Re: Original IIgs ROM

[david@uow.edu.au (David Wilson)]

dempson@actrix.gen.nz (David Empson) writes:
>If you intend to burn a physical ROM from this, my recollection is that
>the high order address bit to the ROM is inverted on the original IIgs
>motherboard, which means that bank FF goes _before_ bank FE in the
>physical ROM.  This isn't obvious from the circuit diagram, but I think
>I discovered it when I tried to read the ROM in an EPROM programmer.
>(This would apply to both ROM 0 and ROM 1.  I've never looked at the
>innards of ROM 3 in this manner.)

I thought that might be the case! I have a book with a cover picture of
a portion of the //gs motherboard showing a 27512 EPROM in the ROM socket.
The //gs technical reference shows the A16 line doubling as /OE (output
enable [active low]). For this to work with a 27512 (during development
of the //gs [perhaps before the toolbox was completed]) bank FF would
have to be configured with A16 low.

I do not know of any 128KB ROMs that could be burned to replace the ROM
already in the socket - 128KB EPROMs have more pins. 
--
David Wilson  School of IT & CS, Uni of Wollongong, Australia