extsb

Extend Sign Byte

extsb RT,RS

Sign extends the low byte of a register to the full width.

Encoding

Binary Layout

31

0:5

RS

6:10

RA

11:15

954

16:20

/

21:31

 

Format X-form

Opcode 0x7C000774

Extension Base

Operands

• RA
  Target
• RS
  Source
• RT
  Target General Purpose Register

Example

extsb r3, r3

Registers Altered

CR0, XER

Related

Across architectures

Sign Extend Byte : how x86, ARM, RISC-V, and PowerISA each do this.

More in Base

Reference

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

Description

The contents of the specified byte (RS)56 are placed into RA56:63, and RA0:55 are filled with a copy of (RS)56.

Operation

if 'extsb' then
    s ← (RS)56
    RA56:63 ← (RS)56:63
    RA0:55 ← 56s
else if 'extsb.' then
    s ← (RS)56
    RA56:63 ← (RS)56:63
    RA0:55 ← 56s

Programming Note

The extsb instruction is commonly used to sign-extend a byte value into a full word. Ensure the source register contains the correct byte to avoid unexpected results. This instruction operates at user privilege level and does not generate exceptions under normal conditions.