Re: Question about keywords and declarations in assembly code

["Jafa" <jafa at silicondust dot com>]

Hi,

You best bet is the online documentation...

http://www.gnu.org/manual
http://www.gnu.org/manual/gas-2.9.1/as.html

Further answers below... I am sure the experts here will correct anything I
have got wrong.

Nick

> Hi,
>
> I've been looking at the assembly code that is produced by
> gcc and have a few questions about some of the declarations
> and keywords found in the assembly file.
>
> When declaring variables in ".Lfe1", what does the following
> mean?
>       .Lfe1:
>            .comm  aVariable,4,1
>            .comm  anotherVariable,4,1
>
> And what function does ".Lfe1" perform?

.Lfe is a label generated by gcc... I don't think it has any meaning to gas
(someone correct me if I am wrong).

.comm = common and they are generated by gcc for global variables that are
not pre-initialized and not static.
The linker will group like-named .comm variables and place them in the .bss
section.

Actually .comm is a PITA because it prevents some tricks you can do
manipulating .o files and causes probems with --gc-sections.

The way I figure it if you have two global variables of the same name then
you should get a linker error... merging the two may be the worst thing it
can do. Maybe there is some obscure C spec that requires this - hopfully
someone will clarify.

> I can see that the second number means the number of bytes
> this variable should take in memory - but what does the
> second number mean, and what does ".comm" stand for?

The alignment. If there is no second number then it means natural alignment
which is usually what you want.

> When a name of a function is declared "extern" in assembly
> what does this mean? i.e.
>        extern aFunction
>
> When there is ".sect" , ".text" , and ".type" at the top of
> the main declaration what do these mean? i.e.
>       .sect  .text
>       .type  main,@function
>
> I'm guessing that the "main,@function" is saying that main
> is a function - is this correct?

Yes.
BTW - .text is the section == code.

> The ".globl" keyword I guess means that this function can be
> called from anywhere in the assembly code - is this correct?
> i.e.
>       .globl aFunction

.global means that the label should be published outside of the context of
the current file.
Any c function that is not static will be marked global for gas - c assumes
global, gas assumes local scope.

> Is there an online manual where I can find answers to
> questions like these above??

http://www.gnu.org/manual
http://www.gnu.org/manual/gas-2.9.1/as.html

> Thankyou for your help with these questions, I really
> appreciate it.
>
> Simon Reynolds.
>
>