RISC-V ISA brief(ASM brief)
本帖最后由 皋陶 于 2020-8-26 16:02 编辑从别处找来的RISCV 指令简介文档，包含了一些常用指令，不全
Table of Contents
1. Architectural State
2. Tiny RISC-V Instruction Overview
3. Tiny RISC-V Instruction Encoding
3.1. Instruction Formats
3.2. Immediate Formats
4. Tiny RISC-V Instruction Details
4.1. Control/Status Register Instructions
4.2. Register-Register Arithmetic Instructions
4.3. Register-Immediate Arithmetic Instructions
4.4. Memory Instructions
4.5. Unconditional Jump Instructions
4.6. Conditional Branch Instructions
5. Tiny RISC-V Privileged ISA
6. Tiny RISC-V Pseudo-Instructions
--------------------------------------------------------------------------
1. Architectural State
--------------------------------------------------------------------------
* Data Formats
- Tiny RISC-V only supports 4B signed and unsigned integer values. There
are no byte nor half-word values and no floating-point.
* General Purpose Registers
- There are 31 general-purpose registers x1-x31 (called x registers),
which hold integer values. Register x0 is hardwired to the constant
zero. Each register is 32 bits wide. Tiny RISC-V uses the same calling
convention and symbolic register names as RISC-V:
+ x0: zero the constant value 0
+ x1: ra return address (caller saved)
+ x2: sp stack pointer (called saved)
+ x3: gp global pointer
+ x4: tp thread pointer
+ x5: t0 temporary registers (caller saved)
+ x6: t1 "
+ x7: t2 "
+ x8: s0/fpsaved register or frame pointer (callee saved)
+ x9: s1 saved register (callee saved)
+ x10 : a0 function arguments and/or return values (caller saved)
+ x11 : a1 "
+ x12 : a2 function arguments (caller saved)
+ x13 : a3 "
+ x14 : a4 "
+ x15 : a5 "
+ x16 : a6 "
+ x17 : a7 "
+ x18 : s2 saved registers (callee saved)
+ x19 : s3 "
+ x20 : s4 "
+ x21 : s5 "
+ x22 : s6 "
+ x23 : s7 "
+ x24 : s8 "
+ x25 : s9 "
+ x26 : s10 "
+ x27 : s11 "
+ x28 : t3 temporary registers (caller saved)
+ x29 : t4 "
+ x30 : t5 "
+ x31 : t6 "
* Memory
- Tiny RISC-V only supports a 1MB virtual memory address space from
0x00000000 to 0x000fffff. The result of memory accesses to addresses
larger than 0x000fffff are undefined.
- A key feature of any ISA is identifying the endianness of the memory
system. Endianness specifies if we load a word in memory, what order
should those bytes appear in the destination register. Assume the
letter A ia at byte address 0x0, the letter B is at byte address 0x1,
the letter C is at byte address 0x2, and the letter D is at byte
address 0x3. If we laod a four-byte word from address 0x0, there are
two options: the destination register can either hold 0xABCD (big
endian) or 0xDCBA (little endian). There is no significant benefit of
one system over the other. Tiny RISC-V uses a little endian memory
system.
--------------------------------------------------------------------------
2. Tiny RISC-V ISA Overview
--------------------------------------------------------------------------
Here is a brief list of the instructions which make up both versions of
the Tiny RISC-V ISA, and then some discussion about the differences
between the two versions.
* TinyRV1
TinyRV1 contains a very small subset of the TinyRV2 ISA suitable for
illustrating how small assembly sequences execute on various
microarchitectures in lecture, problem sets, and exams.
- ADD, ADDI, MUL
- LW, SW
- JAL, JR
- BNE
* TinyRV2
TinyRV2 is suitable for executing simple C programs that do not use
system calls.
- ADD, ADDI, SUB, MUL
- AND, ANDI, OR, ORI, XOR, XORI
- SLT, SLTI, SLTU, SLTIU
- SRA, SRAI, SRL, SRLI, SLL, SLLI
- LUI, AUIPC
- LW, SW
- JAL, JALR
- BEQ, BNE, BLT, BGE, BLTU, BGEU
- CSRR, CSRW (proc2mngr, mngr2proc, stats_en, core_id, num_cores)
* Discussion
TinyRV1 includes the JR instruction, but technically this is not a real
instruction but is instead a pseudo-instruction for the following usage
of JALR:
jalr x0, rs1, 0
The JALR instruction is a bit complicated, and we really only need the JR
functionality to explain function calls in TinyRV1. So TinyRV1 only
includes the JR pseudo-instruction, while TinyRV2 includes the full JALR
instruction.
CSSR and CSRW are also pseudo-instructions in the full RV32IM ISA for
specific usage of the CSRRW and CSRRS instructions. The full CSRRW and
CSRRS instructions are rather complicated and we don't actually need any
functionality beyond what CSSR and CSRW provide. So TinyRV2 only includes
the CSSR and CSRW pseudo-instruction.
--------------------------------------------------------------------------
3. Tiny RISC-V Instruction and Immediate Encoding
--------------------------------------------------------------------------
The Tiny RISC-V ISA uses the same instruction encoding as RISC-V. There
are four instruction types and five immediate encodings. Each instruction
has a specific instruction type, and if that instruction includes an
immediate, then it will also have an immediate type.
--------------------------------------------------------------------------
3.1. Tiny RISC-V Instruction Formats
--------------------------------------------------------------------------
* R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| funct7 | rs2 | rs1 |funct3| rd | opcode |
+------------+---------+---------+------+---------+-------------+
* I-type
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 |funct3| rd | opcode |
+----------------------+---------+------+---------+-------------+
* S-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 |funct3| imm | opcode |
+------------+---------+---------+------+---------+-------------+
* U-type
31 11 7 6 0
+---------------------------------------+---------+-------------+
| imm | rd | opcode |
+---------------------------------------+---------+-------------+
--------------------------------------------------------------------------
3.2. Tiny RISC-V Immediate Formats
--------------------------------------------------------------------------
RISC-V has an asymmetric immediate encoding which means that the
immediates are formed by concatenating different bits in an asymmetric
order based on the specific immediate formats. Note that in RISC-V all
immediates are always sign extended, and the sign-bit for the immediate
is always in bit 31 of the instruction.
The following diagrams illustrate how to create a 32-bit immediate from
each of the five immediate formats. The fields are labeled with the
instruction bits used to construct their value. <-- n is used to indicate
repeating bit n of the instruction to fill that field and z is used to
indicate a bit which is always set to zero.
* I-immediate
31 10 5 4 10
+-----------------------------------------+-----------+-------+--+
| <-- 31 | 30:25 | 24:21 |20|
+-----------------------------------------+-----------+-------+--+
* S-immediate
31 10 5 4 10
+-----------------------------------------+-----------+-------+--+
| <-- 31 | 30:25 | 11:8|7 |
+-----------------------------------------+-----------+-------+--+
* B-immediate
31 12 11 10 5 4 10
+--------------------------------------+--+-----------+-------+--+
| <-- 31 |7 | 30:25 | 11:8|z |
+--------------------------------------+--+-----------+-------+--+
* U-immediate
31 30 20 19 12 11 0
+--+-------------------+---------------+-------------------------+
|31| 30:20 | 19:12 | <-- z |
+--+-------------------+---------------+-------------------------+
* J-immediate
31 20 19 12 11 10 5 4 10
+----------------------+---------------+--+-----------+-------+--+
| <-- 31 | 19:12 |20| 30:25 | 24:21 |z |
+----------------------+---------------+--+-----------+-------+--+
--------------------------------------------------------------------------
4. PARC Instruction Details
--------------------------------------------------------------------------
For each instruction we include a brief summary, assembly syntax,
instruction semantics, instruction and immediate encoding format, and the
actual encoding for the instruction. We use the following conventions
when specifying the instruction semantics:
- R : general-purpose register value for register specifier rx
- CSR : control/status register value for register specifier csr
- sext : sign extend to 32 bits
- M_4B : 4-byte memory value at address addr
- PC : current program counter
- <s : signed less-than comparison
- >=s : signed greater than or equal to comparison
- <u : unsigned less-than comparison
- >=u : unsigned greater than or equal to comparison
- imm : immediate according to the immediate type
Unless otherwise specified assume instruction updates PC with PC+4.
--------------------------------------------------------------------------
4.1. Control/Status Register Instructions
--------------------------------------------------------------------------
* CSRR
- Summary : Move value in control/status register to GPR
- Assembly: csrr rd, csr
- Semantics : R = CSR
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| csr | rs1 | 010| rd | 1110011 |
+----------------------+---------+------+---------+-------------+
The control/status register read instruction is used to read a CSR and
write the result to a GPR. The CSRs supported in TinyRV2 are listed in
Section 5. Note that in RISC-V CSRR is really a pseudo-instruction for a
specific usage of CSRRS, but in TinyRV2 we only support the subset of
CSRRS captured by CSRR. See Section 6 for more details about
pseudo-instructions.
* CSRW
- Summary : Move value in GPR to control/status register
- Assembly: csrw csr, rs1
- Semantics : CSR = R
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| csr | rs1 | 001| rd | 1110011 |
+----------------------+---------+------+---------+-------------+
The control/status register write instruction is used to read a GPR and
write the result to a CSR. The CSRs supported in TinyRV2 are listed in
Section 5. Note that in RISC-V CSRW is really a pseudo-instruction for a
specific usage of CSRRW, but in TinyRV2 we only support the subset of
CSRRW captured by CSRW. See Section 6 for more details about
pseudo-instructions.
--------------------------------------------------------------------------
4.2. Register-Register Arithmetic Instructions
--------------------------------------------------------------------------
* ADD
- Summary : Addition with 3 GPRs, no overflow exception
- Assembly: add rd, rs1, rs2
- Semantics : R = R + R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 000| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
* SUB
- Summary : Subtraction with 3 GPRs, no overflow exception
- Assembly: sub rd, rs1, rs2
- Semantics : R = R - R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0100000 | rs2 | rs1 | 000| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
* AND
- Summary : Bitwise logical AND with 3 GPRs
- Assembly: and rd, rs1, rs2
- Semantics : R = R & R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 111| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
* OR
- Summary : Bitwise logical OR with 3 GPRs
- Assembly: or rd, rs1, rs2
- Semantics : R = R | R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 110| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
* XOR
- Summary : Bitwise logical XOR with 3 GPRs
- Assembly: xor rd, rs1, rs2
- Semantics : R = R ^ R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 100| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
* SLT
- Summary : Record result of signed less-than comparison with 2 GPRs
- Assembly: slt rd, rs1, rs2
- Semantics : R = ( R <s R )
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 010| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
This instruction uses a _signed_ comparison.
* SLTU
- Summary : Record result of unsigned less-than comparison with 2 GPRs
- Assembly: sltu rd, rs1, rs2
- Semantics : R = ( R <u R )
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 011| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
This instruction uses an _unsigned_ comparison.
* SRA
- Summary : Shift right arithmetic by register value (sign-extend)
- Assembly: sra rd, rs1, rs2
- Semantics : R = R >>> R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0100000 | rs2 | rs1 | 101| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
Note that the hardware should ensure that the sign-bit of R is
extended to the right as it does the right shift. The hardware _must_
only use the bottom five bits of R when performing the shift.
* SRL
- Summary : Shift right logical by register value (append zeroes)
- Assembly: srl rd, rs1, rs2
- Semantics : R = R >> R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 101| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
Note that the hardware should append zeros to the left as it does the
right shift. The hardware _must_ only use the bottom five bits of R
when performing the shift.
* SLL
- Summary : Shift left logical by register value (append zeroes)
- Assembly: sll rd, rs1, rs2
- Semantics : R = R << R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | rs2 | rs1 | 001| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
Note that the hardware should append zeros to the right as it does the
left shift. The hardware _must_ only use the bottom five bits of R
when performing the shift.
* MUL
- Summary : Signed multiplication with 3 GPRs, no overflow exception
- Assembly: mul rd, rs1, rs2
- Semantics : R = R * R
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000001 | rs2 | rs1 | 000| rd | 0110011 |
+------------+---------+---------+------+---------+-------------+
--------------------------------------------------------------------------
4.3. Register-Immediate Arithmetic Instructions
--------------------------------------------------------------------------
* ADDI
- Summary : Add constant, no overflow exception
- Assembly: addi rd, rs1, imm
- Semantics : R = R + sext(imm)
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 000| rd | 0010011 |
+----------------------+---------+------+---------+-------------+
* ANDI
- Summary : Bitwise logical AND with constant
- Assembly: andi rd, rs1, imm
- Semantics : R = R & sext(imm)
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 111| rd | 0010011 |
+----------------------+---------+------+---------+-------------+
* ORI
- Summary : Bitwise logical OR with constant
- Assembly: ori rd, rs1, imm
- Semantics : R = R | sext(imm)
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 110| rd | 0010011 |
+----------------------+---------+------+---------+-------------+
* XORI
- Summary : Bitwise logical XOR with constant
- Assembly: xori rd, rs1, imm
- Semantics : R = R ^ sext(imm)
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 100| rd | 0010011 |
+----------------------+---------+------+---------+-------------+
* SLTI
- Summary : Set GPR if source GPR < constant, signed comparison
- Assembly: slti rd, rs1, imm
- Semantics : R = ( R <s sext(imm) )
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 010| rd | 0010011 |
+----------------------+---------+------+---------+-------------+
* SLTIU
- Summary : Set GPR if source GPR is < constant, unsigned comparison
- Assembly: sltiu rd, rs1, imm
- Semantics : R = ( R <u sext(imm) )
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 011| rd | 0010011 |
+----------------------+---------+------+---------+-------------+
* SRAI
- Summary : Shift right arithmetic by constant (sign-extend)
- Assembly: srai rd, rs1, imm
- Semantics : R = R >>> imm
- Format : I-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0100000 | imm | rs1 | 101| rd | 0010011 |
+------------+---------+---------+------+---------+-------------+
Note that the hardware should ensure that the sign-bit of R is
extended to the right as it does the right shift.
* SRLI
- Summary : Shift right logical by constant (append zeroes)
- Assembly: srli rd, rs1, imm
- Semantics : R = R >> imm
- Format : I-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | imm | rs1 | 101| rd | 0010011 |
+------------+---------+---------+------+---------+-------------+
Note that the hardware should append zeros to the left as it does the
right shift.
* SLLI
- Summary : Shift left logical constant (append zeroes)
- Assembly: slli rd, rs1, imm
- Semantics : R = R << imm
- Format : R-type
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| 0000000 | imm | rs1 | 001| rd | 0010011 |
+------------+---------+---------+------+---------+-------------+
Note that the hardware should append zeros to the right as it does the
left shift.
* LUI
- Summary : Load constant into upper bits of word
- Assembly: lui rd, imm
- Semantics : R = imm << 12
- Format : I-type, U-immediate
31 11 7 6 0
+---------------------------------------+---------+-------------+
| imm | rd | 0110111 |
+---------------------------------------+---------+-------------+
* AUIPC
- Summary : Load PC + constant into upper bits of word
- Assembly: auipc rd, imm
- Semantics : R = PC + ( imm << 12 )
- Format : I-type, U-immediate
31 11 7 6 0
+---------------------------------------+---------+-------------+
| imm | rd | 0010111 |
+---------------------------------------+---------+-------------+
--------------------------------------------------------------------------
4.4. Memory Instructions
--------------------------------------------------------------------------
* LW
- Summary : Load word from memory
- Assembly: lw rd, imm(rs1)
- Semantics : R = M_4B[ R + sext(imm) ]
- Format : I-type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 010| rd | 0000011 |
+----------------------+---------+------+---------+-------------+
All addresses used with LW instructions must be four-byte aligned. This
means the bottom two bits of every effective address (i.e., after the
base address is added to the offset) will always be zero.
* SW
- Summary : Store word into memory
- Assembly: sw rs2, imm(rs1)
- Semantics : M_4B[ R + sext(imm) ] = R
- Format : S-type, S-immediate
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 | 010| imm | 0100011 |
+------------+---------+---------+------+---------+-------------+
All addresses used with SW instructions must be four-byte aligned. This
means the bottom two bits of every effective address (i.e., after the
base address is added to the offset) will always be zero.
--------------------------------------------------------------------------
4.5. Unconditional Jump Instructions
--------------------------------------------------------------------------
* JAL
- Summary : Jump to address and place return address in GPR
- Assembly: jal rd, imm
- Semantics : R = PC + 4; PC = PC + sext(imm)
- Format : U-type, J-immediate
31 11 7 6 0
+---------------------------------------+---------+-------------+
| imm | rd | 1101111 |
+---------------------------------------+---------+-------------+
* JR
- Summary : Jump to address
- Assembly: jr rs1
- Semantics : PC = R
- Format : I-Type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| 000000000000 | rs1 | 000| 00000 | 1100111 |
+----------------------+---------+------+---------+-------------+
Note that JR is a "real" instruction in TinyRV1, but it is a
pseudo-instruction for a specific usage of JALR. We don't really worry
about zero-ing out the the least-significant bit to zero in TinyRV1, but
this must be done for TinyRV2.
* JALR
- Summary : Jump to address and place return address in GPR
- Assembly: jalr rd, rs1, imm
- Semantics : R = PC + 4; PC = ( R + sext(imm) ) & 0xfffffffe
- Format : I-Type, I-immediate
31 20 19 15 1412 11 7 6 0
+----------------------+---------+------+---------+-------------+
| imm | rs1 | 000| rd | 1100111 |
+----------------------+---------+------+---------+-------------+
Note that the target address is obtained by adding the 12-bit signed
I-immediate to the value in register rs1, then setting the
least-significant bit of the result to zero. In other words, the JALR
instruction ignores the lowest bit of the calculated target address.
--------------------------------------------------------------------------
4.6. Conditional Branch Instructions
--------------------------------------------------------------------------
* BEQ
- Summary : Branch if 2 GPRs are equal
- Assembly: beq rs1, rs2, imm
- Semantics : PC = ( R == R ) ? PC + sext(imm) : PC + 4
- Format : S-type, B-immediate
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 | 000| imm | 1100011 |
+------------+---------+---------+------+---------+-------------+
* BNE
- Summary : Branch if 2 GPRs are not equal
- Assembly: bne rs1, rs2, imm
- Semantics : PC = ( R != R ) ? PC + sext(imm) : PC + 4
- Format : S-type, B-immediate
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 | 001| imm | 1100011 |
+------------+---------+---------+------+---------+-------------+
* BLT
- Summary : Branch based on signed comparison of two GPRs
- Assembly: blt rs1, rs2, imm
- Semantics : PC = ( R <s R ) ? PC + sext(imm) : PC + 4
- Format : S-type, B-immediate
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 | 100| imm | 1100011 |
+------------+---------+---------+------+---------+-------------+
This instruction uses a _signed_ comparison.
* BGE
- Summary : Branch based on signed comparison of two GPRs
- Assembly: bge rs1, rs2, imm
- Semantics : PC = ( R >=s R ) ? PC + sext(imm) : PC + 4
- Format : S-type, B-immediate
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 | 101| imm | 1100011 |
+------------+---------+---------+------+---------+-------------+
This instruction uses a _signed_ comparison.
* BLTU
- Summary : Branch based on unsigned comparison of two GPRs
- Assembly: bltu rs1, rs2, imm
- Semantics : PC = ( R <u R ) ? PC + sext(imm) : PC + 4
- Format : S-type, B-immediate
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 | 110| imm | 1100011 |
+------------+---------+---------+------+---------+-------------+
This instruction uses an _unsigned_ comparison.
* BGEU
- Summary : Branch based on unsigned comparison of two GPRs
- Assembly: bgeu rs1, rs2, imm
- Semantics : PC = ( R >=u R ) ? PC + sext(imm) : PC + 4
- Format : S-type, B-immediate
31 25 24 20 19 15 1412 11 7 6 0
+------------+---------+---------+------+---------+-------------+
| imm | rs2 | rs1 | 111| imm | 1100011 |
+------------+---------+---------+------+---------+-------------+
This instruction uses an _unsigned_ comparison.本篇完，感谢关注：RISC-V单片机中文网
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