From:	"David Dean - [DS]Deaner666" <david AT deanhouse61 DOT freeserve DOT co DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Virtual Screens
Date:	Thu, 1 Jul 1999 19:28:26 +0100
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To:	djgpp AT delorie DOT com
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Reply-To:	djgpp AT delorie DOT com

I'm currently trying to get to grasps with graphics programming in DJGPP.
I've managed to get round the protected mode quirks using (the albeit
annoying AT&T) assembly, and can now put pixels to the video segment. My
question now regards setting up a _virtual_ screen to put pixels to, so I
can set up the screen in the background and flip it to VGA without flicker.
I'm using mode 13h, so what is the best way of setting up the 64000 (320 *
200) bytes I need and then being able to reference this virtual screen?
Could I combine a function for putting pixels to this virtual screen with my
current function for putting pixels to VGA by adding another parameter?

Here's my current putpixel function:

/*Puts a pixel of colour col to (x,y) on the VGA screen*/
void PutPixelVGA(int x, int y, unsigned char col)
{
short global_selector;
global_selector = _dos_ds;

    __asm__ __volatile__("
    movw %3, %%es\n
    movl $0xA0000, %%edi\n
    movw %0, %%ax\n
    imulw $320, %%ax\n
    addw %1, %%ax\n
    addw %%ax, %%di\n
    movb %2, %%al\n
    stosb"
    :
    :"g" (y), "g" (x), "g" (col), "g" (global_selector)
    );
}

Thanks in advance,

David Dean
(DJGPP really is quirky isn't it?)

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