Date: Thu, 22 Sep 1994 16:54:19 -0700 (PDT)
From: Gordon Hogenson <ghogenso AT u DOT washington DOT edu>
Subject: far pointers 
To: djgpp AT sun DOT soe DOT clarkson DOT edu



(Sorry if this is a repeat, I think it disappeared into the
cyberspace void last time.)

What is the proper usage of the _farsetsel() function ?
Initially, I thought I could just load a segment, i.e.,

main()
{
  _farsetsel(0xb800);
  /* .... */
}

but that generates a General Protection Fault.  So then I
tried the following program:

#include <stdio.h>
#include <go32.h>
#include <sys/farptr.h>

int main()
{
   unsigned long i;
  
   _farsetsel(_go32_conventional_mem_selector());
   for (i = 0xb8000; i < 0xa0000; i++)
     _farnspokew(i, 0x0700 | 65); 
}

If the 'selector' points to the bottom of DOS memory, as returned
by _go32_conventional_mem_selector(), I should be able to put the
absolute address 0xb8000 in for the offset...

I expected the text screen to be filled with the letter 'A' when
run from an 80x25 text screen on a VGA monitor.
However, nothing happened, the program runs and finishes with no
discernible effect.

This is with DJGPP 1.12 / GCC 2.6.0 .  I didn't try far pointers
with DJGPP 1.11 so I am not sure how they are supposed to work.

 _farpeekb(0x40,0x4a) works just fine, returning the
screen width from the BIOS data area.

Any suggestions?

Gordon