From: Shawn Hargreaves <shawn AT talula DOT demon DOT co DOT uk>
To: djgpp AT delorie DOT com
MMDF-Warning:  Parse error in original version of preceding line at punt-1.mail.demon.net
Subject: Re: ALLEGRO internals
Date: Tue, 13 May 1997 09:16:20 +0100
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Krystyna de Heras writes:
> I want to be able to write to the bitmap that ALLEGRO uses to blit to
> screen. Is there a inline asm way to do this? And can I draw to the
> bitmap even if it's target screen is a planar mode-x, in a similar way
> as a 13h-mode for example?

Read the section titled "video memory access" in allegro.txt.

Basically, for a mode 13h or VBE 2.0 linear framebuffer screen, you can do
something like:

   #include <sys/farptr.h>

   void linear_putpixel(BITMAP *bmp, int x, int y, int color)
   {
      _farpokeb(bmp->seg, (unsigned long)bmp->line[y]+x, color);
   }

For a banked SVGA screen (this will also work in linear modes), do:

   void banked_putpixel(BITMAP *b, int x, int y, int color)
   {
      unsigned long address = bmp_write_line(bmp, y);
      _farpokeb(bmp->seg, address+x, color);
   }

To use a planar mode-X screen, you'll have to also set the VGA plane
registers, eg:

   #include <pc.h>
   #include <sys/farptr.h>

   void modex_putpixel(BITMAP *b, int x, int y, int color)
   {
      outportw(0x3C4, (0x100<<(x&3))|2);
      _farpokeb(bmp->seg, (unsigned long)bmp->line[y]+(x>>2), color);
   }


--
Shawn Hargreaves - shawn AT talula DOT demon DOT co DOT uk - http://www.talula.demon.co.uk
Beauty is a French phonetic corruption of a short cloth neck ornament