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From: tjr@ihnet.ATT.COM (Tom Roberts)
Newsgroups: comp.sys.ibm.pc
Subject: C routines to access memory
Message-ID: <584@ihnet.ATT.COM>
Date: Tue, 10-Nov-87 13:56:44 EST
Article-I.D.: ihnet.584
Posted: Tue Nov 10 13:56:44 1987
Date-Received: Fri, 13-Nov-87 07:09:24 EST
Organization: AT&T Bell Laboratories - Naperville, Illinois
Lines: 36

To reference memory on an IBM-compatible PC, using C pointers,
you must be sure that you know which compiler you are using;
THEY DIFFER IN IMPORTANT WAYS!!!

Turbo C:
	/* in L model, you can omit the "far" keyword */

	char far *p;

	p = MK_FP(0xB800,0);
	*p = 'A'; /* sets B800:0000 to 'A' */

	p = (char far *)0xB8000000L; /* equivalent to the MK_FP() above */

Lattice C:
	/* Lattice C has no "far" keyword, use L model (newer versions might) */

	char *p;

	p = 0xB8000L;
	*p = 'A'; /* sets B800:0000 to 'A' */

NOTE: these two compilers use a different mapping between longs and
far pointers: Turbo C simply copies them; Lattice C treats longs as
a flat, uniform address (which I consider much more useful).
If you mix them up, you will clobber random locations in memory.
When I converted from Lattice C (v 2.15) to Turbo C (v 1.00), I did
not know this, AND MY HARD DISK GOT TRASHED SEVERAL TIMES BEFORE I
DISCOVERED THIS DIFFERENCE.

Note also that "far pointer" in Lattice C L model, is similar to the
Turbo C "huge pointer", NOT Turbo C "far pointer" (huge pointers are
normalized after arithmetic, far pointers ARE NOT).

Tom Roberts
ihnp4!ihnet!tjr