Date   : Wed, 15 Feb 1995 17:32:30 +0000
From   : lamcw <lamcw@...>
Subject: Re: Emulating I/O

Neil Pollard wrote:


;Firstly, I think I should get away with only emulating some bits between 
;FE40 and FE4F as far as I/O goes, but how much of this will I need to
do to get
;as far as the startup message? I haven't done any interrupts as yet either.
;At the moment, I'm intercepting OSWRCH to get character output, but I'm 
;only getting odd bits of rubbish through, although that may well be due 
;to bugs of my own. 

If you're only interested in 6502 emulation, then you definitely want to
avoid the 6522, the emulation of which is far from trivial. I think trapping
OSWRCH and OSRDCH(?) is good enough to get the system to a point where
you can type in BASIC commands. However, even if you get BASIC working, it's
far from conclusive that your 6502 emulation is 100% correct. For one thing,
BASIC does not use BCD for maths.


;I've also left BASIC permanently mapped in at 8000 - 
;any ideas whether that might cause problems later?

That's fine.



Chris Lam.