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From: few@quad1.quad.com (Frank Whaley)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Keywords: huge far
Message-ID: <1626@quad1.quad.com>
Date: 11 Dec 88 09:00:37 GMT
References: <597@mks.UUCP>
Reply-To: few@quad1.quad.com (Frank Whaley)
Organization: Quadratron Systems, Westlake Village, CA
Lines: 37

In article <597@mks.UUCP> egisin@mks.UUCP (Eric Gisin) writes:
>How come I can't find a compiler that generates correct
>code for pointer subtraction in C on 8086s?
>struct six {
>	int i[3];
>};
>int diff(struct six far* p, struct six far* q) {
>  	return p - q;
>}
Yet another subtlety of segmented architectures...
Use "huge" instead of "far" -- and return a "long" instead of an "int".
Both "far" and "huge" imply 32-bit values, but since most programs work
with <64K objects, the optimization has been taken towards 16-bit arithmetic
(offset only).  Note that using "huge model" does not make pointers "huge" --
they must be individually specified (in Turbo C at least).

I prefer Lattice's method of handling this via a command line switch, rather
than by requiring source changes.

>The proper method is to propagate the borrow flag into the high
>order 16 bits, like this:
>	mov	ax,WORD PTR [bp+4]	;p
>	sub	ax,WORD PTR [bp+8]	;q
>	sbb	dx,dx			; NOT cwd !!!
>	mov	cx,6
>	idiv	cx
Actually, the correct method is to convert both pointers to 20-bit absolute
addresses, and then perform the arithmetic.  Most compilers provide subroutines
for this, as it's a bit much for inline code.

-- 
Frank Whaley
Senior Development Engineer
Quadratron Systems Incorporated
few@quad1.quad.com

Water separates the people of the world;
Wine unites them.