From: "Johan Venter (aka sphinX)" <^_^@delorie.com>
Newsgroups: comp.os.msdos.djgpp
References: <dMOe2.122$14 DOT 362 AT newsr2 DOT u-net DOT net>
Subject: Re: beginner mode13 question
Date: Tue, 22 Dec 1998 16:41:40 +1100
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

>unsigned char *VGA=(unsigned char *)0xA0000L;

This is not going to work, because what you are specifying here is a near pointer in
32-bit memory without ever turning on nearptr addressing.
To use near pointers in djgpp you need to call the __djgpp_nearptr_enable() function which
is located in the sys/neaprtr.h include file. Be warned though, turning on nearptrs
effectively turns off memory protection, but I think it is probably the easiest way to
access graphics memory (particularly in mode 13h). So, to get your pointer use:

#include <sys/neaptr.h>

unsigned char *vga;

void set_up_graphics(void)
{
   __djgpp_nearptr_enable();
   vga = (unsigned char *)__djgpp_conventional_base + 0xA0000;
}

Then, in each of your functions, you will need to recalculate this address just in case
__djgpp_conventional_base changes which may happen after memory allocs/deallocs.

So, change your putpixel to the following:

void plot_pixel(int x, int y, int colour)
{
  VGA = (unsigned char *)__djgpp_conventional_base + 0xA0000;
  VGA[y*320+x]=colour;
}

When your program exits, don't forget to turn off nearptrs with __djgpp_nearptr_disable();

--
sphinX

e-mail:   sphinx_ AT iname DOT com
icq:   3643877