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Path: utzoo!utgpu!attcan!uunet!husc6!bloom-beacon!mit-eddie!uw-beaver!uw-june!geops!johnf
From: johnf@geops.UUCP (John Firestone)
Newsgroups: comp.misc,comp.arch
Subject: Re: History of PCs (also kind of long)
Summary: History of PC's (yet more on the COSMAC 1802)
Keywords: history, pc, workstation
Message-ID: <426@geops.geops.UUCP>
Date: 4 Aug 88 18:36:01 GMT
References: <5946@venera.isi.edu> <5458@ecsvax.uncecs.edu> <1876@looking.UUCP> <1193@ficc.UUCP>
Organization: Geophysics, U of W, Seattle
Lines: 44

In an article, Karl Lehenbauer writes:
: As I recall, the RCA 1802 has eight 16-bit registers. . . .

As someone who built and programmed an 1802 microcomputer system starting
from a bunch of chips, a surplus teletype, and many pads of paper (boy, I
didn't know what I was getting into), I would like to offer a few corrections
and additions regarding this unusual but neat microprocessor.  As I believe
the part is still being sold, I will use the present tense.

The 1802 actually has sixteen 16-bit registers which are almost completely
interchangeable except that:

	- register R0 becomes a DMA data pointer if you use DMA
	- register R1 becomes the interrupt service program counter and
	  register R2 becomes the stack pointer if you use interrupts
	- register R0 can not be used with the Load via Rn instruction
	  as the required opcode is assigned to the IDLE instruction

One of the most powerful features of the 1802's instruction set is that any
of these registers can serve as a program counter or index register and that
it is very easy to switch the working set.  This makes it easy to use
coroutines (a single instruction switches back and forth) and to write
instruction emulators and threaded/knotted code interpreters.  In fact, often
the best way to program the 1802 is to construct an application-specific,
meta-machine first.  Perhaps this is what makes the 1802 seem "weird".

While the 1802 has no multiply and divide instructions, RCA does sell a
compatible multiply/divide chips.  By dedicating a register or two as
instruction program counters, workable multiply/divide instructions can be
added to the instruction set.

The main drawbacks of the 1802 are that it does not do instruction pipelining
and that you have to route all data through its 8 bit D-register; start
emulating a lot of instructions and it can become very slow.  In some
applications, though, power not speed is important.  Since the 1802 is
constructed in classic CMOS, you almost can reduce its power consumption to
whatever level you want by slowing or stopping its clock.

If you weren't in a hurry, the 1802 was a good chip to look at.

John Firestone
Geophysics, AK-50
University of Washington
Seattle, WA  98195