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From: radzy@calma.UUCP (Tim Radzykewycz)
Newsgroups: net.arch
Subject: Re: Addressing modes (1702)
Message-ID: <192@calma.UUCP>
Date: Mon, 17-Mar-86 19:15:33 EST
Article-I.D.: calma.192
Posted: Mon Mar 17 19:15:33 1986
Date-Received: Wed, 19-Mar-86 04:11:29 EST
References: <187@anwar.UUCP> <1161@mmintl.UUCP> <460@ccivax.UUCP>
Reply-To: radzy@calma.UUCP (Tim Radzykewycz)
Organization: GE/Calma Co., R&D Systems Engineering, Milpitas, CA
Lines: 31

In article <460@ccivax.UUCP> rb@ccivax.UUCP (What's in a name ?) writes:
>For any who might remember the RCA 1702 COSMAC VIP computer, there was
>a strange way to get immediate addressing.  The VIP would let you switch
>which register would work as a program counter and when that register
>was used for instruction fetches or anything else, it would cause the
>register to be incremented.  If R7 was your PC register, you could say
>MOV R6,(R7), and the result was the same as move imediate instruction,
>because R7 would be incremented automatically.

Actually, the fact that the 1702 (and 1802 for that matter) allowed
you to change which register was used as the PC is irrelevant for this.

The PDP-11 used the same method, although the assembler was smart enough
to turn instructions like

MOV #47,R2

into the word-sequence

MOV (PC)+,R2
47

which is exactly what was described above for the 1702.  Immediate
is by definition just a form of PC-relative address, although it
might be implemented as a special case.
-- 
Tim (radzy) Radzykewycz, The Incredible Radical Cabbage.
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