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From: tjr@ihnet.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: Functions to access memory
Message-ID: <585@ihnet.ATT.COM>
Date: Thu, 12-Nov-87 10:20:24 EST
Article-I.D.: ihnet.585
Posted: Thu Nov 12 10:20:24 1987
Date-Received: Sat, 14-Nov-87 12:31:05 EST
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 30


> Use these sparingly!
> 
> #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".

BEWARE! These work fine on a VAX, or other non-segmented machine; on
an 8086-based machine (like an IBM PC or compatible), there is a lot
more going on. A C programmer MUST be aware of the underlying hardware
architecture, and of the different memory models provided by the C
compiler to utilize that hardware. In the most common memory model
(SMALL), the above macros will not let you reference outside of your
DGROUP "segment" (DGROUP is a segment to the HW, but a "group" to the
linker - thanks a bunch Microsoft (:-(); there is almost no point to
such macros in SMALL memory model - PEEK() and POKE() are usually used
to reference the OS or the memory-mapped display - things which are
outside of DGROUP. Writing PEEK() and POKE() macros is compiler
dependent - you are better off using the library routines usually
supplied by the compiler manufacturer.

ANOTHER GOTCHA: different compilers convert (long) to (char far *)
in different ways:
	Turbo C:	0xB8000000L => B800:0000
	Lattice C:	0x000B8000L => B800:0000
(other compilere probably use one of these - I don't know).

Tom Roberts
ihnp4!ihnet!tjr