Xref: utzoo comp.sys.ibm.pc:22782 comp.sys.intel:642
Path: utzoo!utgpu!watmath!mks!egisin
From: egisin@mks.UUCP (Eric Gisin)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Summary: shorter code
Keywords: C pointer math DAMN WELL IS A BUG!!!!
Message-ID: <631@mks.UUCP>
Date: 5 Jan 89 20:46:23 GMT
References: <597@mks.UUCP> <3845@pt.cs.cmu.edu> <18123@santra.UUCP> <51@rpi.edu> <147@bms-at.UUCP>
Organization: Mortice Kern Systems, Waterloo, Ont.
Lines: 17

In article <147@bms-at.UUCP>, stuart@bms-at.UUCP (Stuart Gathman) writes:
< 	
< 	xor	dx,dx
< 	mov	ax,ptr1
< 	sub	ax,ptr2
< 	sbb	dx,0		; carry extend
< 	mov	bx,n
< 	idiv
< 
< For the most common (2^n, 2*n) cases, this takes exactly as many bytes
< and is just as fast as the wrong code.  The general case is just 3 extra
< bytes (and won't occur with word alignment).

You can replace the xor and sbb with one sbb:
	sbb	dx,dx

That's only one byte longer than the incorrect code and it is just as fast.