Question

While going through some C code having inline assembly I came across the .byte (with a Dot at the beginning) directive.

On checking the assembly reference on web I found that it is used to reserve a byte in memory.

But in the code there was no label before the statement. So I was wondering what is use of an unlabeled .byte directive or any other data storage directive for that matter.

For e.g. if i code .byte 0x0a, how can i use it ?

Answer 1

There are a few possibilities… here are a couple I can think of off the top of my head:

1. You could access it relative to a label that comes after the .byte directive. Example:

     .byte 0x0a
   label:
     mov (label - 1), %eax
   

2. Based on the final linked layout of the program, maybe the .byte directives will get executed as code. Normally you’d have a label in this case too, though…

3. Some assemblers don’t support generating x86 instruction prefixes for operand size, etc. In code written for those assemblers, you’ll often see something like:

     .byte 0x66
     mov $12, %eax
   

   To make the assembler emit the code you want to have.