Date   : Wed, 10 Aug 1994 09:23:56 +0100
From   : jfid@... (James Fidell)
Subject: Re: Thicko Strikes Again

"Chris 'Maths Lover' Rae" wrote:

> Yep folks, it's me again with some more "he doesn't even know that?!?" 
> questions for y'all...   ;-)
> 
> I now have the BBC ROMs loaded into my emulator and working okay; I'm 
> working steadily through the ROM code and so far my 6502 appears to be 
> working okay. It's slow progress, though...
> 
> Anyway, I've got a few questions which have arisen since I started trying 
> to get the ROMs working.
> 
> 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?

&258 holds the value from an OSBYTE 200 call. Bit 0 is ESCAPE disable
and bit 1 indicates that memory should be cleared on BREAK.

If it's being read when your emulator ``powers up'', I don't think it
should because the OS should already have worked out that it's in a
power up state by that time.

> 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?

I personally don't think you can.  Certainly I wasn't able to get my
emulator working without emulating some of the system VIA.

Since you mention them,

       FE40    (ORB, IRB) Input/Output register B.  Gets involved in sound,
                       speech, keyboard scanning, screen display, CAPS/SHIFT lock
                       control, joystick FIRE buttons.

       FE42    (DDRB) Data Direction register B.  Sets which lines in FE40 are
                       inputs and outputs.

       FE43    (DDRA) ditto for register A.

       FE4E    (IER) Interrupt Enable register.  Allows/disallows interrupts
                       for a number of different conditions which can occur in the
                       VIA.

       FE4F    A non-handshaking copy of DDRA (which is at FE41).

James.

-- 
 "Yield to temptation --             |
  it may not pass your way again"    |     jfid@...        
                                     |
        - Lazarus Long               |              James Fidell