Xref: news2.mv.net comp.os.msdos.djgpp:6982
From: brennan AT mack DOT rt66 DOT com (Brennan "The Reverend Bas" Underwood)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: buffer -> screen transfers
Date: 8 Aug 1996 10:59:20 -0600
Organization: None, eh?
Lines: 57
Message-ID: <4ud6d8$7j4@mack.rt66.com>
References: <320647A3 DOT 5827 AT cadvision DOT com>
NNTP-Posting-Host: mack.rt66.com
To: djgpp AT delorie DOT com
DJ-Gateway: from newsgroup comp.os.msdos.djgpp

In article <320647A3 DOT 5827 AT cadvision DOT com>,
John Meilleur  <meilleuj AT cadvision DOT com> wrote:
>Can anyone help me speed up this bit of code.  I'm writing some hi-res graphics routines and would like to know
>how i can speed up clearing the screen buffer and the buffer transfer.  I know I should be able to do 32-bit
>transfering but I've had no luck with it sofar.  The "video_ds" is the selector to video memory and starts at
>offset 0.  Can any show me how to speed this up alot? TIA 
>
>char *screen = malloc(640*480);
>while (!kbhit()) 
>{
>  for (i=0;i<640*480;i++)
>    screen[i]=0;

asm volatile (
  "rep\n\t"
  "stosl"
  :
  : "a" (0), "c" (640*480/4), "D" (screen)
  : "%edi", "%ecx" );

>  drawstuff(screen);
>
>  _farsetsel(video_ds);
>  for (i=0;i<640*480;i++)
>    _farnspokeb(i,screen[i]);	

asm volatile (
  "movw %%es, %%dx\n\t"
  "movw %%bx, %%es\n\t"
  "rep\n\t"
  "movsl\n\t"
  "movw %%dx, %%es\n\t" /* put things back where you found 'em */
  :
  : "b" (video_ds), "c" (640*480/4), "S" (screen), "D" (0)
  : "%ebx", "%ecx", "%edx", "%esi", "%edi" );

>
>}


I realize there are other tiny little optimizations, but they don't
amount to more than a cycle or 3 per page.

If at all possible, arrange to not have to clear the parts of the
screen that are overwritten anyway. 640*480 makes for a lot of bytes.
Don't write to them twice if you can.

For clarity's sake, I'd just use memset instead of the first asm,
and fmemcpy on the second. Well , *I* wouldn't, but I recommend it
anyway.


--Brennan
-- 
brennan AT rt66 DOT com  |  fsck /u

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