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From: jhunter AT kendaco DOT telebyte DOT com (Joel Hunter)
Newsgroups: comp.os.msdos.djgpp
Subject: Re: GPF when accessing video memory directly
Date: Sat, 19 Aug 1995 01:09:09 GMT
Organization: Telebyte North West Silverdale, Wa
Lines: 32
References: <DDMG1t DOT Bwr AT jade DOT mv DOT net>
Nntp-Posting-Host: kendaco.telebyte.com
To: djgpp AT sun DOT soe DOT clarkson DOT edu
Dj-Gateway: from newsgroup comp.os.msdos.djgpp

taps AT cyberstore DOT ca (Tapio Vocadlo) wrote:

>Using the _farpokeb and related functions in sys/farptr.h
>compiles fine, but then GPFs with a 'unsupported int 0x0D'
>message when run. All I'm doing is

>#include <sys/farptr.h>

>unsigned char gc=65;

>_farpokeb(0xb800,0,gc);


>I'm obviously doing something wrong, since far mem i/o
>has to work, but what..?

  A _selector_ isn't the same thing as a segment.  You need to either
use a linear memory selector
(_go32_info_block.selector_for_linear_memory) and use 0xB8000 as an
offset, or create a custom selector for your video memory, which you
can do using the 
__dpmi_segment_to_descriptor().  Like this:

  unsigned short videoSelector;
  unsigned char gc=65;

  videoSelector=__dpmi_segment_to_selector(0xB800);
  _farpokeb(videoSelector,0,gc);