x86_64 assembler "movb" instruction

External Since: Jun 18, 2005 Posts: 24

The as assembler accept "movb %al,0(%rsi)"
but not "movb %r8,0(%rsi)" (invalid r8 register...)

Why ?

How to put (least significative) byte in r8 to memory 0(%rsi) ? (with a single instruction...)

External Since: Jun 18, 2005 Posts: 24

In article ,
mhz wrote:

> The as assembler accept "movb %al,0(%rsi)"
> but not "movb %r8,0(%rsi)" (invalid r8 register...)
>
> Why ?
>
> How to put (least significative) byte in r8 to memory 0(%rsi) ? (with a single instruction...)

Solved: add b suffix to register name

movb %r8d,0(%rsi)

It is a little bit redundant...

mov %r8d,0(%rsi)

is also valid.