Bit Shifts, Significant Bits, and Extracting Bytes (Intel Syntax)

Concepts

Bytes in a 64-bit register: rdi = 64 bits = 8 bytes.
- 1st least significant byte (LSB) → bits 0–7
- 2nd byte → bits 8–15
- 3rd byte → bits 16–23
- 4th byte → bits 24–31
- 5th byte → bits 32–39

On x86-64 in Intel syntax, the sh family of instructions is:

shl reg, imm → shift left (multiply by 2^imm)
shr reg, imm → shift right logical (divide unsigned by 2^imm)
sar reg, imm → shift right arithmetic (divide signed by 2^imm, preserving sign bit)
To isolate a byte using only shifts:
1. Shift right to bring target byte into the lowest 8 bits.
2. Shift left to clear higher bits and leave only that byte.

Example

Extract the 5th least significant byte of rdi (bits 32–39).
Place result in rax.
Allowed instructions: mov, shr, shl.

.intel_syntax noprefix
.global _start
.section .text
_start:
    mov rax, rdi        ; copy input
    shr rax, 32         ; shift right 32 bits → 5th byte now in lowest 8 bits
    shl rax, 56         ; clear out everything except that byte
    shr rax, 56         ; move it back down into lowest 8 bits
    ; now rax holds only the 5th byte

or

.intel_syntax noprefix
.global _start
_start:
	shl rdi, 24
	shr rdi, 56
	mov rax, rdi

shr rax, 32 → moves byte 5 down into position 0.
shl rax, 56 → clears all bits except that byte (forces others out).
shr rax, 56 → moves it back down to the LSB for a clean result.

Final: rax contains the 5th least significant byte of rdi.

Resources

Resource	Description
1.2 - Bitwise Logic in x86 (Intel Syntax)	Next note