Risc-V支持16位压缩格式，压缩格式立即数位数更少，能使用的寄存器也比较少，有些指令只能用常用8个整数寄存器（x8-x15）或者(f8-f15)。16 位指令只对汇编器和链接器可见，并且是否以短指令取代对应的宽指令由它们决定。编译器编写者和汇编语言程序员可以幸福地忽略 RV32C 指令及其格式，他们能感知到的则是最后的程序大小小于大多数其它 ISA 的程序。
为了能在一系列的程序上得到良好的代码压缩效果， RISC-V 架构师精心挑选了 RVC扩展中的指令。同时，基于以下的三点观察，架构师们成功地将指令压缩到了 16 位。第一，对十个常用寄存器（a0-a5， s0-s1， sp 以及 ra）访问的频率远超过其他寄存器；第二，许多指令的写入目标是它的源操作数之一；第三，立即数往往很小，而且有些指令比较喜欢某些特定的立即数。因此，许多 RV32C 指令只能访问那些常用寄存器；一些指令隐式写入源操作数的位置；几乎所有的立即数都被缩短了， load 和 store 操作只使用操作数整数倍尺寸的无符号数偏移量。
下图为RVC指令格式：
其中rs1’,rs2’, rd’如下图所示，只能用x8-x15,f8-f15这些寄存器。但有些指令可以使用所有通用寄存器作为操作数。
每个RVC指令都有对应的32位指令，下面列出所有的RV32C指令。
c.addispn
c.addi4spn rd’, uimm //x[8+rd’] = x[2] + uimm
加 4 倍立即数到栈指针 (Add Immediate, Scaled by 4, to Stack Pointer, Nondestructive). RV32IC and RV64IC.
扩展形式为 addi rd, x2, uimm, 其中 rd=8+rd’. uimm=0 时非法。
| imm | |||||||||||||||||
| func3 | 5 | 4 | 9 | 8 | 7 | 6 | 2 | 3 | rd' | opcode | |||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.addi4spn | CIW | 0 | 0 | 0 | 0 | 0 | |||||||||||
示例：
to do
c.fld
c.fld rd’, uimm(rs1’) //f[8+rd’] = M[x[8+rs1’] + uimm][63:0]
浮点双字加载 (Floating-point Load Doubleword). RV32DC and RV64DC.
扩展形式为 fld rd, uimm(rs1), 其中 rd=8+rd’, rs1=8+rs1’
| imm | imm | ||||||||||||||||
| func3 | 5 | 4 | 3 | rs1' | 7 | 6 | rd' | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.fld | CL | 0 | 0 | 1 | 0 | 0 | |||||||||||
示例：
to do
c.fsd
c.fsd rs2’, uimm(rs1’) //M[x[8+rs1’] + uimm][63:0] = f[8+rs2’]
浮点双字存储 (Floating-point Store Doubleword). RV32DC and RV64DC.
扩展形式为 fsd rs2, uimm(rs1), 其中 rs2=8+rs2’, rs1=8+rs1’
| imm | imm | ||||||||||||||||
| func3 | 5 | 4 | 3 | rs1' | 7 | 6 | rd' | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.fsd | CL | 1 | 0 | 1 | 0 | 0 | |||||||||||
示例：
to do
c.lw
c.lw rd’, uimm(rs1’) //x[8+rd’] = sext(M[x[8+rs1’] + uimm][31:0])
字加载 (Load Word). RV32IC and RV64IC.
扩展形式为 lw rd, uimm(rs1), 其中 rd=8+rd’, rs1=8+rs1’.
| imm | imm | ||||||||||||||||
| func3 | 5 | 4 | 3 | rs1' | 2 | 6 | rd' | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.lw | CL | 0 | 1 | 0 | 0 | 0 | |||||||||||
示例：
to do
c.addi
c.addi rd, imm //x[rd] = x[rd] + sext(imm)
加立即数 (Add Immediate). RV32IC and RV64IC.
扩展形式为 addi rd, rd, imm.
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.addi | CI | 0 | 0 | 0 | 0 | 1 | |||||||||||
示例：
to do
c.jal
c.jal offset //x[1] = pc+2; pc += sext(offset)
链接跳转 (Jump and Link). RV32IC.
扩展形式为 jal x1, offset.
| imm | |||||||||||||||||
| func3 | 11 | 4 | 9 | 8 | 10 | 6 | 7 | 3 | 2 | 1 | 5 | opcode | |||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.jal | CJ | 0 | 0 | 1 | 0 | 1 | |||||||||||
示例：
to do
c.li
c.li rd, imm //x[rd] = sext(imm)
立即数加载 (Load Immediate). RV32IC and RV64IC.
扩展形式为 addi rd, x0, imm.
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.li | CI | 0 | 1 | 0 | 0 | 1 | |||||||||||
示例：
to do
c.lui
c.lui rd, imm //x[rd] = sext(imm[17:12] << 12)
高位立即数加载 (Load Upper Immediate). RV32IC and RV64IC.
扩展形式为 lui rd, imm. 当 rd=x2 或 imm=0 时非法。
| imm | imm | ||||||||||||||||
| func3 | 17 | rd | 16 | 15 | 14 | 13 | 12 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.lui | CI | 0 | 1 | 1 | 0 | 1 | |||||||||||
示例：
to do
c.srli
c.srli rd’, uimm //x[8+rd’] = x[8+rd’] >>u uimm
立即数逻辑右移 (Shift Right Logical Immediate). RV32IC and RV64IC.
扩展形式为 srli rd, rd, uimm, 其中 rd=8+rd’
| imm | imm | ||||||||||||||||
| func3 | 5 | rd' | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.srli | CA | 1 | 0 | 0 | 0 | 0 | 0 | 1 | |||||||||
示例：
to do
c.srai
c.srai rd’, uimm //x[8+rd’] = x[8+rd’] >>s uimm
立即数算术右移 (Shift Right Arithmetic Immediate). RV32IC and RV64IC.
扩展形式为 srai rd, rd, uimm, 其中 rd=8+rd’.
| imm | imm | ||||||||||||||||
| func3 | 5 | rd' | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.srai | CA | 1 | 0 | 0 | 0 | 1 | 0 | 1 | |||||||||
示例：
to do
c.andi
c.andi rd’, imm //x[8+rd’] = x[8+rd’] & sext(imm)
与立即数 (AND Immediate). RV32IC and RV64IC.
扩展形式为 andi rd, rd, imm, 其中 rd=8+rd’.
| imm | imm | ||||||||||||||||
| func3 | 5 | rd' | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.andi | CA | 1 | 0 | 0 | 1 | 0 | 0 | 1 | |||||||||
示例：
to do
c.sub
c.sub rd’, rs2’ //x[8+rd’] = x[8+rd’] - x[8+rs2’]
减 (Subtract). RV32IC and RV64IC.
扩展形式为 sub rd, rd, rs2. 其中 rd=8+rd’, rs2=8+rs2’.
| rd' | rs2' | opcode | |||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.sub | CA | 1 | 0 | 0 | 0 | 1 | 1 | 0 | 0 | 0 | 1 | ||||||
示例：
to do
c.xor
c.xor rd’, rs2’ //x[8+rd’] = x[8+rd’] ^ x[8+rs2’]
异或 (Exclusive-OR). RV32IC and RV64IC.
扩展形式为 xor rd, rd, rs2, 其中 rd=8+rd’, rs2=8+rs2’.
| rd' | rs2' | opcode | |||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.xor | CA | 1 | 0 | 0 | 0 | 1 | 1 | 0 | 1 | 0 | 1 | ||||||
示例：
to do
c.or
c.or rd’, rs2’ //x[8+rd’] = x[8+rd’] | x[8+rs2’]
或 (OR). RV32IC and RV64IC.
扩展形式为 or rd, rd, rs2, 其中 rd=8+rd’, rs2=8+rs2’.
| rd' | rs2' | opcode | |||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.or | CA | 1 | 0 | 0 | 0 | 1 | 1 | 1 | 0 | 0 | 1 | ||||||
示例：
to do
c.and
c.and rd’, rs2’ //x[8+rd’] = x[8+rd’] & x[8+rs2’]
与 (AND). RV32IC and RV64IC.
扩展形式为 and rd, rd, rs2, 其中 rd=8+rd’, rs2=8+rs2’
| rd' | rs2' | opcode | |||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.and | CA | 1 | 0 | 0 | 0 | 1 | 1 | 1 | 1 | 0 | 1 | ||||||
示例：
to do
c.subw
c.subw rd’, rs2’ //x[8+rd’] = sext((x[8+rd’] - x[8+rs2’])[31:0])
减字 (Subtract Word). RV64IC.
扩展形式为 subw rd, rd, rs2. 其中 rd=8+rd’, rs2=8+rs2’. .
| rd' | rs2' | opcode | |||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.subw | CA | 1 | 0 | 0 | 1 | 1 | 1 | 0 | 0 | 0 | 1 | ||||||
示例：
to do
c.addw
c.addw rd’, rs2’ //x[8+rd’] = sext((x[8+rd’] + x[8+rs2’])[31:0])
加字 (Add Word). RV64IC.
扩展形式为 addw rd, rd, rs2, 其中 rd=8+rd’, rs2=8+rs2’.
| rd' | rs2' | opcode | |||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.addw | CA | 1 | 0 | 0 | 1 | 1 | 1 | 0 | 1 | 0 | 1 | ||||||
示例：
to do
c.j
c.j offset //pc += sext(offset)
跳转 (Jump). RV32IC and RV64IC.
扩展形式为 jal x0, offset.
| imm | |||||||||||||||||
| func3 | 11 | 4 | 9 | 8 | 10 | 6 | 7 | 3 | 2 | 1 | 5 | opcode | |||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.j | CJ | 1 | 0 | 1 | 0 | 1 | |||||||||||
示例：
to do
c.bnez
c.bnez rs1’, offset //if (x[8+rs1’] ≠ 0) pc += sext(offset)
不等于零时分支 (Branch if Not Equal to Zero). RV32IC and RV64IC.
扩展形式为 bne rs1, x0, offset, 其中 rs1=8+rs1’.
| imm | imm | ||||||||||||||||
| func3 | 8 | 4 | 3 | rs1' | 7 | 6 | 2 | 1 | 5 | opcode | |||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.bnez | CI | 1 | 1 | 1 | 0 | 1 | |||||||||||
示例：
to do
c.beqz
c.beqz rs1’, offset //if (x[8+rs1’] == 0) pc += sext(offset)
等于零时分支 (Branch if Equal to Zero). RV32IC and RV64IC.
扩展形式为 beq rs1, x0, offset, 其中 rs1=8+rs1’.
| imm | imm | ||||||||||||||||
| func3 | 8 | 4 | 3 | rs1' | 7 | 6 | 2 | 1 | 5 | opcode | |||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.beqz | CI | 1 | 1 | 0 | 0 | 1 | |||||||||||
示例：
to do
c.slli
c.slli rd, uimm //x[rd] = x[rd] << uimm
立即数逻辑左移 (Shift Left Logical Immediate). RV32IC and RV64IC.
扩展形式为 slli rd, rd, uimm.
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.slli | CI | 0 | 0 | 0 | 1 | 0 | |||||||||||
示例：
to do
c.fldsp
c.fldsp rd, uimm(x2) //f[rd] = M[x[2] + uimm][63:0]
栈指针相关浮点双字加载 (Floating-point Load Doubleword, Stack-Pointer Relative). RV32DC
and RV64DC.
扩展形式为 fld rd, uimm(x2).
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 8 | 7 | 6 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.fldsp | CI | 0 | 0 | 1 | 1 | 0 | |||||||||||
示例：
to do
c.lwsp
c.lwsp rd, uimm(x2) //x[rd] = sext(M[x[2] + uimm][31:0])
栈指针相关字加载 (Load Word, Stack-Pointer Relative). RV32IC and RV64IC.
扩展形式为 lw rd, uimm(x2). rd=x0 时非法。
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 2 | 7 | 6 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.lwsp | CI | 0 | 1 | 0 | 1 | 0 | |||||||||||
示例：
to do
c.flwsp
c.flwsp rd, uimm(x2) //f[rd] = M[x[2] + uimm][31:0]
栈指针相关浮点字加载 (Floating-point Load Word, Stack-Pointer Relative). RV32FC.
扩展形式为 flw rd, uimm(x2).
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 2 | 7 | 6 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.flwsp | CI | 0 | 1 | 1 | 1 | 0 | |||||||||||
示例：
to do
c.mv
c.mv rd, rs2 //x[rd] = x[rs2]
移动 (Move). RV32IC and RV64IC.
扩展形式为 add rd, x0, rs2. rs2=x0 时非法
| rd | rs2 | opcode | |||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.mv | CA | 1 | 0 | 0 | 0 | 1 | 0 | ||||||||||
示例：
to do
c.fsdsp
c.fsdsp rs2, uimm(x2) //M[x[2] + uimm][63:0] = f[rs2]
栈指针相关浮点双字存储 (Floating-point Store Doubleword, Stack-Pointer Relative). RV32DC and RV64DC.
扩展形式为 fsd rs2, uimm(x2).
| imm | |||||||||||||||||
| 5 | 4 | 3 | 8 | 7 | 6 | rs2 | opcode | ||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.fsdsp | CSS | 1 | 0 | 1 | 1 | 0 | |||||||||||
示例：
to do
c.fswsp
c.fswsp rs2, uimm(x2) //M[x[2] + uimm][31:0] = f[rs2]
栈指针相关浮点字存储 (Floating-point Store Word, Stack-Pointer Relative). RV32FC.
扩展形式为 fsw rs2, uimm(x2)
| imm | |||||||||||||||||
| 5 | 4 | 3 | 2 | 7 | 6 | rs2 | opcode | ||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.fswsp | CSS | 1 | 1 | 1 | 1 | 0 | |||||||||||
示例：
to do
c.nop
空操作，只增加PC值。对应 addi x0,x0,0
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.nop | CI | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 |
示例：
to do
c.ebreak
c.ebreak RaiseException(Breakpoint)
环境断点 (Environment Breakpoint). RV32IC and RV64IC.
扩展形式为 ebreak.
| imm | imm | ||||||||||||||||
| func3 | 5 | rd | 4 | 3 | 2 | 1 | 0 | opcode | |||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.nop | CI | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 |
示例：
to do
c.addi16sp
c.addi16sp imm //x[2] = x[2] + sext(imm)
加 16 倍立即数到栈指针 (Add Immediate, Scaled by 16, to Stack Pointer). RV32IC and RV64IC.
扩展形式为 addi x2, x2, imm. imm=0 时非法。
| func3 | 9 | 4 | 6 | 8 | 7 | 5 | opcode | ||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.addi16spn | CIW | 0 | 1 | 1 | 0 | 0 | 0 | 1 | 0 | 0 | 1 | ||||||
示例：
to do
c.jr
c.jr rs1 //pc = x[rs1]
寄存器跳转 (Jump Register). RV32IC and RV64IC.
扩展形式为 jalr x0, 0(rs1). 当 rs1=x0 时非法。
| func3 | 9 | rs1 | opcode | ||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.jr | CI | 1 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | |||||
示例：
to do
c.jalr
c.jalr rs1 t = pc+2; pc = x[rs1]; x[1] = t
寄存器链接跳转 (Jump and Link Register). RV32IC and RV64IC.
扩展形式为 jalr x1, 0(rs1). 当 rs1=x0 时非法
| func3 | 9 | rs1 | opcode | ||||||||||||||
| name | type | 15 | 14 | 13 | 12 | 11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
| c.jalr | CI | 1 | 0 | 0 | 1 | 0 | 0 | 0 | 0 | 0 | 1 | 0 | |||||
示例：
to do
xxxxx
示例：
to do
xxxxx
示例：
to do