Xref:	news2.mv.net comp.os.msdos.djgpp:3293
From:	Shawn Hargreaves <slh100 AT york DOT ac DOT uk>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Re: Yet about MK_FP, pokeb, and peekb
Date:	Tue, 30 Apr 1996 11:35:12 +0100
Organization:	The University of York, UK
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To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

On Mon, 29 Apr 1996, Bruno Kraychete da Costa wrote:

> 	Guys, sorry if I seem to insist at the same point (about 
> MK_FP). What I'd like to have is the facility of refering to a memory 
> position using a pointer, instead of a function (like dosmemput/get). On 
> the Borland version of a program of mine I had:

The problem is that memory addresses in djgpp aren't absolute, because it 
is running in protected mode. In Borland C real mode you can specify a 
physical address, like 0xA0000, and know exactly where it will go. In 
protected mode, however, all addresses are relative to your program's 
virtual address space. When you create a pointer to 0xA0000, the 
processor treats it as being relative to the base address of your data 
segment, rather than absolute, plus all addresses go through the page 
mapping tables before they are actually used to refer to physical bits of 
ram (check a book on 386 asm for more detail on this).

If you want to refer to a specific physical address, you have to load a 
different value into one of the segment registers, which means either 
using asm or the farptr functions. Alternatively there is a trick you can 
use with near pointers, by setting an infinite limit on your data 
segment and then using negative offsets (look at nearptr.h), but this 
disables memory protection and won't work under all DPMI providers.

I know this can be a pain, but it's what you get for using protected mode :-)


Shawn Hargreaves.        Why is 'phonetic' spelt with a ph?
Check out Allegro and FED on http://www.york.ac.uk/~slh100/

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