From: _ <1s AT ge1ties DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Help! Confused by problems with VESA programming.
Date: Wed, 09 Jul 1997 11:48:36 +0200
Organization: Aktiefront Red de Eikel.
Message-ID: <33C35E74.79EA@ge1ties.com>
References: <5psfn0$mpi$1 AT pulp DOT ucs DOT ualberta DOT ca> <33C24657 DOT 4AF6 AT cornell DOT edu> <5pvbbs$j1q$1 AT pulp DOT ucs DOT ualberta DOT ca>
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Jon Martin wrote:
> 
> "A. Sinan Unur" <asu1 AT cornell DOT edu> writes:
> 
> >Jon Martin wrote:
> >>
> >> int main(void)
> >>         {
> >>         VBE_VgaInfo vgainfo;
> >>         VBE_ModeInfo modeinfo;
> >>         unsigned char colour;
> >>         int done = 0;
> >>         int y;
> >>         unsigned char virtscreen[307200];
> 
> >i haven't checked the rest of the code but this probably is the culprit.
> >the default stack size for djgpp programs is 256K. you are asking for
> >about 300K. you could stubedit this to 1 Mb and see if it makes a
> >difference. however, a better programming practice would be to allocate
> >the memory for the virtual screen from the heap (using malloc).
> 
>      I tried this (using malloc), but it did not change anything (the bugs
> scurried elsewhere).  I stripped it down to a bare minimum, to help reduce
> clutter.  The printf in the loop reveals that everything works fine up to
> pixel 3921 (the 3922nd iteration of the loop).  Then it crashes, often locking
> my computer _hard_.  Here is the new, stripped down main(), everything else
> remains unchanged.
> 
> int main(void)
>         {
>         VBE_VgaInfo vgainfo;
>         VBE_ModeInfo modeinfo;
>         int y;
> 
>         printf("finished startup stuff\n");
>         fflush(stdout);
> 
>         if (VBE_detect(&vgainfo) < 200)
>                 {
>                 printf("Your card does not support VESA 2.0\n");
>                 fflush(stdout);
>                 exit(1);
>                 }
> 
>         printf("finished checking for VBE 2\n");
>         fflush(stdout);
> 
>         VBE_getmodeinfo(0x101, &modeinfo);
> 
>         printf("finished getting modeinfo\n");
>         fflush(stdout);
> 
>         if (modeinfo.ModeAttributes & 0x8)
>                 {
>                 printf("Linear frame buffer found\n");
>                 fflush(stdout);
>                 }
>         else
>                 {
>                 printf("Linear frame buffer not found\n");
>                 fflush(stdout);
>                 exit(1);
>                 }
> 
>         printf("finished checking for LFB\n");
>         fflush(stdout);
> 
>         VBE_get_linear_address(0x101);
> 
>         printf("finished getting linear address for 0x101\n");
>         fflush(stdout);
> 
>         make_selector();
> 
>         printf("finished making selector\n");
>         fflush(stdout);
> 
>         VBE_setmode(0x4101);
> 
>         printf("finished setting mode 0x4101\n");
>         fflush(stdout);
> 
>         for (y = 0; y < 307200; ++y)
>                 {
>                 _farpokeb(video_selector, y, 12);
> 
>                 printf("finished setting pixel %d\n", y);
>                 fflush(stdout);
>                 }
> 
>         printf("finished looping thing\n");
>         fflush(stdout);
> 
>         getchar();
> 
>         printf("finished getting a char\n");
>         fflush(stdout);
> 
>         textmode(LASTMODE);
> 
>         printf("finished returning to text mode (via textmode())\n");
>         fflush(stdout);
> 
> /*
>         VBE_setmode(0x03);
> 
>         printf("finished returning to text mode (via VBE_setmode())\n");
>         fflush(stdout);
> */
> 
>         return(0);
>         }
> --
> Serve Gonk.
> Jon Martin  jmartin AT obed DOT cs DOT ualberta DOT ca  <-- finger for more info
> http://elycion.geology.ualberta.ca/~jon/
> Department of Computing Science at University of Alberta.


int VBE_detect(VBE_VgaInfo *vbeinfo)
        {
        __dpmi_regs regs;

        assert(sizeof(*vbeinfo) <
_go32_info_block.size_of_transfer_buffer);

        regs.x.ax = 0x4F00;
        regs.x.di = __tb & 0x0F;
        regs.x.es = (__tb >> 4) & 0xFFFF;

This should be changed to 

    regs.x.di = __tb & 0xfffF;
    regs.x.es = (__tb & 0xFFFF0000)>>4;

Do this also in function VBE_getmodeinfo(0x101, &modeinfo);

I've got some example source code that's almost the same as yours, but
works for me at 
http://www.geocities.com/ResearchTriangle/5603/famebuffer_example.html