Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site floyd.UUCP 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