Path: utzoo!attcan!uunet!mcvax!ukc!stl!stc!idec!camcon!mrh
From: mrh@camcon.co.uk (Mark Hughes)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Microsoft Quickc - Pointers to arbitrary memory location
Message-ID: <1920@titan.camcon.co.uk>
Date: 8 Sep 88 08:36:47 GMT
References: <666@dogie.edu>
Organization: Cambridge Consultants Ltd., Cambridge, UK
Lines: 41

From article <666@dogie.edu>, by dorl@vms.macc.wisc.edu (Michael Dorl - MACC):
> I've been trying to reference screen memory directly from Quickc.
> The stuff I've tried looks something like this
> 
>   char far *p;
>   long i;
> 
>   p = (char far *)0xb8000;
>   for (i=0; i<0x8000; i++) {
>    *p = '\000';
>    p++;
>   }
> 
Try defining a macro like:-

#define fp(a) ( (( (long)(a) & 0xf0000 )<< 12) | ( (long(a) & 0xffff ) )

and replacing your pointer assignment with:-

p = (char far *)( fp( 0xb8000 ) );

The problem is caused by the way that pointers are stored, i.e. most
significant word contains segment address and l.s.w. contains the offset within 
the segment. Unfortunately, Microsoft C does not realise that you want to
point to the address 0xb8000, and simply interprets this as segment=0xb,
offset 0x8000. What you want is segment=0xb000, offset=0x8000. The macro
performs this conversion for you.

This is a consequence of the architecture of the 8086 (etc.), but I'm not sure
if the compiler should interpret your assignment in the way that it does.
Intuitively, I would say that the compiler should realise that you want to
point to a memory location and implement the assignment appropriately instead
of blindly stuffing the value into your variable.

Does anyone know what the standards say (if anything) about this?

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