-s9}, {ra, s0-s11}
** note that {ra, s0-s10} is _not_ valid, giving 12 lists not 13 for better encoding
* _stack_adj_ is the total size of the stack frame. 
** valid values vary with register list length and the specific encoding, see the instruction pages for details.

[%header,cols="^1,^1,4,8"]
|===
|RV32
|RV64
|Mnemonic
|Instruction

|yes     
|yes     
|cm.push _{reg_list}, -stack_adj_
|<<#insns-cm_push>> 

|yes     
|yes     
|cm.pop _{reg_list}, stack_adj_
|<<#insns-cm_pop>> 

|yes     
|yes     
|cm.popret _{reg_list}, stack_adj_
|<<#insns-cm_popret>> 

|yes     
|yes     
|cm.popretz _{reg_list}, stack_adj_
|<<#insns-cm_popretz>> 

|yes     
|yes     
|cm.mva01s _rs1', rs2'_
|<<#insns-cm_mva01s>> 

|yes     
|yes     
|cm.mvsa01 _r1s', r2s'_
|<<#insns-cm_mvsa01>> 

|===

<<<

[#Zcmt]
=== Zcmt

Zcmt adds the table jump instructions and also adds the JVT CSR. The JVT CSR requires a 
state enable if Smstateen is implemented. See <<csrs-jvt>> for details.

This extension reuses some encodings from _c.fsdsp_.  Therefore it is _incompatible_ with <<Zcd>>,
 which is included when C and D extensions are both present. 

NOTE: Zcmt is primarily targeted at embedded class CPUs due to implementation complexity. Additionally, it is not compatible with architecture class profiles.

The Zcmt extension depends on the <<Zca>> and Zicsr extensions.

[%header,cols="^1,^1,4,8"]
|===
|RV32
|RV64
|Mnemonic
|Instruction

|yes     
|yes     
|cm.jt _index_
|<<#insns-cm_jt>> 

|yes     
|yes     
|cm.jalt _index_
|<<#insns-cm_jalt>> 

|===

[#Zc_formats]
=== Zc instruction formats

Several instructions in this specification use the following new instruction formats.

[%header,cols="2,3,2,1,1,1,1,1,1,1,1,1,1"]
|=====================================================================
| Format | instructions       | 15:10  | 9  | 8 | 7   | 6 | 5 | 4   | 3 | 2  | 1 | 0
| CLB    | c.lbu              | funct6 3+| rs1'    2+| uimm    3+| rd'    2+| op
| CSB    | c.sb               | funct6 3+| rs1'    2+| uimm    3+| rs2'   2+| op
| CLH    | c.lhu, c.lh        | funct6 3+| rs1'    | funct1 | uimm 3+| rd'  2+| op
| CSH    | c.sh               | funct6 3+| rs1'    | funct1 | uimm 3+| rs2' 2+| op
| CU     | c.[sz]ext.*, c.not | funct6 3+| rd'/rs1' 5+| funct5            2+| op
| CMMV   | cm.mvsa01 cm.mva01s| funct6 3+| r1s'     2+| funct2 3+| r2s'   2+| op
| CMJT   | cm.jt cm.jalt      | funct6 8+| index                          2+| op
| CMPP   | cm.push*, cm.pop*  | funct6 2+| funct2 4+| urlist   2+| spimm  2+| op
|=====================================================================

NOTE: c.mul uses the existing CA format

[#Zcb_instructions]
== Zcb instructions

include::c_lbu.adoc[]
include::c_lhu.adoc[]
include::c_lh.adoc[]
include::c_sb.adoc[]
include::c_sh.adoc[]

include::c_zext_b.adoc[]
include::c_sext_b.adoc[]
include::c_zext_h.adoc[]
include::c_sext_h.adoc[]
include::c_zext_w.adoc[]
include::c_not.adoc[]
include::c_mul.adoc[]

include::pushpop.adoc[]
include::cm_push.adoc[]
include::cm_pop.adoc[]
include::cm_popretz.adoc[]
include::cm_popret.adoc[]
include::cm_mvsa01.adoc[]
include::cm_mva01s.adoc[]

include::tablejump.adoc[]
include::jvt_csr.adoc[]
include::cm_jt.adoc[]
include::cm_jalt.adoc[]

