Date   : Tue, 26 Oct 2010 05:46:05 +0100
From   : afra@... (Phill Harvey-Smith)
Subject: Reversing the Tube ULA (destructively)

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
flash based.

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 
simplify things.

Cheers.

Phill.

-- 
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !

"You can twist perceptions, but reality won't budge" -- Rush.