Re: Beyond 8 meg limit?

[pubpc1@library.ucla.edu]

midd33@aol.com (Midd33) 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

Andy, the Applied Engineering RAM Keeper board can do this.  It plugs
into the expansion slot and you can in turn plug in two narrow profile
(eg. Sequential Systems' RAM GS Plus) 8MB memory cards into the two
RAM Keeper pockets.  The GS will then have 16MB on board.  Of the 16MB,
8MB will be usable as system RAM.  The other 8MB can be divided up
between the RAM disk and the ROM disk.  The ROM disk is backed up by
a battery.  [To be exact, you'll be able to recognize slightly less
that 16MB because some of the GS' 24-bit memory space is reserved,
but the point is you can get LOTS of memory!]



Eric Jacobs <no@no.no> wrote:
>Joseph Turner wrote: [regarding 8meg limit on regular GS RAM]
>> 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

Yes.  The GS supports ROM expansion through the memory expansion slot.
Specifically, the computer divides up its 24-bit memory address space
into 8MB for RAM and 8MB for ROM, with some reserved.  The RAM Keeper
card from Applied Engineering supported plugging in two narrow body
8MB expansion cards to give the GS 16MB of memory, divided between
RAM and ROM.  It basically supported ROM expansion by providing a
"ROM disk," a battery backed virtual disk.

-Scott G.