Re: request for assistance in understanding undocumented gas directives

[Nicholas Clifton <nickc at redhat dot com>]

Hi Daniel,

> Unfortunately, it also contains uses of odd assembler directives that
> I can't find explained in the online GNU documentation:

Note - the 2,22 documentation is now out of date.  Assuming that you are 
using the latest release of the binutils, you should be looking at the 
2.24 documentation, which can be found here:

  https://sourceware.org/binutils/docs-2.24/

> .abicalls,
> .fmask,

Not that the 2.24 documentation will help you with these.  This is a 
snafu - the directives should be documented.  As usual though, the 
source code is your friend.  In gas/config/tc-mips.c there is this comment:

   The following pseudo-ops from the Kane and Heinrich MIPS book are
   not MIPS CPU specific, but are also not specific to the object file
   format.  This file is probably the best place to define them, but

Which implies that you need to obtain a copy of "MIPS RISC Architecture" 
in order to proceed.  But later on there is this:

  /* Handle the .abicalls pseudo-op.  I believe this is equivalent to
   .option pic2.  It means to generate SVR4 PIC calls.  */

Which might help a bit.

The .fmask directive appears to be part of some kind of debug 
information generation facility, although again documentation is 
severely lacking.


> a use of .previous in what seems to be the first section,
> which doesn't make sense as far as I understand.

It is probably there for safety,  In case there was a previous section. 
 Ie the .previous is probably being generated by code in the compiler 
which might be used anywhere, not just at the start of the assembler 
output.  So using a .previous to restore a saved current section is just 
good programming.


> Also, much of the
> debugging information is encoded in blocks of binary.

Presumably this debug information is being generated by the *compiler* 
and not the *assembler*.  Thus you need to tell the compiler to annotate 
its debugging output.  With gcc this would be via  the -dA command line 
option.


Cheers
  Nick