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From: ron@brl-sem.ARPA (Ron Natalie <ron>)
Newsgroups: net.lang.c
Subject: Re: The 68000 as PDP-11 Clone
Message-ID: <542@brl-sem.ARPA>
Date: Tue, 19-Nov-85 04:41:49 EST
Article-I.D.: brl-sem.542
Posted: Tue Nov 19 04:41:49 1985
Date-Received: Wed, 20-Nov-85 21:08:54 EST
References: <3360@brl-tgr.ARPA>
Organization: Ballistic Research Lab
Lines: 20

> 
> Of course there are *some* differences. And of course the 68000 is a 
> 32-bit machine (crammed onto a 16-bit bus) while the pdp is a 16-bit one.

And it has the bytes in the wrong order.

> As far as relative code, the pdp is missing something. How do you
> load the address of a variable relative to the PC? This way:
> 
> 		mov	pc,r0			; address of `here'
> 	here:	add	#foobar-here,r0		; make relative
> 
Bad example, now what are you going to do with this loaded
address?  PDP-11 instructions allow things like

		tst	foobar(pc)

which is exactly equivalent to
		
		tst	foobar