Date   : Sat, 16 Feb 2008 12:01:30 +1100
From   : msmcdoug@... (Mark McDougall)
Subject: BBC FPGA Boots to BASIC... almost...

Tom Walker wrote:

> At this stage it is most likely the System VIA that is causing problems. MOS
> doesn't need interrupts working to get to the prompt, but the IO ports do
> have to work. Probably worth checking that the speech/joystick/keyboard
> inputs are all simulated as 'not pressed' or 'no event' or whatever.

The keyboard is "no event" atm - I know that because it reads the option
links for MODE 6 correctly, and I just fixed a bug in which it looped
forever waiting for the user to release the CTRL key...

IIRC the joystick fire buttons are active low, but I'm not sure of the sense
of the two speech inputs... since the schematics don't designate active low
naming I'm assuming they're active high??? I guess I should check the MESS
source further...

I also notice in the MOS disassembly the comment that the VIA is set to
interrupt on the negative edge of VYSNC and the VSYNC output of the 6845 is
active high - which of course means that the interrupt happens on the
falling edge of VSYNC. I would've thought you'd design it to interrupt on
the leading edge, to give a few more scanlines of time in an ISR before the
video started to redraw...?!?

Regards,

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