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Path: utzoo!linus!wivax!decvax!harpo!floyd!crc
From: crc@floyd.UUCP
Newsgroups: net.arch
Subject: Re: uP architecture - (nf)
Message-ID: <1750@floyd.UUCP>
Date: Wed, 6-Jul-83 10:45:34 EDT
Article-I.D.: floyd.1750
Posted: Wed Jul  6 10:45:34 1983
Date-Received: Thu, 7-Jul-83 11:37:33 EDT
References: <416@umcp-cs.UUCP>
Organization: Bell Labs, Whippany, NJ
Lines: 34

The COSMAC 1802 is produced by the Retrograde Corporation of America: RCA.

The 1802 is not only elegant, it is painful to program. Subroutine return
addresses are stored in one of 16 cpu registers. To address a location in
memory, one must go indirect through a register. To load a register with
a 16 bit address, one must do two ( load accumulator, transfer acc to
register half). That's FOUR instructions to put an address in a register.

	# I don't remember the memnonics, but this is what you did.
	LDA low-half-of-address #byte loaded to acc
	STRL R1			#put acc in register low half
	LDA high-half-of-address #byte loaded to acc.
	STRH R1			#put acc in register high half.
				# now you can reference the memory location

Electrically the COSMAC 1802 has a cute *feature*. The little beast does
not have a 'memory select' line. It has 'read select' and 'write select'
which it asserts when it is serious about an address on the address bus.
Early COSMAC documention did not mention that during operations such as
increment register, it throws the register contents out on the address
bus. (presumably for debugging.) It is up to memory devices to restrain
themselfs until 'memory read' or 'memory write' is asserted.

I used one in a school project and found that I couldn't get anyone to
program the bloody thing.

The COSMAC is well suited for use in applications where there is little or
no RAM and its registers can be used instead. AND where the programs involved
are very simple, such as the triggers of Nuclear bombs which is an application
where I believe it is used. (It was one of the first microprocessers available
with radiation hardening.)

			Not afraid to dislike simple and crude architectures:
			Charles Colbert