Date   : Sun, 17 Feb 2008 11:46:38 +1100
From   : msmcdoug@... (Mark McDougall)
Subject: BBC FPGA Boots to BASIC... almost...

Tom Seddon wrote:

> I concur... The OS sets the PCR to have bit 0 clear, so the interrupt 
> occurs on negative edge of CA1. The 6845 sets CA1 high at start of vsync 
> and puts it low at the end, with position determined by 6845 R7 
> (specifies character row as I recall, the vsync starting on line 0 of 
> this row) and length of pulse according to upper nybble of 6845 R3.

Yup!

> And finally one resource you may find handy is the fully-commented OS 
> ROM disassembly, if you haven't already seen it. 

Been pouring over it the last week or so - thanks!!!

It's alive!!!
<http://members.iinet.net.au/~msmcdoug/pace/platforms/pace-p2/bbc-micro.jpg>
That's the FPGA board in the bottom left with a few LEDs...

I was returning an ACIA interrupt active due to not decoding it and 
returning &FE for all unimplemented SHEILA accessess... fixing that and I 
can now type in and run a BASIC program!

Thanks to all on this list who have chipped-in so far!
Still plenty of work to do!

Regards,


-- 
|              Mark McDougall                | "Electrical Engineers do it
|  <http://members.iinet.net.au/~msmcdoug>   |   with less resistance!"