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Path: utzoo!linus!philabs!micomvax!musocs!mcgill-vision!mouse
From: mouse@mcgill-vision.UUCP (der Mouse)
Newsgroups: net.arch
Subject: Re: What I miss in micro-processors (fairly long)
Message-ID: <142@mcgill-vision.UUCP>
Date: Tue, 10-Sep-85 17:38:04 EDT
Article-I.D.: mcgill-v.142
Posted: Tue Sep 10 17:38:04 1985
Date-Received: Thu, 12-Sep-85 21:48:10 EDT
References: <1517@umcp-cs.UUCP>
Organization: McGill University, Montreal
Lines: 56

> > ...  It would be so nice to end each routine with a CASE (R7),TABLE
> > and have control passed to the routine who's address is at TABLE+R7*size.
>
> How about
>
>	movl	table[r7],pc
>
> or something like that?

     That's what I thought too.  I considered trying something like

	jmp	*_table[r7]

but unfortunately the way indexed mode works, that  comes out grouped as
(*_table)[r7]  instead of  *(_table[r7]).    So  I  wrote the  following
routine:

	.globl	_callroutine
	.globl	_table
_callroutine:
	.word	0xffc
	movl		4(ap),r0
	addl3		$2,_table[r0],pc

This assembled silently.   I  wrote a C test  driver for it,  ran it and
boom, illegal instruction.  Adb told me pc was just after the addl3 (the
instruction  there  was a "halt").  Put in "calls $0, _abort" there so I
could tell, and sure enough, this time it called abort before dying.  So
the  addl3 wasn't functioning as a  jump.   I  look  at the architecture
handbook:

3.4 GENERAL MODE ADDRESSING FORMATS

3.4.1 Register Mode

     [...]. PC may  not be used  in register mode  addressing.  If PC is
read,  the  value read  is UNPREDICTABLE.   If PC  is  written, the next
instruction  executed  or the next operand specified  is  UNPREDICTABLE.
[...]

     So I guess we can't do that.  We seem to be reduced to

	movl			_table[r7],r0
	jmp (calls, etc)	(r0)

which I haven't tried, but I feel certain would work.

     Ugh.
-- 
					der Mouse

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