- 16 general-purpose registers (x0–x15), where x0 is a dedicated zero register. When read, its value is always 0x00000000. Whenever there is an attempt to write to x0, the data is simply discarded.
- Registers are 32 bits wide.
- There is a program counter (PC) register 32 bits wide.
- Little endian architecture.
- Multiply and divide instructions are not present
- “Control and Status Registers” (CSRs)
- Single and double precision Floating point instructions are not present
- Atomic instructions are not present
Jump Unconditional change
Branch Conditional change| Register | ABI | Use by convention | Preserved? |
|---|---|---|---|
| x0 | zero | hardwired to 0, ignores writes | n/a |
| x1 | ra | return address for jumps | no |
| x2 | sp | stack pointer | yes |
| x3 | gp | global pointer | n/a |
| x4 | tp | thread pointer | n/a |
| x5 | t0 | temporary register 0 | no |
| x6 | t1 | temporary register 1 | no |
| x7 | t2 | temporary register 2 | no |
| x8 | s0 or fp | saved register 0 or frame pointer | yes |
| x9 | s1 | saved register 1 | yes |
| x10 | a0 | return value or function argument 0 | no |
| x11 | a1 | return value or function argument 1 | no |
| x12 | a2 | function argument 2 | no |
| x13 | a3 | function argument 3 | no |
| x14 | a4 | function argument 4 | no |
| x15 | a5 | function argument 5 | no |
| x16 | a6 | function argument 6 | no |
| x17 | a7 | function argument 7 | no |
| x18 | s2 | saved register 2 | yes |
| x19 | s3 | saved register 3 | yes |
| x20 | s4 | saved register 4 | yes |
| x21 | s5 | saved register 5 | yes |
| x22 | s6 | saved register 6 | yes |
| x23 | s7 | saved register 7 | yes |
| x24 | s8 | saved register 8 | yes |
| x25 | s9 | saved register 9 | yes |
| x26 | s10 | saved register 10 | yes |
| x27 | s11 | saved register 11 | yes |
| x28 | t3 | temporary register 3 | no |
| x29 | t4 | temporary register 4 | no |
| x30 | t5 | temporary register 5 | no |
| x31 | t6 | temporary register 6 | no |
| pc | (none) | program counter | n/a |
Operands:
RegD,Reg1,Reg2
Example:
ADD x4,x6,x8 # x4 = x6+x8
Operands:
RegD,Reg1,Immed-12
Examples:
ADDI x4,x6,123 # x4 = x6+123
LW x4,8(x6) # x4 = Mem[8+x6]
Operands:
Reg1,Reg2,Immed-12
Example:
SW x4,8(x6) # Mem[8+r6] = x4 (word)
Operands:
Reg1,Reg2,Immed-12
Example:
blt x4,x6,loop # if x4<x6, goto offset(pc)
The only difference between S-type and B-type instructions is how the 12-bit immediate value is handled. In an B-type instruction, the immediate value is multiplied by 2 (i.e., shifted left 1 bit) before being used. In the S-type instruction, the value is not shifted. In both cases, sign-extension occurs. In particular, the bits to the left are synthesized by 2illing them in with a copy of the most signi2icant bit actually present.
Actual value used (where s=sign-extension):
ssss ssss ssss ssss ssss VVVV VVVV VVVV
Range of values:
-2,048 .. +2,047
0xFFFFF800 .. 0x000007FF
Actual value used (where s=sign-extension):
ssss ssss ssss ssss sssV VVVV VVVV VVV0
Range of values:
-4,096 .. +4,094 (in multiples of 2)
0xFFFFF000 .. 0x00000FFE
Operands:
RegD,Immed-20
Example:
LUI x4,0x12AB7 # x4 = value<<12
AUIPC x4,0x12AB7 # x4 = (value<<12) + pc
Operands:
RegD,Immed-20
Example:
jal x4,foo # call: pc=offset+pc; x4=ret addr
The only difference between U-type and J-type instructions is how the 20-bit immediate value is handled. In a U-type instruction, the immediate value is shifted left by 12 bits to give a 32 bit value. In other words, the immediate value is placed in the uppermost 20 bits, and the lower 12 bits are zero-2illed.
Actual value used:
VVVV VVVV VVVV VVVV VVVV 0000 0000 0000
The value is always aligned to a multiple of 4,096.
Actual value used (where s=sign-extension):
ssss ssss sssV VVVV VVVV VVVV VVVV VVV0
Range of values:
-1,048,576 .. +1,048,574 (in multiples of 2)
0xFFF00000 .. 0x000FFFFE
Next, we give the encodings for the different types of instructions. In the following, each letter represents a single bit, according to the following legend:
DDDDD = RegD 11111 = Reg1 22222 = Reg2 VVVVV = Immediate value XXXXX = Op-code / function code
Operands:
RegD,Reg1,Reg2
Encoding:
XXXX XXX2 2222 1111 1XXX DDDD DXXX XXXX
Operands:
RegD,Reg1,Immed-12
Encoding:
VVVV VVVV VVVV 1111 1XXX DDDD DXXX XXXX
Operands:
Reg1,Reg2,Immed-12
Encoding:
VVVV VVV2 2222 1111 1XXX VVVV VXXX XXXX
Operands:
RegD,Immed-20
Encoding:
VVVV VVVV VVVV VVVV VVVV DDDD DXXX XXXX
| Instruction | Type | Description | RTL |
|---|---|---|---|
| add rd, rs1, rs2 | R | Add | rd ← rs1 + rs2, pc ← pc+4 |
| or rd, rs1, rs2 | R | Or | rd ← rs1 |
| sll rd, rs1, rs2 | R | Shift Left Logical | rd ← rs1 << (rs2%XLEN), pc ← pc+4 , page52 |
| slt rd, rs1, rs2 | R | Set If Less Than (Signed) | rd ← (rs1 < rs2) ? 1 : 0, pc ← pc+4 , page 61 |
| sltu rd, rs1, rs2 | R | Set If Less Than (Unsigned) | rd ← (rs1 < rs2) ? 1 : 0, pc ← pc+4 |
| sra rd, rs1, rs2 | R | Shift Right Arithmetic | rd ← rs1 >> (rs2%XLEN), pc ← pc+4 , page54 |
| srl rd, rs1, rs2 | R | Shift Right Logical | rd ← rs1 >> (rs2%XLEN), pc ← pc+4 , page53 |
| xor rd, rs1, rs2 | R | Exclusive Or | rd ← rs1 ^ rs2, pc ← pc+4 |
| sub rd, rs1, rs2 | R | Subtract | rd ← rs1 - rs2, pc ← pc+4 |
| and rd, rs1, rs2 | R | And | rd ← rs1 & rs2, pc ← pc+4 |
| addi rd, rs1, imm | I | Add Immediate | rd ← rs1 + imm i, pc ← pc+4 |
| andi rd, rs1, imm | I | And Immediate | rd ← rs1 & imm i, pc ← pc+4 |
| jalr rd, imm(rs1) | I | Jump And Link Register | rd ← pc+4, pc ← (rs1+imm i)&~1 , page73 |
| lb rd, imm(rs1) | I | Load Byte | rd ← sx(m8(rs1+imm i)), pc ← pc+4 , page78 |
| lbu rd, imm(rs1) | I | Load Byte Unsigned | rd ← zx(m8(rs1+imm i)), pc ← pc+4 , page78 |
| lh rd, imm(rs1) | I | Load Halfword | rd ← sx(m16(rs1+imm i)), pc ← pc+4 |
| lw rd, imm(rs1) | I | Load Word | rd ← sx(m32(rs1+imm i)), pc ← pc+4 , page 80 |
| ori rd, rs1, imm | I | Or Immediate | rd ← rs1 |
| slli rd, rs1, shamt | I | Shift Left Logical Immediate | rd ← rs1 << shamt i, pc ← pc+4 |
| lhu rd, imm(rs1) | I | Load Halfword Unsigned | rd ← zx(m16(rs1+imm i)), pc ← pc+4 |
| slti rd, rs1, imm | I | Set Less Than Immediate (Signed) | rd ← (rs1 < imm i) ? 1 : 0, pc ← pc+4 |
| sltiu rd, rs1, imm | I | Set Less Than Immediate Unsigned | rd ← (rs1 < imm i) ? 1 : 0, pc ← pc+4 , Page 62 |
| srai rd, rs1, shamt | I | Shift Right Arithmetic Immediate | rd ← rs1 >> shamt i, pc ← pc+4 , page54 |
| xori rd, rs1, imm | I | Exclusive Or Immediate | rd ← rs1 ^ imm i, pc ← pc+4 |
| srli rd, rs1, shamt | I | Shift Right Logical Immediate | rd ← rs1 >> shamt i, pc ← pc+4 |
| auipc rd, imm | U | Add Upper Immediate to PC | rd ← pc + imm u, pc ← pc+4 , page59 |
| beq rs1, rs2, pcrel 13 | B | Branch Equal | pc ← pc + ((rs1==rs2) ? imm b : 4) , page65 |
| bge rs1, rs2, pcrel 13 | B | Branch Greater or Equal | pc ← pc + ((rs1>=rs2) ? imm b : 4) , page68 |
| bgeu rs1, rs2, pcrel 13 | B | Branch Greater or Equal Unsigned | pc ← pc + ((rs1>=rs2) ? imm b : 4) ,page70 |
| blt rs1, rs2, pcrel 13 | B | Branch Less Than (Signed) | pc ← pc + ((rs1<rs2) ? imm b : 4) |
| bltu rs1, rs2, pcrel 13 | B | Branch Less Than Unsigned | pc ← pc + ((rs1<rs2) ? imm b : 4) , page69 |
| bne rs1, rs2, pcrel 13 | B | Branch Not Equal | pc ← pc + ((rs1!=rs2) ? imm b : 4) , page67 |
| jal rd, pcrel 21 | J | Jump And Link | rd ← pc+4, pc ← pc+imm j , page71 |
| lui rd, imm | U | Load Upper Immediate | rd ← imm u, pc ← pc+4 , page52 |
| sb rs2, imm(rs1) | S | Store Byte | m8(rs1+imm s) ← rs2[7:0], pc ← pc+4 , page 83 |
| sh rs2, imm(rs1) | S | Store Halfword | m16(rs1+imm s) ← rs2[15:0], pc ← pc+4 , page 84 |
| sw rs2, imm(rs1) | S | Store Word | m32(rs1+imm s) ← rs2[31:0], pc ← pc+4 , page 85 |
| Instruction | Operation | Description |
|---|---|---|
| neg rd,rs2 | Negate | NEG x4,x9 , x4 = -x9 This is a special case of a more general instruction. This instruction is assembled identically to: SUB RegD,x0,Reg |
| not rd,rs1 | Not | NOT x4,x9 , x4 = ~x9 , This is a special case of a more general instruction. This instruction is assembled identically to: XORI RegD,Reg1,-1 # Note that -1 = 0xFFFFFFFF |
| nop | Nop | NOP , #Do nothing , This is a special case of a more general instruction. This instruction is assembled identically to: ADDI x0,x0,0 |
| mv rd,rs1 | Move | MV x4,x9 , x4 = x9 , This is a special case of a more general instruction. This instruction is assembled identically to: ADDI x4,x9,0 |
| LI rd,Immed-32 | Load Immediate | LI x4,123 , x4 = 0x0000007B , This is a special case of more general instructions and is assembled differently depending on the actual value present. If the immediate value is in the range of -2,048 .. +2,047, then it can be assembled identically to: ADDI RegD,x0,Immed If the immediate value is not within the range of -2,048 .. +2,047 but is within the range of a 32-bit number (i.e., -2,147,483,648 .. +2,147,483,647) then it can be assembled using this two-instruction sequence: LUI RegD,Upper-20 ADDI RegD,RegD,Lower-12 where “Upper-20” represents the uppermost 20 bits of the value and“Lower-12” represents the least signi2icant 12-bits of the value. Page 59 |
| LA rd,Address | Load Address | LA x4,MyVar # x4 = &MyVar, There is no actual “load address” instruction; instead the assembler substitutes a sequence of two instructions to achieve the same effect. The “address” can refer to any location within the 32-bit memory space. The address is converted to a PC-relative address, with an offset of 32 bits. This offset is then broken into two pieces: a 20-bit piece and a 12-bit piece. The instruction is assembled using these two instructions: AUIPC RegD,Upper-20 ADDI RegD,RegD,Lower-12. Page 60 |
| SGT RegD,Reg1,Reg2 | Set If Greater Than (Signed) | SGT x4,x9,x13 # x4 = (x9>x13) ? 1 : 0 This is a special case of a different instruction. This instruction is assembled identically to: SLT RegD,Reg2,Reg1 # Note: regs are switched. The contents of Reg1 is compared to the contents of Reg2 using signed comparison. If the value in Reg1 is greater than the value in Reg2, the value 1 is stored in RegD. Otherwise, the value 0 is stored in RegD. Page 62 |
| SGTU RegD,Reg1,Reg2 | Set If Greater Than (UnSigned) | SGTU x4,x9,x13 # x4 = (x9>x13) ? 1 : 0 This is a special case of a different instruction. This instruction is assembled identically to: SLTU RegD,Reg2,Reg1 # Note: regs are switched. The contents of Reg1 is compared to the contents of Reg2 using unsigned comparison. If the value in Reg1 is greater than the value in Reg2, the value 1 is stored in RegD. Otherwise, the value 0 is stored in RegD. Page 63 |
| SEQZ RegD,Reg1 | Set If Equal To Zero | SEQZ x4,x9 # x4 = (x9==0) ? 1 : 0. This is a special case of a more general instruction. This instruction is assembled identically to: SLTIU RegD,Reg1,1. This instruction is implemented with an unsigned comparison against 1. Using unsigned numbers, the only value less than 1 is 0. Therefore if the less-than condition holds, the value in Reg1 must be 0. If the value in Reg1 is zero, the value 1 is stored in RegD. Otherwise, the value 0 is stored in RegD. Page 63 |
| SNEZ RegD,Reg2 | Set If Not Equal To Zero | SNEZ x4,x9 # x4 = (x9≠0) ? 1 : 0. This is a special case of a more general instruction. This instruction is assembled identically to: SLTU RegD,x0,Reg2. This instruction is implemented with an unsigned comparison against 0. Using unsigned numbers, the only value not less than 0 is 0. Therefore if the less�than condition holds, the value in Reg2 must be not be 0. If the value in Reg2 is not zero, the value 1 is stored in RegD. Otherwise, the value 0 is stored in RegD. Page 63 |
| SLTZ RegD,Reg1 | Set If Less Than Zero (signed) | SLTZ x4,x9 # x4 = (x9<0) ? 1 : 0. This is a special case of a more general instruction. This instruction is assembled identically to: SLT RegD,Reg1,x0. If the value in Reg1 is less than zero (using signed arithmetic), the value 1 is stored in RegD. Otherwise, the value 0 is stored in RegD. Page 64 |
| SGTZ RegD,Reg1 | Set If Greater Than Zero (signed) | SGTZ x4,x9 # x4 = (x9>0) ? 1 : 0. This is a special case of a more general instruction. This instruction is assembled identically to: SLT RegD,x0,Reg2. This is a special case of a more general instruction. This instruction is assembled identically to: SLT RegD,x0,Reg2. Page 64 |
| BLE Reg1,Reg2,Immed-12 | Branch if Less Than Or Equal (Signed) | BLE x4,x9,MyLabel # If x4<=x9 goto MyLabel. This is a special case of another instruction. This instruction is assembled identically to: BGE Reg2,Reg1,Immed-12 # Note: regs are swapped. The contents of Reg1 is compared to the contents of Reg2. If Reg1 is less than or equal to Reg2 (using signed comparison), control jumps to a PC-relative target address. |
| BGT Reg1,Reg2,Immed-12 | Branch if Greater Than (Signed) | BGT x4,x9,MyLabel # If x4>x9 goto MyLabel. This is a special case of another instruction. This instruction is assembled identically to: BLT Reg2,Reg1,Immed-12 # Note: regs are swapped. The contents of Reg1 is compared to the contents of Reg2. If Reg1 is greater than Reg2 (using signed comparison), control jumps to a PC-relative target address. |
| BLEU Reg1,Reg2,Immed-12 | Branch if Less Than Or Equal (Unsigned) | BLEU x4,x9,MyLabel # If x4<=x9 goto MyLabel. This is a special case of another instruction. This instruction is assembled identically to: BGEU Reg2,Reg1,Immed-12 # Note: regs are swapped. The contents of Reg1 is compared to the contents of Reg2. If Reg1 is less than or equal to Reg2 (using unsigned comparison), control jumps to a PC-relative target address. |
| BGTU Reg1,Reg2,Immed-12 | Branch if Greater Than (Unsigned) | BGTU x4,x9,MyLabel # If x4>x9 goto MyLabel. This is a special case of another instruction. This instruction is assembled identically to: BLTU Reg2,Reg1,Immed-12 # Note: regs are swapped. The contents of Reg1 is compared to the contents of Reg2. If Reg1 is greater than Reg2 (using unsigned comparison), control jumps to a PC-relative target address. |
| J Immed-20 | Jump (Short-Distance) | J MyLabel # Goto MyLabel. This is a special case of another instruction. This instruction is assembled identically to: JAL x0,Immed-20 # Discard return address. The target address is given as a PC-relative offset. The effective range is ±1 MiByte, i.e., -1,048,576 .. 1,048,574 (in multiples of 2), relative to the PC. Page 73 |
| JR Reg1 | Jump Register | JR Reg1 # Goto *Reg1, i.e., PC = Reg1. This is a special case of another instruction. This instruction is assembled identically to: JALR x0,Reg1,0 # Discard ret addr; offset=0. Jump to the address in Reg1. Page 73 |
| RET | Return | RET # Goto *x1, i.e., PC = x1. This is a special case of another instruction. This instruction is assembled identically to: JALR x0,x1,0 # PC=x1+0; don’t save prev PC. By convention, x1 is used as the “link register” and will hold a return address. This instruction returns from a subroutine/function. Page 74 |
| CALL Immed-32 | Call Faraway Subroutine | CALL MyFunct # PC = new addr; x1 = ret addr. In order to deal with the larger distance to the subroutine, this “synthetic” instruction will be assembled using the following two-instruction sequence. The target address can be expressed as a 32-bit offset from the Program Counter. This offset is broken into two pieces, which are added to the PC in two steps. AUIPC x6,Immed-20 JALR x1,x6,Immed-12 The AUIPC instruction adds the PC to the upper 20-bit portion of the 32-bit offset and places the result in a temporary register. The JALR instruction adds in the lower 12 bits of the 32-bit offset and transfers control by loading the sum into the PC. It also saves the return address in x1. The CALL instruction makes use of the convention that x1 is the link register. It also uses x6, which is a “caller-saved temporary register” by convention. By convention, x1 is used as the “link register” and will hold a return address. This instruction calls a subroutine/function using a PC-relative scheme, where the subroutine offset from the CALL instruction exceeds the 20-bit limit (i.e., ±1 MiByte) of the JAL instruction. This instruction modi2ies register x6. Page 75 |
| TAIL Immed-32 | Tail Call (Faraway Subroutine) / Long-Distance Jump | TAIL MyFunct # PC = new addr; Discard ret addr. This instruction is used to jump to a distant location using a PC-relative scheme, where the displacement from the TAIL instruction to the target instruction exceeds the 20-bit limit (i.e., ±1 MiByte) of the J instruction (jump short distance). This instruction modi2ies register x6. This “synthetic” instruction will be assembled using the following two�instruction sequence. AUIPC x6,Immed-20 JALR x0,x6,Immed-12 See the comments for the CALL instruction. The only difference is that here the return address is discarded (x0), instead of being saved in x1. Page 77 |
| Pseudoinstruction | Base Instruction(s) | Meaning | Comment |
|---|---|---|---|
| la rd, symbol | auipc rd, symbol[31:12]; addi rd, rd, symbol[11:0] | Load address | |
| l{b|h|w} rd, symbol | auipc rd, symbol[31:12]; l{b|h|w|d} rd, symbol[11:0](rd) | Load global | |
| s{b|h|w} rd, symbol, rt | auipc rt, symbol[31:12]; s{b|h|w|d} rd, symbol[11:0](rt) | Store global | |
| nop | addi x0, x0, 0 | No operation | |
| li rd, immediate | Myriad sequences | Load immediate | |
| mv rd, rs | addi rd, rs, 0 | Copy register | |
| not rd, rs | xori rd, rs, -1 | Ones’ complement | |
| neg rd, rs | sub rd, x0, rs | Two’s complement | |
| negw rd, rs | subw rd, x0, rs | Two’s complement word | |
| sext.b rd, rs | slli rd, rs, XLEN - 8; srai rd, rd, XLEN - 8 | Sign extend byte | It will expand to another instruction sequence when B extension is available*[1] |
| sext.h rd, rs | slli rd, rs, XLEN - 16; srai rd, rd, XLEN - 16 | Sign extend half word | It will expand to another instruction sequence when B extension is available*[1] |
| sext.w rd, rs | addiw rd, rs, 0 | Sign extend word | |
| zext.b rd, rs | andi rd, rs, 255 | Zero extend byte | |
| zext.h rd, rs | slli rd, rs, XLEN - 16; srli rd, rd, XLEN - 16 | Zero extend half word | It will expand to another instruction sequence when B extension is available*[1] |
| zext.w rd, rs | slli rd, rs, XLEN - 32; srli rd, rd, XLEN - 32 | Zero extend word | It will expand to another instruction sequence when B extension is available*[1] |
| seqz rd, rs | sltiu rd, rs, 1 | Set if = zero | |
| snez rd, rs | sltu rd, x0, rs | Set if != zero | |
| sltz rd, rs | slt rd, rs, x0 | Set if < zero | |
| sgtz rd, rs | slt rd, x0, rs | Set if > zero | |
| beqz rs, offset | beq rs, x0, offset | Branch if = zero | |
| bnez rs, offset | bne rs, x0, offset | Branch if != zero | |
| blez rs, offset | bge x0, rs, offset | Branch if ≤ zero | |
| bgez rs, offset | bge rs, x0, offset | Branch if ≥ zero | |
| bltz rs, offset | blt rs, x0, offset | Branch if < zero | |
| bgtz rs, offset | blt x0, rs, offset | Branch if > zero | |
| bgt rs, rt, offset | blt rt, rs, offset | Branch if > | |
| ble rs, rt, offset | bge rt, rs, offset | Branch if ≤ | |
| bgtu rs, rt, offset | bltu rt, rs, offset | Branch if >, unsigned | |
| bleu rs, rt, offset | bgeu rt, rs, offset | Branch if ≤, unsigned | |
| j offset | jal x0, offset | Jump | |
| jal offset | jal x1, offset | Jump and link | |
| jr rs | jalr x0, rs, 0 | Jump register | |
| jalr rs | jalr x1, rs, 0 | Jump and link register | |
| ret | jalr x0, x1, 0 | Return from subroutine | |
| call offset | auipc x6, offset[31:12]; jalr x1, x6, offset[11:0] | Call far-away subroutine | |
| tail offset | auipc x6, offset[31:12]; jalr x0, x6, offset[11:0] | Tail call far-away subroutine | |
| fence | fence iorw, iorw | Fence on all memory and I/O |
