cmpb

Compare Bytes

cmpb RA, RS, RB

Compares bytes in two GPRs, result is byte mask.

Encoding

Binary Layout

31

0:5

RS

6:10

RA

11:15

RB

16:20

508

21:30

/

31

 

Format X-form

Opcode 0x7C0003F8

Extension Base

Operands

• RA
  Target GPR
• RS
  Src A
• RB
  Src B

Example

cmpb r4, r3, r5

Related

More in Base

Reference

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

Description

Compares each byte of register RS with the corresponding byte in register RB and stores a byte-mask result in register RA. For each byte, 0xFF is written to RA if the bytes are equal, and 0x00 is written if they differ. No condition register fields are updated.

Operation

for i in 0 to 7 do
  if RS[i*8:i*8+7] = RB[i*8:i*8+7] then
    RA[i*8:i*8+7] ← 0xFF
  else
    RA[i*8:i*8+7] ← 0x00
  end
end

Programming Note

The cmpb instruction is useful for performing byte-wise comparisons between two registers. Ensure that the input registers (RS and RB) are properly aligned to avoid unexpected results. The output register (RA) will contain 0xFF in bytes where the comparison was equal, and 0x00 elsewhere. This instruction operates at user privilege level.