[vcf-midatlantic] Working on a historical microprocessor exhibt

Dave McGuire mcguire at neurotica.com
Sun Feb 9 14:24:31 EST 2020

On 2/9/20 2:02 PM, Adam Michlin via vcf-midatlantic wrote:
> Ooo.. how could I forget Fairchild? Thanks!
> The AMD bit slice is new to me. Am I understanding correctly that it is
> not a discrete CPU but rather parts of a CPU that could be used
> together? Probably not a candidate for this exhibit, but I'm already
> having ideas for the next one!

  Bit-slice is great stuff.  There are many bit-slice chips, but the
Am2901 essentially owned the market.  It is a four-bit unit.

  Basically it's a processor building block used to construct a
micromachine.  You stack them up to whatever bit width you want, and
program them with microcode to implement whatever macro-architecture you
want.  There are companion chips, like the Am2910 sequencer, etc.  These
are not required but come in handy.  The sequencer can implement
constructs like a microprogram counter, branches and jumps, etc.

  A few examples you've heard of: The VAX-11/730 uses a row of eight
Am2901s to implement a 32-bit VAX processor, and the DECsystem-2020 uses
a row of nine to implement a 36-bit PDP-10 processor.  Both use
RAM-based control stores, so when they power up they don't speak the
VAX/PDP-10 instruction sets until after the microcode is loaded.

  This is generally the smallest/cheapest approach to build a large
processor.  The VAX-11/730 and DECsystem-2020 are very small, very
low-end members of their respective families.  But the bit-slice chips
themselves are quite fast for their day, being big hot-running 1970s
bipolar chips.

  It's also possible to use Am2901s (or other bit-slice chips) as
processors in their own right.  For example, many, many third-party disk
controllers for DEC machines use a pair or a quad of Am2901s to
implement their functionality.  In this application they typically
aren't running microcode, but are used as more of a microcontroller with
firmware.  The distinction is largely semantic, though, of course.

  It's great stuff.  If you want to learn more about how all of this
works, I recommend picking up a copy of "Bit-Slice Microprocessor
Design" by John Mick and James Brick.  This book is/was known in the
industry as "Mick & Brick".

> I'd love to find a Zilog Z8000, but don't imagine period correct Z8000s
> are so easy to come by. We have a Z8000 machine in the warehouse, but
> one of the big rules for this product is that no vintage computers will
> be harmed to make it happen.

  Are you talking about just a Z8000 chip?  I can provide that for the
exhibit.  It will need ESD protection, of course.


Dave McGuire, AK4HZ
New Kensington, PA

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