RISC-V AMOMAX.B Instruction Details

Instruction ManualR-type

Atomic byte signed max: rd = *rs1 (sign-ext), *rs1 = max(*rs1, rs2[7:0])

Instruction Syntax

amomax.b rd, rs2, (rs1)

Operand Breakdown

Destination rd: register receiving the operation result.

Source rs1: register holding the first operand.

Source rs2: register holding the second operand.

ZabhaAtomic

Related Search Terms

amomax.b instruction RISC-V amomax.b byte AMO Zabha amomax.b byte signed atomic maximum amomax.b aq rl

Instruction Behavior

AMOMAX.B (Zabha extension) atomically loads a byte from the address in rs1 into rd (sign-extended), compares with the low byte of rs2, stores signed maximum back. Supports aq/rl bits.

Quick Understanding & Search Notes

AMOMAX.B is a Zabha 8-bit signed atomic maximum instruction. It atomically reads the old memory value, performs the read-modify-write at 8-bit width, and writes the sign-extended old value to rd.

Only the 8-bit subword is operated on; upper bits of rs2 are ignored.

rd receives the old memory value sign-extended to XLEN, not the newly updated value.

The address must be naturally aligned to the operand size; aq/rl bits can provide release-consistency ordering semantics.

RISC-V Unprivileged ISA Manual: Zabha byte and halfword AMO extension RISC-V Unprivileged ISA Manual: A atomic extension

Common Usage Scenarios

Atomic & Sync

Understand this scenario with real code like «amomax.b a0, a2, (a1)».

Network & Byte Order

Understand this scenario with real code like «amomax.b a0, a2, (a1)».

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

rd returns old memory value (sign-extended to XLEN), not the operation result

Signed maximum comparison; note difference from AMOMAXU (signed vs unsigned)

Atomicity depends on natural alignment and implementation support

FAQ

Does AMOMAX.B return the post-operation value?

No. Like A-extension AMOs, rd receives the old value loaded from memory.

Can AMOMAX.B affect neighboring bytes?

Architecturally it modifies only the byte operand; Zabha provides native byte-granule atomic read-modify-write operations.