AMOCAS.W

Atomic Compare and Swap Word

AMOCAS.W rd, rs2, (rs1)

Atomically compares memory at rs1 with rd; if equal, writes rs2 to memory. Returns original value in rd.

Encoding

Binary Layout

00101

31:27

aq

26

rl

25

rs2

24:20

rs1

19:15

010

14:12

rd

11:7

0101111

6:0

 

Format R-Type (Atomic)

Opcode 0x2800202F

Extension Zacas

Operands

• rd
  Dest/Compare
• rs2
  Swap Value
• rs1
  Address

Example

AMOCAS.W t0, a1, a0

Related

Across architectures

Atomic Compare and Swap : how x86, ARM, RISC-V, and PowerISA each do this.

More in Zacas

Reference

View in RISC-V Unprivileged ISA Specification ↗ RISC-V ISA Manual

Description

AMOCAS.W (Zacas) atomically compares the word at the address in rs1 with rs1 (the compare value), and if equal, writes rs2 to that address. The original value is returned in rd.

Operation

atomic { if(M[rs1]==R[rd]) M[rs1]=R[rs2]; R[rd]=M[rs1]; }