X86 Assembly/Logic

Logical instructionsEdit

The instructions on this page deal with bit-wise logical instructions. For more information about bit-wise logic, see Digital Circuits/Logic Operations.

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

binary operationsEdit

These instructions require two operands.

logical andEdit

and mask, destination GAS Syntax
and destination, mask Intel Syntax


operationEdit

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

side effectsEdit

See below.

exampleEdit
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
applicationEdit
  • 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   operation, that is the remainder of integer division. For that, mask has to contain the value   (i. e. all lower bits set until a certain threshold), where   equals your desired divisor.

logical orEdit

or addend, destination GAS Syntax
or destination, addend Intel Syntax


operationEdit

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

side effectsEdit

See below.

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

logical xorEdit

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


operationEdit

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

side effectsEdit

See below.

exampleEdit
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
applicationEdit
  • 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 remarksEdit

side effects for and, or, and xorEdit
  • 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 operationsEdit

logical notEdit

not argument

operationEdit

Performs a bit-wise inversion of argument.

side-effectsEdit

None.

exampleEdit
movl $0x1, %edx ; edx ≔ 1
notl %edx ; edx ≔ ¬edx
; here edx would be 0xFFFFFFFE because a bitwise NOT 0x00000001 = 0xFFFFFFFE
applicationEdit
  • not is frequently used to get a register with all bits set.