Re: Beyond 8 meg limit?

[Eric Jacobs <no@no.no>]

Joseph Turner wrote:
> 
> Midd33 (midd33@aol.com) wrote:
> :
> : I was reading Steve Weyhrich's report on the Apple II History.  In it he
> : discusses memory expansion and that with some sort of patch, the GS can
> : recognize beyond 8 megs of ram. I think it was up to 12.
> :
> : I was wondering if anyone has ever looked into this. I have since sold my
> : GS and went to a Mac, but I'm still interested to know.
> :
> :
> : Andy
> 
> I got the good impression the *meg limit is die to the hardware
> configuartion, i.e. not enouogh lines to decode extra banks.

I very much doubt that. The 65816 puts the bank address on the data lines during
the first half of every cycle. Since all data lines are accessible to memory card,
it would be possible to decode all 256 banks.

You can see this for yourself. Go in the monitor and try to access a bank that you
don't have. You'll get back the bank number in return.

Probably what Apple did is use the high order bit to decide whether to route the
access to expansion RAM (basic 256k and up, banks $00-$7F), or to the motherboard
(slow RAM, and ROM, banks $E0,$E1,$FE, and $FF). Still, as I recall there was some
mechanism for expansion ROM addressing in the memory card slot. It may be possible 
to use this for adding more RAM instead.

-ej