Bit-Shifting Operations

SHL/SAL -> Shift Left operation shifts the most significant bit to left one position.

In this operation, Carry Flag is used to store the shifted bit. And as we can see the LSB is auto-populated with 0.

SHR -> Shift Right

In this, LSB is shifted to carry flag and MSB is auto-populated with 0.

SAR-> Shift Arithmetic Right. This is very interesting. If the operand is positive, 0 is populated in the MSB but if it is negative, 1 is populated.

ROR-> Rotate Right. Some bits are rotated from LSB to MSB by a particular value.

Here is a demo:

global _start			



section .text

_start:



	mov rax, 0x00000000ffffffff

	sal rax, 32

	sal rax, 1



	clc	

	mov rax, 0x00000000ffffffff

	shr rax, 1

	shr rax, 31



	clc

	mov rax, 0x00000000ffffffff

	sar rax, 1

	clc

	mov rax, 0xffffffffffffffff

	sar rax, 1



	clc

	mov rax, 0x0123456789abcdef

	ror rax, 8

	ror rax, 12

	ror rax, 44





	; exit the program gracefully  



	mov rax, 0x3c

	mov rdi, 0		

	syscall



section .data



	var1 dq 0x1111111111111111

	var2 dq 0x0

1. Shift left by 0x20 (32 bits decimal)

2. SHL by 1 bit again. See CF is now 1 as 1 bit from ff is taken which becomes fe.

Continue and inspect this behavior.