Xref: utzoo comp.sys.ibm.pc:23157 comp.sys.intel:664
Path: utzoo!attcan!uunet!lll-winken!ames!amdahl!drivax!alexande
From: alexande@drivax.UUCP (Mark Alexander)
Newsgroups: comp.sys.ibm.pc,comp.sys.intel
Subject: Re: correct code for pointer subtraction
Summary: look at the actual assembly output
Message-ID: <4121@drivax.UUCP>
Date: 12 Jan 89 18:07:17 GMT
References: <597@mks.UUCP> <3845@pt.cs.cmu.edu> <18123@santra.UUCP> <142@bms-at.UUCP> <6604@killer.DALLAS.TX.US>
Reply-To: alexande@drivax.UUCP (Mark Alexander)
Organization: Bob-ist Temple of Monterey
Lines: 20
Keywords:


When all else fails, look at the actual code generated by your
compiler to see where the problem lies.  The problem people are having
with Turbo C and MS C doing incorrect pointer subtraction is probably
due to the compiler generating a SIGNED shift (SAR) instead of an
UNSIGNED shift (SHR) when subtracting INT pointers.  Datalight C 3.14
has this problem, as shown below.

With DLC, the code generated to do the pointer subtraction look like this:

	mov	BX,offset DGROUP:U0EA60h	; &a[30000]
	mov	SI,offset DGROUP:U0		; &a[0]
	sub	BX,SI				; &a[30000] - &a[0]
	sar	BX,1				; convert bytes to words

That last 'sar' instruction should really be a 'shr'.  I'm sure Turbo C
and MS C have a similar problem.
-- 
Mark Alexander	(UUCP: amdahl!drivax!alexande)
"Bob-ism: the Faith that changes to meet YOUR needs." --Bob (as heard on PHC)