Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!uw-beaver!uw-june!kolding
From: kolding@uw-june (Eric Koldinger)
Newsgroups: comp.arch
Subject: Re: while (*p++ = *p++);  (was Re: Endian wars)
Message-ID: <7211@june.cs.washington.edu>
Date: 8 Feb 89 23:16:25 GMT
Sender: kolding@june.cs.washington.edu
Reply-To: kolding@uw-june (Eric Koldinger)
Distribution: usa
Organization: U of Washington, Computer Science, Seattle
Lines: 37

In article <775@atanasoff.cs.iastate.edu> hascall@atanasoff.cs.iastate.edu (John Hascall) writes:
>In article <3300050@m.cs.uiuc.edu> gillies@m.cs.uiuc.edu writes:
>
>>instruction (or something amazingly short) on the PDP-11, C's mother
>>machine:
>
>>while (*p++ = *q++);
>
>>This is perhaps part of the reason why strings were designed with
>>null-termination
>
>  First, I assume you mean:
>
>     char   *p,*q;
>     while (*p++ == *q++);
>
>  I can see no way to code this in 1 PDP instruction, the best I can see
>  is (assume R1 is p, R2 is q):

Piece of cake:
For the loop as he put it, with your assumptions:
loop:	movb	(r2)+, (r1)+
	bneq	loop

Two instructions for the loop.  The loop as you wrote it (which doesn't really
do very much, but here goes), again with  your assumptions:
loop:	cmpb	(r2)+, (r1)+
	beq	loop
    
God it's good to write some PDP-11 code again.  Now there was a great machine
even if it did only have 64K of memory.

------
	_   /|				Eric Koldinger
	\`o_O'				University of Washington
  	  ( )     "Gag Ack Barf"	Department of Computer Science
       	   U				kolding@cs.washington.edu