Date   : Wed, 15 Apr 2009 19:06:14 +0100
From   : jumbos.bazzar@... (Mark Haysman)
Subject: ReCo6502 now available

>> It also requires you to remove the ULA from your existing 6502 second
>> processor
>
> Has anybody tried to engineer a replacement for the Tube ULA?
>
> I was playing with an XMOS XS1-G4 development kit over Christmas. The
> XS1-G4 is a cheap 400MHz quad-core microprocessor with extremely fancy
> IO facilities that's designed for doing FPGA-like stuff. The thought
> occurred at the time that it probably wouldn't be too hard to knock up
> an XS1 coprocessor for the BBC, with the chip emulating the ULA (and any
> external CPU you like) in software...

You're not going to need anything so fancy to just do the ULA (altho the 
complete caboodle with it sounds good!). A CPLD may do it, but not one of 
the smaller ones. Register 1 is a 24 byte FIFO (and Reg 3 is  a 2 byte 
FIFO), so that's 208 registers before you even start on the others, status 
registers and control logic, altho the app notes say that R1 is only used 
for 10 byte VDU command length transfer, so maybe the size of it could be 
reduced without too much affect on performance. You could always place the 
RAM outside the CPLD, but that then defeats the "single chip" neatness. 
There are certainly chips that it will fit into with ease, it's just a case 
of balancing the cost to make it worthwhile doing.

Mark.