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From: mac3n@babbage.acc.virginia.edu (Alex Colvin)
Newsgroups: comp.sys.ibm.pc,comp.lang.c
Subject: Re: Looking for C functions to access PC memory
Message-ID: <127@babbage.acc.virginia.edu>
Date: Fri, 13-Nov-87 10:13:32 EST
Article-I.D.: babbage.127
Posted: Fri Nov 13 10:13:32 1987
Date-Received: Sun, 15-Nov-87 16:38:08 EST
References: <24261F3U@PSUVMB> <6662@brl-smoke.ARPA>
Organization: University of Virginia
Lines: 34
Summary: questionable cast
Xref: mnetor comp.sys.ibm.pc:10151 comp.lang.c:5412

> #define	PEEK(loc)	(*(char *)(loc))
> #define	POKE(loc,value)	*(char *)(loc) = (value)
> 
> To access a wide datum than a byte, change "char" to "short" or "long".

These will work in SOME program models with SOME compilers. 

In the small model, a (char *) can only point within the data segment. 
What's probably wanted here is a pointer that can reach anywhere in
memory. 

Because this issue isn't well defined by PDP-11 C, most PC compilers
have their own way of doing this.  In CII's C86 there is the movblock
routine, whose arguments are offsets and segments.  In uSoft C you can
declare (char far*) pointers, which can point outside your data segment.

Casting (loc) to a pointer is unreliable.  The meaning of a such a cast
is not well-defined.  If loc is a 16-bit int or short, this won't get
you beyond the data segment, but is probably reliable for references
(offsets) within it.  If loc is a 32-bit int or long (depends on your
compiler), it can hold a full address.  Now the question is, what's an
address?  Some compilers view such a cast as a simple trick on the type
system, in which case (loc) should consist of a segment in the most
significant 16 bits and an offset in the least significant 16 (the
actual format of a far pointer).  Other compilers try to make casts into
conversions, and assume (loc) is a 32-bit offset from the beginning of
memory, in which case (loc) has the segment number * 16 + the offset, in
32 bits. 

Best to find out what your compiler manual tells you.  Then decorate the
code with warnings and #ifdefs

			been burned before...
			mac@cs.virginia.edu