Re: Computing Editorial (please read)

[et472@FreeNet.Carleton.CA (Michael Black)]

Ruud Dingemans (rtech@home.nl) writes:
> Lee Hart wrote:
> 
>> Some computers really *are* simpler than radios and audio amplifiers. In
>> 1978 I built an 1802 "ELF" computer. You can't get much simpler, even
>> today!
> 
> Hm, from what I can see those still have about 9 - 11 chips, while on 
> the ol' ZX81 PCB I count only five (ROM, CPU, ULA and 2x 0.5k RAM :)
> 
But the 1802 was designed so it was easy to bootstrap.  I forget
the details, but you could load the RAM from toggle switches without
a lot of complicated hardware, as something like the S-100 bus required.
So you could take the 1802, some RAM, some switches and readouts, and
a handful of simple logic ICs, and it would be up and running.  You didn't
need a ROM or EPROM programming, you didn't even need to have a monitor
to start with.  You just needed to wire it up, and learn from scratch
how to program.  It was used in an amateur satellite or two, precisely
because of that simple ability to load the RAM directly.

The ZX81 might have had a lower parts count but it was infinitely
more complicated.  You couldn't duplicate it at home without a lot more
circuitry, because the ULA took the place of the various circuitry needed
to implement a more complicated computer, and you couldn't buy the ULA buy
itself.  And you'd need that ROM, or something someone else developed,
to run it all (and if you don't have that monitor ROM, you'd need fancy
hardware to develop it).

The analog would be that you could still build an Apple II today,
though it would be tedious wiring it all together.  But the later
versions of the II, while using fewer parts, uses parts that you couldn't
buy except from Apple, and they likely wouldn't sell.

The 1802 would still be a great today for a beginner, except that
it was never all that common and available, and that would be
moreso now.

There was an article in Byte about 1980, or maybe 81, about an
8085 that could be bootstrapped in a similar way.  It was meant
for someone to throw together and learn from.  Considering that there
were a lot more 8085's sold, and hence they are more common, it
would still make a good project today.  And static memory has become
more dense, it would take even less wiring.

For that matter, I have a book about the 68000 put out by Dr. Dobbs,
mostly reprints from the magazine, that shows a similar way of getting
a 68000 going, though it wasn't quite as neat in the simple peripheral
area.

   Michael