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From: rb@ccivax.UUCP
Newsgroups: net.arch
Subject: Re: Addressing modes (1702)
Message-ID: <460@ccivax.UUCP>
Date: Tue, 11-Mar-86 11:12:52 EST
Article-I.D.: ccivax.460
Posted: Tue Mar 11 11:12:52 1986
Date-Received: Sat, 15-Mar-86 04:06:22 EST
References: <187@anwar.UUCP> <1161@mmintl.UUCP>
Reply-To: rb@ccivax.UUCP (What's in a name ?)
Organization: CCI Telephony Systems Group,  Rochester NY
Lines: 20
Summary: A sneaky way to use only two modes.

In article <1161@mmintl.UUCP> franka@mmintl.UUCP (Frank Adams) writes:
>In article <187@anwar.UUCP> mlopes@anwar.UUCP (Marco Lopes) writes:
>>In my earlier posting, I assumed the existence of absolute addressing.
>>Therefore, a minimum set of addressing modes is:
>>		register
>>		register indirect
>>Another functionally complete set would have immediate instead of absolute.
>You should seriously consider using immediate instead of absolute.
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.
This may have been a bug, but it was a useful bug (All instructions
had to be a uniform number of cycles, and immediate mode took too many
cycles).  Actually, the 1702 had a lot of "cute" features, it would be
interesting to see a 32 bit version.  Does anybody familiar with this
chip consider it RISC?  It was certainly a good learning tool.