Re: AE RamKeeper?

[dempson@actrix.gen.nz (David Empson)]

Edhel Iaur, Esq. <edhel@LOSETHECAPITALWORDSbigfoot.com> wrote:

> Doesn't all the IIgs's ROM count as part of the 24-bit (.'. 16MB RAM)
> address limitation, making a limit?  I'm guessing the IIgs doesn't
> have 8MB of ROM, but a ROM-switching-in-and-out thingiey like in the
> //e et al. was probably never added to the IIgs, making the bulk of
> the IIgs's ROM pretty much stuck in place.  Assuming I'm right (am
> I?), what would the theoretical RAM limit be (I mean GS RAM, of
> course, not "Slinky", Titan, et al.)?

I've discussed this before.  One reason for the 8 MB RAM limit is that
this is all the Memory Manager knows about (it doesn't check banks
higher than $7F for RAM at startup time).  This could be patched after
boot, so I don't think this is a major problem.

A second reason (and the main one) is that the motherboard hardware
almost certainly doesn't produce memory decoding signals when accessing
memory above bank $7F, except for the areas which are designated for the
Mega II chip (banks $E0 and $E1) and the ROM (banks $F0 through $FF).

I say "almost certainly" because I've never actually verified this using
a logic analyser.  However, if this were not the case, I'm sure one of
the third-party RAM card designers would have busted the 8MB limit at
some point.  This has not happened.

For all IIgs software to be able to deal with extra memory, it needs to
be directly addressable in the 16MB address space of the IIgs, without
bank switching.  This rules out any IIe-like techniques.

(The areas of the IIgs which emulate the IIe bank switching are used by
operating system code or toolbox code that knows how and when to switch
banks.  General application code never gets to use these areas.)

It would certainly be possible for a special type of memory card to be
implemented which supports some kind of bank switching technique, but
support for it would be limited to those applications which could deal
with it (similar to the situation with the multi-bank auxiliary slot RAM
cards for the IIe).

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand