Re: Trying to cram GS/OS into a walnut

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

Supertimer <supertimer@aol.com> wrote:

> jonINTERNETrelayCHAT@napaVALLEYnet.CLARInet (Jon Bettencourt) wrote:
> 
> >If I were to make a ROM disk for the IIgs, how would I do it? Can it be as
> >big as 6.5MB? In what format should I save it in? Would it be the same as
> >an HDV file? I'm feeling like putting the complete GS/OS, plus a few extra
> >things, on there and booting off it.
> 
> There are two types of ROM disks.  Apple IIGS ROM disks and
> Apple II ROM disks.  Apple IIGS ROM disks are addressed
> through the memory expansion slot.  The ROM disk can be up
> to a little less than 8MB in size (8MB minus the amount of
> ROM on the IIGS).

Are you sure about that?  The IIgs memory slot design allows for a ROM
disk which occupies banks $F0 through $F7, and it can be extended up to
the motherboard ROM (which breaks Apple's rules for reserved pages).

This means that a ROM disk can officially occupy 512K, and unofficially
768K (ROM 3) or 896K (ROM 1).

A specially designed card could use bank switching to increase the
amount of ROM available, but it can't use banks below $F0, because the
enable signal doesn't go active for these banks.

> I don't think there were any genuine ROM disks,

There was at least one early card which had genuine ROM disk support.
There was also the OctoRAM ESP, which was a static RAM disk with battery
backup which pretended to be a ROM disk (and could be write-protected
with a jumper).  It was a piggyback card for the OctoRAM.

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