From: ranla AT post DOT tau DOT ac DOT il (Tal Lavi)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Extended ASM (Was: misc questions)
Date: Fri, 4 Dec 1998 18:10:47 LOCAL
Organization: Tel-Aviv University Computation Center
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>If you have already studied those, you will 
>have to ask further questions here.

Here's one.
I need a FlushPage routine(macro?) for a 640x480x16bpp mode written in asm.
Until now, I used a C(++) routine which uses _farsetsel & _farnspokel.
All my experiments in writing such a routine with the extended ASM failed.
Here's one that SEEM to be alright to me, the unexperienced ASM programer.
Please, show me a better one(that works).
And, PLEASE, mention what the not so obvious parts of your code does.

void FlushPage(unsigned short C)
{
  __asm__ __volatile__ ("
    movw %0,%%fs\n
    .byte 0x64\n
    movl $0,%%edi\n
    movl $153600, %%ecx\n
    rep\n
    stosl"
  :
  :"r"(LFBSelector[ScreenNum]),
   "a"((long(C)<<16)+C)
  :"ax","cx","di","memory"
  );
}

I also have a couple of questions:

1) Should I use a rep stos, or maybe movl with a branch?

2)
> The first are constraints again, and ".byte 0x64" causes the assembler to 
> emit 0x64 into the binary code.  0x64 is the op-code for FS: prefix 
> override (meaning the next instruction uses offsets into the segment 
> whose selector is in the FS register).  sys/farptr.h uses a byte constant 
> because early versions of Gas didn't support prefixes (I'm not sure how 
> the things are with Binutils 2.8.1).

Why should you write(and know) the opcode? Isn't there an ASM instruction for that purpose?
(movl %something, %%fs(%something))?


	Thanks in advance,

		Tal Lavi		ranla AT post DOT tau DOT ac DOT il