Re: RAM limit for IIGS?

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

In article <4jjc97$ht9@borg.svpal.org>,
Paul Creager <wily@svpal.svpal.org> wrote:
> Ben Lai (Ben_Lai@mindlink.bc.ca) wrote:
> : What is the maximium MB of RAM that is usable by an Apple 
> : IIGS?  8 Megabytes?
> 
> : I have been trying to figure this out for a while but 
> : I keep getting different answers from people.
> 
> 8 MB is correct.  What may be confusing is that the 65816 has a 16 MB 
> directly addressable limit (24-bit address bus), but Apple only 
> implemented 23 lines, thus limiting it to 8 Megs.

Wrong.  The IIgs only supports the lower 8 megabytes for fast RAM, but
there are parts of the upper 8 megabytes of the address space: banks
$E0 and $E1 contain the 128K of slow RAM, and banks $FE and $FF
contain the ROM (on the ROM 1 - ROM 3 also uses banks $FC and $FD).
It is also possible for a card in the memory expansion slot to provide
additional ROM (in the form of a "ROM disk") in banks $F0 through $F7,
possibly also $F8 to the bank before the ROM.

One reason for the 8 megabyte limit is that the IIgs Memory Manager
assumes that the fast RAM cannot go past bank $7F.

I suspect that the hardware also has a limitation: it probably doesn't
provide row and column address strobe signals to the memory expansion
slot if you try to access banks $80 through $DF, which prevents the
circuitry on the card from receiving an address.

I haven't verified this (it would require the use of a logic analyser,
and work's one is away getting fixed at the moment) but it seems
likely.  The hardware would have to prevent the fast RAM from being
selected if you access banks $E0, $E1 or $F0 through $FF, and the
easiest way would be to prevent the generation of the strobe signals
for an access with bit 23 set ($800000) set.

> I _think_ you can also have up to 1 or 2 Megs of ROM in addition to the 
> RAM.  However the memory card has to be specially designed to provide the 
> ROM and I'm aware of none ever being made for that purpose.  (ROM and RAM 
> on the same card.)

Definitely were some early ones - I remember seeing a picture of one
card that had four EPROM sockets as well as 1 megabyte of RAM.

The only card I know of that went anywhere with it was the OctoRAM ESP,
a piggy-back card for the OctoRAM, which provides up to 512K of
battery-backed static RAM as a "ROM disk", with a jumper to
write-protect the disk.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand