Mail Archives: djgpp/1997/10/18/16:02:26

Date: Sat, 18 Oct 1997 13:00:54 -0700 (PDT)
Message-Id: <>
Mime-Version: 1.0
To: waage AT brainerd DOT net (waage), djgpp AT delorie DOT com
From: Nate Eldredge <eldredge AT ap DOT net>
Subject: Re: wirting to a virtual screen - HELP!

At 12:16  10/17/1997 GMT, waage wrote:
>I am having problems copying info from a virtual screen allocated with
>malloc and copying it to the VGA 0xa000.  The code looks like this,
>when compiled it says no errors but when I try to write to the virtual
>screen with my put_pixel it crashes.
>This is the function that allocates the virtual screen and (I hope)
>returns the address in memory where its located.
>unsigned long set_virtual(int w, int h) {
>    unsigned long v_size = w * h;
>    unsigned long vir_scr;
>    (void *)vir_scr = malloc(v_size);
>    if(vir_scr != NULL) {
>      printf("Allocated memory for virtual screen.\n");
>      return vir_scr;
>      }
>    else {
>      printf("Unable to allocate %U bytes for virtual screen.\n",
>      return 0;
>      }
>This is my put_pixel function, where is the address in memory that it
>wants to write to.
>void put_pixel(int where, int x, int y, unsigned char *col) {
>    _farpokeb(_dos_ds, where*16+(x+(y*320)), col);
>    }
>This does not work with the virtual screen.  Can anyone tell me if it
>is because the virtual screen has a different selector than the a000
>(the info said that _dos_de has a limit of 1MB)?  Does malloc(...) use
>memory above 1mb and do I need to use a different selector?  I thought
>of this and came up with this:
That's it exactly. All your program's code and data is addressed relative to
the _my_ds() selector. You don't need to use farptr functions to access it.
>short get_virtual_discriptor(unsigned long vir_scr) {
>    return(__dpmi_segment_to_descriptor(vir_scr));
>	}
>void put_vir_pixel(short sel, unsigned long where, int x, int y,
>unsigned char col) {
>    _farpokeb(sel, where*16+(x+(y*320)), col);
>    }
Do this to put_pixel your vir_scr:
*(unsigned char *)vir_scr+x+(y*32) = col;

Nate Eldredge
eldredge AT ap DOT net

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