# 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 ${\text{destination}}\!\!\!\!\mod {\text{mask}}$  operation, that is the remainder of integer division. For that, mask has to contain the value $2^{n}-1$  (i. e. all lower bits set until a certain threshold), where $2^{n}$  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.