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9.38.3 Instruction Formats

The RISC-V Instruction Set Manual Volume I: User-Level ISA lists 12 instruction formats where some of the formats have multiple variants. For the ‘.insn’ pseudo directive the assembler recognizes some of the formats. Typically, the most general variant of the instruction format is used by the ‘.insn’ directive.

The following table lists the abbreviations used in the table of instruction formats: 

     
opcode Unsigned immediate or opcode name for 7-bits opcode.
     
opcode2 Unsigned immediate or opcode name for 2-bits opcode.
     
func7 Unsigned immediate for 7-bits function code.
     
func6 Unsigned immediate for 6-bits function code.
     
func4 Unsigned immediate for 4-bits function code.
     
func3 Unsigned immediate for 3-bits function code.
     
func2 Unsigned immediate for 2-bits function code.
     
rd Destination register number for operand x, can be GPR or FPR.
     
rd' Destination register number for operand x,
     only accept s0-s1, a0-a5, fs0-fs1 and fa0-fa5.
     
rs1 First source register number for operand x, can be GPR or FPR.
     
rs1' First source register number for operand x,
     only accept s0-s1, a0-a5, fs0-fs1 and fa0-fa5.
     
rs2 Second source register number for operand x, can be GPR or FPR.
     
rs2' Second source register number for operand x,
     only accept s0-s1, a0-a5, fs0-fs1 and fa0-fa5.
     
simm12 Sign-extended 12-bit immediate for operand x.
     
simm20 Sign-extended 20-bit immediate for operand x.
     
simm6 Sign-extended 6-bit immediate for operand x.
     
uimm8 Unsigned 8-bit immediate for operand x.
     
symbol Symbol or lable reference for operand x.

The following table lists all available opcode name:

C0
C1
C2
Opcode space for compressed instructions.
LOAD
Opcode space for load instructions.
LOAD_FP
Opcode space for floating-point load instructions.
STORE
Opcode space for store instructions.
STORE_FP
Opcode space for floating-point store instructions.
AUIPC
Opcode space for auipc instruction.
LUI
Opcode space for lui instruction.
BRANCH
Opcode space for branch instructions.
JAL
Opcode space for jal instruction.
JALR
Opcode space for jalr instruction.
OP
Opcode space for ALU instructions.
OP_32
Opcode space for 32-bits ALU instructions.
OP_IMM
Opcode space for ALU with immediate instructions.
OP_IMM_32
Opcode space for 32-bits ALU with immediate instructions.
OP_FP
Opcode space for floating-point operation instructions.
MADD
Opcode space for madd instruction.
MSUB
Opcode space for msub instruction.
NMADD
Opcode space for nmadd instruction.
NMSUB
Opcode space for msub instruction.
AMO
Opcode space for atomic memory operation instructions.
MISC_MEM
Opcode space for misc instructions.
SYSTEM
Opcode space for system instructions.
CUSTOM_0
CUSTOM_1
CUSTOM_2
CUSTOM_3
Opcode space for customize instructions.

An instruction is two or four bytes in length and must be aligned on a 2 byte boundary. The first two bits of the instruction specify the length of the instruction, 00, 01 and 10 indicates a two byte instruction, 11 indicates a four byte instruction.

The following table lists the RISC-V instruction formats that are available with the ‘.insn’ pseudo directive:

R type: .insn r opcode, func3, func7, rd, rs1, rs2
     +-------+-----+-----+-------+----+-------------+
     | func7 | rs2 | rs1 | func3 | rd |      opcode |
     +-------+-----+-----+-------+----+-------------+
     31      25    20    15      12   7             0

R type with 4 register operands: .insn r opcode, func3, func2, rd, rs1, rs2, rs3
R4 type: .insn r4 opcode, func3, func2, rd, rs1, rs2, rs3
     +-----+-------+-----+-----+-------+----+-------------+
     | rs3 | func2 | rs2 | rs1 | func3 | rd |      opcode |
     +-----+-------+-----+-----+-------+----+-------------+
     31    27      25    20    15      12   7             0

I type: .insn i opcode, func3, rd, rs1, simm12
     +-------------+-----+-------+----+-------------+
     |      simm12 | rs1 | func3 | rd |      opcode |
     +-------------+-----+-------+----+-------------+
     31            20    15      12   7             0

S type: .insn s opcode, func3, rd, rs1, simm12
     +--------------+-----+-----+-------+-------------+-------------+
     | simm12[11:5] | rs2 | rs1 | func3 | simm12[4:0] |      opcode |
     +--------------+-----+-----+-------+-------------+-------------+
     31             25    20    15      12            7             0

SB type: .insn sb opcode, func3, rd, rs1, symbol
SB type: .insn sb opcode, func3, rd, simm12(rs1)
B type: .insn s opcode, func3, rd, rs1, symbol
B type: .insn s opcode, func3, rd, simm12(rs1)
     +------------+--------------+-----+-----+-------+-------------+-------------+--------+
     | simm12[12] | simm12[10:5] | rs2 | rs1 | func3 | simm12[4:1] | simm12[11]] | opcode |
     +------------+--------------+-----+-----+-------+-------------+-------------+--------+
     31          30            25    20    15      12           7            0

U type: .insn u opcode, rd, simm20
     +---------------------------+----+-------------+
     |                    simm20 | rd |      opcode |
     +---------------------------+----+-------------+
     31                          12   7             0

UJ type: .insn uj opcode, rd, symbol
J type: .insn j opcode, rd, symbol
     +------------+--------------+------------+---------------+----+-------------+
     | simm20[20] | simm20[10:1] | simm20[11] | simm20[19:12] | rd |      opcode |
     +------------+--------------+------------+---------------+----+-------------+
     31           30             21           20              12   7             0

CR type: .insn cr opcode2, func4, rd, rs2
     +---------+--------+-----+---------+
     |   func4 | rd/rs1 | rs2 | opcode2 |
     +---------+--------+-----+---------+
     15        12       7     2        0

CI type: .insn ci opcode2, func3, rd, simm6
     +---------+-----+--------+-----+---------+
     |   func3 | imm | rd/rs1 | imm | opcode2 |
     +---------+-----+--------+-----+---------+
     15        13    12       7     2         0

CIW type: .insn ciw opcode2, func3, rd, uimm8
     +---------+--------------+-----+---------+
     |   func3 |          imm | rd' | opcode2 |
     +---------+--------------+-----+---------+
     15        13             7     2         0

CA type: .insn ca opcode2, func6, func2, rd, rs2
     +---------+----------+-------+------+--------+
     |   func6 | rd'/rs1' | func2 | rs2' | opcode |
     +---------+----------+-------+------+--------+
     15        10         7       5      2        0

CB type: .insn cb opcode2, func3, rs1, symbol
     +---------+--------+------+--------+---------+
     |   func3 | offset | rs1' | offset | opcode2 |
     +---------+--------+------+--------+---------+
     15        13       10     7        2         0

CJ type: .insn cj opcode2, symbol
     +---------+--------------------+---------+
     |   func3 |        jump target | opcode2 |
     +---------+--------------------+---------+
     15        13             7     2         0

For the complete list of all instruction format variants see The RISC-V Instruction Set Manual Volume I: User-Level ISA.