From: "Patrick Smith" <patricks AT travsoft DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: Re: Graphics under DJGPP V2.01
Date: 12 Dec 1996 20:01:10 GMT
Organization: Traveling Software
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To: djgpp AT delorie DOT com
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Thomas,

Glad to see you are doing some graphics programming with DJGPP...  Its
pretty fun stuff, ya know.  :)

Anyways, lets see if I can help you out a little bit.  For your first
question about faster methods for moving data to video ram:  It looks like
you are aware of the selector you need to use to access dos memory,
_dos_ds... that's good!  Now if you want to do your own blitters, I'm
afraid you need to use assembly.  I'm sorry, I don't know any other way to
use the selector in C to access dos memory.  (Besides the built in
functions like you were using)

Basically, when you want to read/write to dos memory, you need to load the
_dos_ds (data segment selector) into the es register.  Then, to offset into
that memory, you need to take the real mode segement, multiply it by 16 (or
left shift by 4) and add the real mode offset.  For VGA memory, the result
would be 0xA0000.  Notice there are 4 zeros instead of the 3 you are used
to (that's 'cause I multiplied it (the segment) by 16).  You need to move
this offset value into the edi register (if you are writing to memory, if
you are reading, use esi).  Now if you need to offset furthur into VGA
memory, just add the offset value to the edi register, for example, 0xA0140
(0xA0000 + 320) would reference the first pixel of the second scan line. 
Here's a quick and dirty function to write a pixel using DJGPP and VGA.

short _iDosMem = _dos_ds;

void PlotPixel (char chColor, short iXPos, short iYPos)
{

    asm volatile ("

        push    %%es
        push    %%edi
        push    %%eax

        movw    _iDosMem, %%es
        movl    $0xA0000, %%edi

        movw    %2, %%ax
        imulw   $320, %%ax
        addw    %1, %%ax
        addw    %%ax, %%di

        movb    %0, %%al
        movb    $0, %%ah

        stosb

        pop     %%eax
        pop     %%edi
        pop     %%es

    " 
        :                                           // No Outputs
        : "g" (chColor), "g" (iXPos), "g" (iYPos)   // Inputs
    );

    return ;
}

You could easily expand this function to do whatever blitting you need,
(transparent, RLE, whatever).  I noticed that you are already using the
seg*16+offset rule (0xA0000) -- thats great!  I hope this example has
helped a little bit...

Now for your second question...  How to set the VGA mode 13h?  How 'bout
this?

void SetVGAMode (void)
{
    __dpmi_regs tRegs;
    memset (&tRegs, 0, sizeof (tRegs));

    tRegs.x.ax = 0x13;
    __dpmi_int(0x10, &tRegs);

    return;
}

To switch back to DOS 80x25 mode, just set the ax register to 0x03 instead
of 0x13.  :)

I hope this helps...  I'm certainly no expert in the area, but I believe
everything I told you is correct.

Happy Coding!

Patrick Smith
Traveling Software


Thomas Harte <T DOT Harte AT btinternet DOT com> wrote in article
<01bbe78f$57f16cc0$LocalHost AT default>...
> 	I am fairly new to this compiler, but so far I have liked everything I
> have seen. My previous C compiler was a Borland DOS version, so I am very
> new to protected mode. As such, I am having trouble with graphics. At the
> minute (in DJGPP) I access the VGA board using the code :-
> 
>    for(temp=0;temp<64000;temp++)
>      {
>       _farpokeb(_dos_ds, 0xA0000 + temp, doublebuffer[temp]);
>       doublebuffer[temp]=background[temp];
>      }    
> 
> which, as I'm sure you can imagine is VERY slow, in that it copies each
> pixel individually. Anyway, my question is :-
> 
> 	Which faster methods can I use to copy the contents of an integer into
the
> video ram, starting at 0xA0000 ?
> 
> Also :-
> 
> 	How would I switch graphic modes (to and from mode 13h) under DJGPP ?
(at
> the moment I have code from the old compiler which changes modes running
> before and after the new programs in a batch file) 
> 
> Thanks,
> 
> 		-Thomas
>