Date   : Tue, 23 Aug 1994 11:14:51 +0000
From   : lamcw <lamcw@...>
Subject: Re: Mine did something! Eeek!

Bonfield James <jkb@...> wrote:


;Anyways, who's to say which emulator is correct? I guess if there are any
;differences then you can at least say something's wrong somewhere!

this is a snag. it is possible to get the thing *working* with a whole host
of bugs in the 6502 emulation - i know from experience! when i get my
mac wired to the beeb, i'll be able to zap over loads of progs and we'll
soon see whether it really works or not.


;I've finally remembered to bring my Adv U.G. in today. I'll have another bash
;at finishing my 6522 code ;-) Has anyone got the datasheets for it? Does
;anyone know the difference between ORA and ORA non-handshake? The AUG has a
;bit about it in section 22.4.2, but is pretty wooly. It seems to be implying
;that non handshake mode has does not use the CB2 interrupt flag. Is this only
;on outputs? Is it possible to input by writing to a non-handshake pin on the
;6522 that doesn't generate a CB2 interrupt? It'd help enormously if the AUG
;had bothered to give the pin out for the chip.

i feel this is all rather irrelevant. i've managed to get
interrupts and the 'hardware' keyboard working by emulating just 4 
system VIA loactions. these are &FE45 (i think) to load T1 counter;
&FE4D the IFR, &FE4E the IER and &FE4F which is the port to the keyboard.
all other locations are just treated as RAM.

beeb snapshot formats were discussed a while back. naturally the contents
of the paged io needs to be dumped but what is dumped will depend on
how you emulate the hardware. therefore to get one snapshot to work on
more than one emulator is probably unlikely unless an overall strategy is
agreed upon.

my philosophy for handling the paged io is to do all the emulation when
values are STORED to sheila. therefore any reads from sheila will be 
correct and one does not need to 'intercept' the read.

a really invaluable (but somewhat unheard of) book is 'guide to the bbc
roms' by don thomasson, published by melbourne books. there wasn't a hope
in hell of me getting this far without it.


chris lam.