x86 Assembly/Logic

Logical instructions

All logical instructions presented in this section are executed in the, as the name already suggests, the arithmetic logic unit.

binary operations

These instructions require two operands.

logical and

operation

and performs a bit-wise and of the two operands, and stores the result in destination.

example
movl $0x1, %edx ; edx ≔ 1 movl$0x0, %ecx ; ecx ≔ 0
andl %edx, %ecx ; ecx ≔ edx ∧ ecx
; here ecx would be 0 because 1 ∧ 0 ⇔ 0

application
• An and can be used to calculate the intersection of two “sets”, or a value representing a “mask”. Some programming language require that Boolean values are stored exactly as either 1 or 0. An and rax, 1 will ensure only the LSB is set, or not set.
• If partial register addressing is not available in the desired size, an and can be used for a ${\displaystyle {\text{destination}}\!\!\!\!\mod {\text{mask}}}$ operation, that is the remainder of integer division. For that, mask has to contain the value ${\displaystyle 2^{n}-1}$ (i. e. all lower bits set until a certain threshold), where ${\displaystyle 2^{n}}$ equals your desired divisor.

logical or

operation

The or instruction performs a bit-wise or of the two operands, and stores the result in destination.

example
movl $0x1, %edx ; edx ≔ 1 movl$0x0, %ecx ; ecx ≔ 0
orl  %edx, %ecx ; ecx ≔ edx ∨ ecx
; here ecx would be 1 because 1 ∨ 0 ⇔ 1

application
• An or can be used to calculate the union of two “sets”, or a value representing a “mask”.

logical xor

 xor flip, destination GAS Syntax xor destination, flip Intel Syntax

operation

Performs a bit-wise xor of the two operands, and stores the result in destination.

example
movl $0x1, %edx ; edx ≔ 1 movl$0x0, %ecx ; ecx ≔ 0
xorl %edx, %ecx ; ecx ≔ edx ⊕ ecx
; here ecx would be 1 because 1 ⊕ 0 ⇔ 1

application
• xor rax, rax (or any GPR twice) will clear all bits. It is a specially recognized word. However, since xor affects flags it might introduce bogus dependencies.

common remarks

side effects for and, or, and xor
• OF ≔ 0
• CF ≔ 0
• SF becomes the value of the most significant bit of the calculated result
• ZF ≔ result = 0
• PF is set according to the result

unary operations

logical not

not argument

operation

Performs a bit-wise inversion of argument.

None.

example
movl \$0x1, %edx ; edx ≔ 1
notl %edx ; edx ≔ ¬edx
; here edx would be 0xFFFFFFFE because a bitwise NOT 0x00000001 = 0xFFFFFFFE

application
• not is frequently used to get a register with all bits set.