cmp

Compare

cmp BF, L, RA, RB

Compares two registers as signed integers and records the result in the specified Condition Register Field.

Encoding

Binary Layout

0:5

6:8

11:15

16:20

0000000000

21:30

Format X-form

Opcode 0x7C000000

Extension Base

Operands

BF
Condition Register Field (0-7)
L
Operand Size (0=32-bit, 1=64-bit)
RA
Source Register 1
RB
Source Register 2

Example

cmp cr7, 0, r3, r4

// Compare r3 vs r4 (32-bit signed), result in cr7.

Registers Altered

Across architectures

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

Reference

View in PowerISA v3.1C Specification ↗ OPF Power ISA v3.1C

Description

The fixed-point Compare instructions compare the contents of register RA with either the sign-extended value of the SI field, the zero-extended value of the UI field, or the contents of register RB. The comparison is signed for cmpi and cmp, and unsigned for cmpli and cmpl. The L field determines whether the operands are treated as 32-bit or 64-bit quantities.

Operation

if (RA) < (RB) then CR[BF] <- 0b1000 (LT)...

Programming Note

The cmp instruction is commonly used for signed comparisons between two registers. Ensure that the operands are correctly aligned and consider the L field to specify whether the comparison should be performed on 32-bit or 64-bit values. The result of the comparison updates the Condition Register (CR), so always check CR[BF] after executing cmp to determine the outcome.

cmp