AMD x86 manual Use XOR Instruction to Clear Integer Registers, Efficient 64-Bit Integer Arithmetic

AMD Athlon™ Processor x86 Code Optimization 22007E/0 — November 1999

Use XOR Instruction to Clear Integer Registers

To clear an integer register to all 0s, use “XOR reg, reg”. The AMD Athlon processor is able to avoid the false read dependency on the XOR instruction.

Example 1 (Acceptable):

MOV REG, 0

Example 2 (Preferred):

XOR REG, REG

Efficient 64-Bit Integer Arithmetic

This section contains a collection of code snippets and subroutines showing the efficient implementation of 64-bit arithmetic. Addition, subtraction, negation, and shifts are best handled by inline code. Multiplies, divides, and remainders are less common operations and should usually be implemented as subroutines . If these subroutines are used often, the programmer should consider inlining them. Except for division and remainder, the code presented works for both signed and unsigned integers. The division and remainder code shown works for unsigned integers, but can easily be extended to handle signed integers.

Example 1 (Addition):

;add operand in ECX:EBX to operand EDX:EAX, result in

;EDX:EAX

ADD	EAX,	EBX
ADC	EDX,	ECX

Example 2 (Subtraction):

;subtract operand in ECX:EBX from operand EDX:EAX, result in

;EDX:EAX

SUB	EAX,	EBX
SBB	EDX,	ECX

Example 3 (Negation):

;negate	operand in EDX:EAX
NOT	EDX
NEG	EAX
SBB	EDX, –1 ;fixup: increment hi-word if low-word was 0

86 Use XOR Instruction to Clear Integer Registers