From: "Max Erhard" <erhard AT banstead DOT u-net DOT com>
Newsgroups: comp.os.msdos.djgpp
Subject: beginner mode13 question
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Date: Sat, 19 Dec 1998 14:29:22 -0000
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To: djgpp AT delorie DOT com
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I am having trouble getting mode13 programs to work. Plotting pixels using
bios interupts works OK. But as soon as I try to directly address the video
memory, I start to get horrible GPFs. I have tried A000, A0000 and A0000000
as the address for video memory but no luck. I have read about special
commands for near pointers or something but they need <sys/nearptr.h> and I
do not have this file in my /include/sys folder, only farptr.h. Also is
there a memory model setting for gcc that I need to know about.

Errors look like this:-

General Protection Fault at eip=211
eax=00000001 ebx=00051d00 ecx=000a0141 edx=000a0141 esi=00000000
edi=00000000
ebp=00051d50 esp=00051d4c cs=bf ds=b7 es=b7 fs=b7 gs=b7 ss=c7 cr2=00001fec
Call frame traceback EIPs:
  0x00000211
  0x00000285

My code is:- (which should make a rainbow of all of the 256 colours on
screen.)

#include <stdio.h>
#include <bios.h>
#include <dos.h>

int a;
int dx;
int dy;
unsigned char *VGA=(unsigned char *)0xA0000L;

void set_mode13()
{
 union REGS regs;
 regs.h.ah = 0x00;
 regs.h.al = 0x13;
 int86(0x10,&regs,&regs);
}

void set_textmode()
{
 union REGS regs;
 regs.h.ah = 0x00;
 regs.h.al = 0x03;
 int86(0x10,&regs,&regs);
}

void plot_pixel(int x, int y, int colour)
{
 VGA[y*320+x]=colour;
}

void main ()
{
 set_mode13();
 for(a=0;a<=256;a+1)
 {
  for(dx=1;dx<=320;dx++)
  {
   for(dy=1;dy<=200;dy++)
   {
    plot_pixel(dx,dy,a);
   }
  }
 }
 set_textmode();
}