X86 Assembly/Arithmetic

Arithmetic instructions

Arithmetic instructions take two operands: a destination and a source. The destination must be a register or a memory location. The source may be either a memory location, a register, or a constant value. Note that at least one of the two must be a register, because operations may not use a memory location as both a source and a destination.

add src, dest	GAS Syntax
add dest, src	Intel syntax

This adds src to dest. If you are using the NASM syntax, then the result is stored in the first argument, if you are using the GAS syntax, it is stored in the second argument.

sub src, dest	GAS Syntax
sub dest, src	Intel syntax

Like ADD, only it subtracts source from destination instead. In C: dest -= src;

mul arg

This multiplies "arg" by the value of corresponding byte-length in the AX register.

operand size	1 byte	2 bytes	4 bytes
other operand	AL	AX	EAX
higher part of result stored in:	AH	DX	EDX
lower part of result stored in:	AL	AX	EAX

In the second case, the target is not EAX for backward compatibility with code written for older processors.

imul arg

As MUL, only signed.

div arg

This divides the value in the dividend register(s) by "arg", see table below.

divisor size	1 byte	2 bytes	4 bytes
dividend	AX	DX:AX	EDX:EAX
remainder stored in:	AH	DX	EDX
quotient stored in:	AL	AX	EAX

The colon (:) means concatenation. With divisor size 4, this means that EDX are the bits 32-63 and EAX are bits 0-31 of the input number (with lower bit numbers being less significant, in this example).

As you typically have 32-bit input values for division, often need to use CDQ to sign-extend EAX into EDX just before the division.

If quotient does not fit into quotient register, arithmetic overflow interrupt occurs. All flags are in undefined state after the operation.

idiv arg

As DIV, only signed.

neg arg

Arithmetically negates the argument (i.e. two's complement negation).

Carry Arithmetic Instructions

adc src, dest	GAS Syntax
adc dest, src	Intel syntax

Add with carry. Adds src + carry flag to dest, storing result in dest. Usually follows a normal add instruction to deal with values twice as large as the size of the register. In the following example, source contains a 64-bit number which will be added to destination.

mov eax, [source] ; read low 32 bits
mov edx, [source+4] ; read high 32 bits
add [destination], eax ; add low 32 bits
adc [destination+4], edx ; add high 32 bits, plus carry

sbb src, dest	GAS Syntax
sbb dest, src	Intel syntax

Subtract with borrow. Subtracts src + carry flag from dest, storing result in dest. Usually follows a normal sub instruction to deal with values twice as large as the size of the register.

Increment and Decrement

inc arg

Increments the register value in the argument by 1. Performs much faster than ADD arg, 1.

dec arg

Decrements the register value in the argument by 1. Performs much faster than SUB arg, 1.

Pointer arithmetic

The lea instruction can be used for arithmetic, especially on pointers. See X86 Assembly/Data Transfer#Load Effective Address.