Classic Computer Rescue Squad

From: Allison J Parent <allisonp_at_world.std.com>
Date: Sun Nov 9 14:03:46 1997

<It had no RAM, clock, or anything, it was simply an ALU wired to 2 banks o
<4 switches, and a rotary switch configured to select one of the 8 or 16
<operations supported by the ALU. If I'd built it I'm sure I would've tired
<of 4 bit boolean operations pretty quickly.
<
<Kevin

But the step from that to a rudimentary computer was fairly short. What
was missing was the sequential(or microprogrammed) logic to get from
switches to Fetch an instruction from memory and Execute it. The simple
machine in Byte had the microprogrammed part of the system to do that
albeit crudely. What was significant was how few parts it took to do
that.

To make a point. The DEC RX01 and RX02 disk systems use a crude
microprogrammed computer to do all the disk formatting and control work.
The RX01 more specifically is easier to understand as the biggest chips
used are bipolar proms, 7489 (16x4 rams) and a 74181 alu chip. Its logic
contained on one board of 16"x6" and if the RX01 specific logic were left
out it would be 2/3rds that. It could be programmed to behave differently
and I have used the basic logic design to create other similar high speed
specialized controllers. the later rx02 is similar but they use the 2901
bit slice with is really a 16x4 ram, ALU and gating logic in one chip.
Oh, the alu is identical to the 74181. FYI: the RX01 controller has 5
basic microinstructions as it's all that were needed to implement the
needed programming. General purpose no, it didn't need to be It had to
be fast enough to run bits at 500khz.

Needless to say that Byte article I refer to would be of interest as it's
simple enough to explain and build with current components. I plan to dig
for it but I am not sure I still have it.

the latter 1985 article is more ambitious and most would have trouble
fabricating it as it's a fairly large number of chips (60-100 ttl).
The design would have to be revamped to use more common chips and then
simulated/breadboarded and tested for logic/timing errors. A PCboard
for it would be several cards and likely several hundred dollars for
the set in small quantity. Then there is programming as it's a unique
machine with no existant programming available. That means cross
assembler would be required to get initial code going for a basic monitor
program. the article is a worthwhile read as it does explain how a computer
is devloped and composed. It's in Byte September (VOL 10, number9) and
October (vol 10, number 10) and they are available from BYTE at $3.00 each.

I have no connection to byte other than having those on the shelf and
handy. I've used them to present on computer design.

Allison
Received on Sun Nov 09 1997 - 14:03:46 GMT