[BBC-Micro] Reversing the Tube ULA (destructively)
afra at aurigae.demon.co.uk
Tue Oct 26 05:46:05 BST 2010
Rick Murray wrote:
> On 26/10/2010 05:53, Phill Harvey-Smith wrote:
>> I believe his clone, doesn't have as many bytes in it's FIFOs,
> Yes, I remember, though this could just be a limitation of the
> programmable logic he used - that adding a larger FIFO would not have
> provided suitable gains for the work involved?
I think he used a 64 or 128 macrocell CPLD, so it's probably a
limitation of not enough flipflops, using a small FPGA would have solved
this though brought it's own problems too, such as needing level
shifters as most FPGAs operate at 3.3V or less, and needing a
configuration prom, CPLDs generally don't need this as they are mostly
I just wish someone did a CPLD with lots of logic but not using stupid
numbers of pins :(
>> and I seem to remember him saying that it didn't do DMA I think,
> The Tube does DMA!?
I think it supports it on the tube side, I could be miss remembering, I
know John said that there was another limitation.
> I suppose the question is, do we want an EXACT replica, or "something
> that works"?
That's the difference between an exact clone of the hardware and a
logical clone. For example if you took my Atom clone, put it in an Atom
case and screwed it shut you hopefully woudn't know it was a clone, as
it's logically identical (or near as damn it !), but it's not a literal
clone of Acorn's circuitry as it uses CPLDs, modent static ram etc to
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
"You can twist perceptions, but reality won't budge" -- Rush.
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