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From: ado@elsie.UUCP (Arthur David Olson)
Newsgroups: comp.arch
Subject: Re: JSR
Message-ID: <7391@elsie.UUCP>
Date: Tue, 21-Apr-87 20:02:06 EST
Article-I.D.: elsie.7391
Posted: Tue Apr 21 20:02:06 1987
Date-Received: Wed, 22-Apr-87 06:15:18 EST
References: <12884@watnot.UUCP> <1061@aw.sei.cmu.edu> <1070@osiris.UUCP>
Organization: NIH-LEC, Bethesda, MD
Lines: 18

> . . .the PDP-11 JSR instruction allows you to specify *any* of the eight
> "GPRs" to hold the current PC contents, after pushing the old value of the
> specified register onto the stack.  (Among other purposes, this was used. . .
> for passing the address of an in-line FORTRAN parameter address block. . .

Yet another purpose:  if you wanted to push register r0 onto the stack
without changing the processor status word (as low-level interrupt handlers
were wont to do), then instead of using the instruction
			mov	r0,-(sp)
you could use the instruction
			jsr	r0,(pc)
to turn the trick.

You did clobber register zero--but you had just saved it away, yes?
-- 
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