Date   : Wed, 27 Feb 2008 13:49:54 +1100
From   : msmcdoug@... (Mark McDougall)
Subject: BBC FPGA and B flags

Hi,

Thought I'd start a new thread with some good news!

It appears to be the case that my woes over the last few weeks have indeed 
been caused by a bug in the 6502 core that I'm using.

I did a quick "hack" to the core (as I don't know how to fix it properly) 
and to my relief, the behaviour of the BBC has improved! Generating a 
Mistake, for example, does eventually return to the prompt, albeit with a 
few lines of BASIC keywords listed. After a few times, it hangs.

However, and perhaps more tellingly, my "soft reset" will actually re-boot 
the BBC to a clean BASIC prompt - something which has eluded me before now.

Looking at the B "flag" on the logic analyzer output after compiling my 
"fix" shows the flag toggling in various states somewhat erratically, so I'm 
not at all surprised at the observed behaviour. At least now it's not stuck ON!

I'm eager to hear from the 6502 core author - and hopefully he'll have a fix 
that will result in a reasonably functional BBC-on-a-chip!

Functionality lacking atm includes:

* cursor - I intend to add this to the 6845 core and video ULA core
* MODE 7 - I can force it into a hybrid non-interlaced mode atm, but the 
6845 core doesn't support interlace mode, so proper support is pending
* sound - pending
* user VIA - no need for this without attached peripherals atm
* disk controller
* cassette input
* tube, speech, and other fancy bits

Regards,

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