Re: Beyond 8 meg limit?

[jkeim@unlinfo.unl.edu (James Keim)]

In article <33668F37.2B50@no.no>, Eric Jacobs <no@no.no> wrote:

> 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

Hmmm, I'd like someone to explain the possibility of going
beyond the 8 meg limit.  I think I've asked this same question
myself at some point in the past.  I don't remember what I was
told, though.  I've been under the impression that it was a
hardware limit.  However, I've also been told that the problem
would lie in a major rewrite of the memory manager.  I kinda doubt
anyone would be interested in taking on that task.

Now, an interesting point to ponder is how the folks at Apple
who are creating GUS, the IIgs emulator, managed to break the
8 meg limit in emulation.  GUS reportedly will allow you to run
an emulated accelerated GS with up to 14 megs of memory accessible.

Now this fascinates me because I am not technically proficient
enough to even begin guessing what allows them to do this.  
I'm going to though.  ;-)

   1. They have addressed the hardware limit in emulation. 
      (and current software still works)
   2. They have rewritten/revised the memory manager for the
      emulator [not sure where the main memory manager code is
      located ROM/RAM, so I don't know if this possible].
      (and current software doesn't break)
   3. Some combination of the above.
      (and current software still works)

Any other possibilities?  Hope no one minds me speculating here--
I really have no idea what I am talking about.

So, now I am wondering if any emulator "enhancements" could ever
trickle down to hardware/software improvements for the GS.