Home/Instructions/VBREV8-V

VBREV8.V

RISC-V VBREV8.V Instruction Details

Instruction ManualR-type

Vector bit-reverse within bytes: reverse bits in each byte, byte order preserved

Instruction Syntax

vbrev8.v vd, vs2, vm

Operand Breakdown

vd: destination vector register group.

vs2/vs1 or scalar source: selected by suffixes such as .vv, .vx, .vi, or .vf.

vm: when present, vm=0 uses v0 as the execution mask and vm=1 is unmasked.

ZvbbVector Bit Manipulation

Related Search Terms

vbrev8.v instruction RISC-V vbrev8.v vbrev8.v syntax vbrev8.v encoding Zvbb vector reverse bits in bytes

Instruction Behavior

vbrev8.v is the Zvbb vector byte-internal bit-reverse. Groups each element into bytes, reverses bits within each byte (bit[0]↔bit[7], etc.), preserving byte order. Distinct from vbrev.v (full element reverse).

Quick Understanding & Search Notes

VBREV8.V is a Zvbb vector instruction for vector reverse bits in bytes. This page is checked against the official vector crypto extension and V-extension execution model.

Reverses bit order within each byte only; byte positions do not change.

It follows V-extension vl, vstart, vtype, and optional vm mask rules.

RISC-V Unprivileged ISA Manual: Vector cryptography extensions 1.0 RISC-V Unprivileged ISA Manual: V standard vector extension

Vector Execution Context

When reading VBREV8.V, do not stop at the mnemonic. Official V-extension semantics also depend on the current vl, vtype, and mask state. The suffix and operand form determine whether sources are vector, scalar, or immediate values.

Check vl first

The current vl determines the number of body elements. Typical code executes vsetvli, vsetivli, or vsetvl before this instruction.

Then check vtype

The current vtype supplies SEW, LMUL, tail policy, and mask policy; these affect element width, register-group size, and inactive/tail destination elements.

Then check vm/v0

For ordinary vector instructions with vm, vm=0 uses v0 as the execution mask and vm=1 is unmasked. A few forms such as VMERGE use v0 as data-selection input.

Official source: RISC-V V Standard Extension for Vector Operations

Common Usage Scenarios

Crypto & Security

Understand this scenario with real code like «vbrev8.v vd, vs2».

Vector Acceleration

Understand this scenario with real code like «vbrev8.v vd, vs2».

Pre-Use Checklist

Syntax Check

Confirm the current instruction format is R-type.
Confirm the operand order matches the example.

Semantic Check

Ensure the destination register usage is compatible with the calling convention.
Confirm this is not the lower-level form of a pseudo-instruction expansion.

Pitfalls / Common Confusions

Reverses within each byte only; byte order unchanged. Use vbrev.v for full element reverse.

FAQ

Can VBREV8.V always use a v0.t mask?

It can use the vm mask operand shown in the syntax; omitting it gives the unmasked form.

What determines the element width for VBREV8.V?

The current vtype SEW determines it, subject to any instruction-specific SEW restrictions in the extension.