OT: how big would it be?

From: Richard Erlacher <edick_at_idcomm.com>
Date: Wed Oct 20 10:43:03 1999

These days, no one would seriously attempt to build a CPU equivalent using
TTL SSI/MSI components simply because the packaging gets too much in the way
of smooth data flow. I once built a 650x CPU equivalent on a 4"x6" wirewrap
board using TTL SSI/MSI, but it was my goal to build one using the logic
available to the original designers. That was not easy, but it fit only
because of the efficiencies inherent in the 650x series timing and
instruction set.

If one were to do the same thing with an 8080 CPU design, I suspect it could
be forced to fit on a board like an S-100 card. It might be difficult to do
in "period" family logic, i.e. with the parts that were available to the
designers of the 8080, though. If you'd like to convince yourself of this,
take a look at the '72 TTL data book from T.I. or Signetics. If one were
limited to 28-pin FPLD's and smaller, I think it could be done easily
enough. That would take the emphasis off parts search and the occasional
unavailability of some functions, while allowing some random logic to be
localized in a single device rather than requiring several different flavors
of AOI gates, expanders, etc. The internal data paths of the 8080 could
more easily be done with today's tristate devices than back in '72-'74 with
muxes of various types.

There are FPGA cores, though . . .

I personally think it would be fun to build an S-100 card to replace the
8080 LSI. It might well be possible to replace all the external support
logic with the hardware that goes "inside" the device, and you could fiddle
with the instruction set more or less like the folks with wire-wrapped PDP8
processors did from time to time.

Dick

-----Original Message-----
From: Eric Smith <eric_at_brouhaha.com>
To: Discussion re-collecting of classic computers
<classiccmp_at_u.washington.edu>
Date: Wednesday, October 20, 1999 12:25 AM
Subject: Re: OT: how big would it be?


>> Say someone were to home-build a CPU from scratch using only individual
>> components, no ICs only modern descrete(?) components. How big would the
>> CPU be? For comparison lets say it would be an 8080 clone. Any guesses?
>
>One of the vendors of bit-slice components (AMD 2900 or clone, I think)
>offered a board that emulated an 8080. In addition to being faster than
>an 8080, you could of course modify the microcode to add or change
>instructions. IIRC, it looked like the board had about fifty chips.
>
>If you didn't use a bit-slice, you'd have to use separate ALU, memory,
>and shifter chips, and you'd wind up with even more.
>
>The early microprocessor architectures were designed based on minimal
>transistor count for a single-chip implementation. This does not result
>in minimal chip count if you implement the equivalent in 7400-series
>chips. It is quite possible to design useful processors with a lot fewer
>TTL chips.
Received on Wed Oct 20 1999 - 10:43:03 BST