The trivial way to accomplish this is to use a 64Kx8 SRAM and simply disable it
when a ROM or other memory device is selected.
Scratch building a system with 64Kx8 SRAM and a 64Kx8 EPROM that copies itself
in to the RAM, then disconnects itself, would take, perhaps an hour or two. A
video circuit, with or without an RF modulator, and an FDC might take a mite
Sifting through the parts in a kit to verify they're all there and studying the
instructions so you get it right the first time would probably take longer than
that. Confidence in the end=product is a valuable part of the mix, however.
----- Original Message -----
From: "Tony Duell" <ard(a)p850ug1.demon.co.uk>
Sent: Saturday, September 29, 2001 1:09 PM
Subject: Re: ZX81 kits (was Re: Keep or assemble???)
does a lot more than handle the display ;>)
CPU clock generation (which would be useful). Address decoding (which I'd
want to modify if I was changing the amount of
Very interesting -- how much RAM are you talking about adding that you have
to modify the address decoding?
Well, assuming I could fit Forth into 8K, then I see no reason not to
have 56K RAM :-).
Maybe I could use the existing address decoder, but I'd want to extend it
for the extra RAM so it wouldn't be much more work to design the whole
thing from scratch...
not ;>) But a ZX81 kit is still faster than starting from
Is it? It's not going to take long to wire-wrap a clock circuit and
address decoder, and stick in the Z80. It depends _very much_ on how much
of the ZX81 you want to use...
Which in turn depends on what the intended use of the finished device is.
Since I want a "general purpose"
computer the ROM BASIC functions are very
Sure, if you want to kepe the ZX81 BASIC then it makes sense to start
from a ZX81. As I said, I'd probably not want that...
convenient, and fast if called directly. Other
applications of the Z80
would not need this code so a quickie Z80 board would be fine.
desoldering the RF modulator from a ZX81 board ;>)
Well, I had never tried to do it before, so I grabbed a ZX81,
What, do you have a pile of these?
I have a couple I think. I knew where one of them was, and I don't
normally mind dismantling computers for experimentation. I was
sufficiently confident of my soldering skills to believe that either I'd
be able to remove the modulator without damage or that I'd stop
attempting to remove it before any damage was done if there was a problem.
the case and attacked it with soldering iron and sucker.
What's the catch?
In the US versions which have 3 thin wires, they sometimes become brittle
The UK version only has 2 wires. Still, I guess they could break off as
well. It wouldn't be that hard to repair that sort of damage The
modulator can can be taken a part and the video input wire re-soldered to
the PCB inside. I am sure the power wire to the feedthrough capacitor
could be repaired as well.
Tony (and others), a question: in your opinion, which microprocessor was
the most well-designed (even if not implemented)?
For the 8 bitters, probably the 6809.
For 16 bitters, one of the PDP11 ones (T11 or something), possibly.