Date   : Wed, 10 Aug 1994 10:08:17 +0100 (BST)
From   : amh15@... (Alan Hart)
Subject: Re: Thicko strikes again

> What is stored at &258? As far as I have got in the emulator, it is 
> loaded but nothing has been saved in it - does it hold something special 
> or am I getting things wrong again?

The AUG says that &236-28F are used to store the values of the *FX settings 
for OSBYTE calls &A6 to &FF. Thus &258 would be OSBYTE &C8 or *FX200, which 
controls whether Break clears memory or not. A 1 in bit 1 indicates memory 
should be cleared; bit 0 indicates whether Esc is disabled or not 
(1=disabled).

> Do I *really* not have to pay attention to Sheila peeks/pokes? I was told 
> that I just had to start up the emulator with BASIC loaded in at the 
> paged ROM area (&8000-&BFFF) but is this really true? So far I have come 
> across addressing of FE4E, FE42, FE40, FE43, FE4F as well as FE30 (the 
> only one which I know what it does!). Can I really ignore all of these?

These appear to be the system VIA, which controls "keyboard, sound, speech, 
joystick fire buttons etc."

As far as I can tell, here are the Sheila addresses you refer to for the
system VIA:

40      Output/Input Reg B
41      Output/Input Reg A
42      Data Direction Reg B
43      Data Direction Reg A
...
4E      Interrupt Enable Register
4F      As Sheila &41 above, but no handshaking, whatever that means.

I think you should get a copy of the AUG ASAP!

Alan

 Alan Hart  -  amh15@...  -  University of Cambridge, UK