Does BREV8 access memory?
No. It only rearranges bit fields within each byte of an integer register.
BREV8
RISC-V BREV8 Instruction Details
Instruction ManualI-typebit reverse in bytes (Zbkb).
Instruction Syntax
brev8 rd, rs1
Operand Breakdown
rd: receives the XLEN-bit result with each byte's bit order reversed.
rs1: supplies the XLEN-bit source grouped into 8-bit bytes.
Bits 31..20 are fixed to 011010000111; this is the BREV8 encoding selector, not an assembly immediate.
ZbkbBit Manipulation
Instruction Behavior
BREV8 reverses bits within each byte of rs1 (bit 0↔7, 1↔6, ... per byte), writing the result to rd. Part of the Zbkb scalar bit-manipulation extension.
BREV8 Decode & Execute Animation
Shows only official encodings, conditions, and architectural results; it does not imply a microarchitecture.
brev8 a0, a1
Execution context
x[a0] <- 0x80C4A2E691D5B3F7
Encoding fields
31..20
imm[11:0]
011010000111
19..15
rs1
01011
14..12
funct3
101
11..7
rd
01010
6..0
opcode
0010011
Encoding fields identify the instruction.
0x0123456789ABCDEF -> 0x80C4A2E691D5B3F7 (each byte reverses independently)
x[a0] <- 0x80C4A2E691D5B3F7
Architectural state has not been committed.
Quick Understanding & Search Notes
BREV8 belongs to Zbkb and operates on a 32/64-bit integer source on RV32/RV64. It reverses bits independently in each byte while keeping byte positions unchanged.
Within every byte, bits 0/7, 1/6, 2/5 and 3/4 exchange positions.
The encoding uses OP-IMM, imm[11:0]=011010000111 and funct3=101; this is not a variable shift.
Common Usage Scenarios
byte-local bit permutation
Understand this scenario with real code like «brev8 a0, a1 # Reverse bits in each byte of a1».
per-byte bit reversal
Understand this scenario with real code like «brev8 a0, a1 # Reverse bits in each byte of a1».
Pre-Use Checklist
Syntax Check
- Confirm the assembly operands are only rd, rs1; bits 31..20 value 011010000111 is a fixed encoding selector, not an immediate.
- Confirm the target implements Zbkb and reads rs1 and writes rd at the current XLEN.
Semantic Check
- Confirm each 8-bit byte reverses bit[7:0] independently; no byte positions are exchanged.
- Confirm this is a register-only bit permutation: it neither accesses memory nor treats the fixed encoding bits as runtime input.
Pitfalls / Common Confusions
Do not confuse it with REV8: BREV8 reverses bits within bytes, not byte order.
RV32/RV64 operate on XLEN=32/64 bits respectively; it accesses integer registers, not memory.
FAQ
Which extension contains BREV8?
It belongs to the Zbkb cryptographic bit-manipulation extension.