Re: Which //gs to buy?

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

Francine Smit <francine@xs4all.nl> wrote:

> Supertimer <supertimer@aol.com> wrote:
> 
> > The ROM 3 will also allow you to plug a card into slot 4
> > and still use the ADB mouse in GS/OS.  That effectively
> > gains you one expansion slot, which is very very useful.
> 
> yes, this sounds interesting. i have a home build card which
> uses slot 4 on a ][+ and i can place it in slot 4 of the IIgs
> and still use the mouse then.
> the card is adressed hard wired on the ][+ and cannot be used
> in an other slot.
> will the card work the same way on the IIgs?

It depends how you are decoding the address.  If you have hardware to
explicitly decode $C4xx or $C0Cx from the address bus and you are
completely ignoring the DEVSEL and IOSEL signals, then it will not work.

The IIgs doesn't output a valid address to the slot bus during all
cycles, particularly when running in Fast mode.  Even in slow mode, you
could see what you think is your address but the code might be accessing
another RAM bank (the I/O space only exists in banks $00, $01, $E0 and
$E1).

All cards must use DEVSEL, IOSEL or IOSTROBE to decode addresses, or use
the IIgs-specific M2SEL (Mega II Select) to identify when an access is
being performed to the appropriate areas.  M2SEL is on pin 39, which was
the microprocessor sync for the II+ and IIe.

There is also a M2B0 signal, which can be used to distinguish between
banks 0 and 1 or $E0 and $E1.  This signal is only present on slot 3 of
the original IIgs motherboard (ROM 0 or ROM 1), but is on all slots
except 7 for a ROM 3.

(This signal is mainly needed by video cards such as the Video Overlay
Card and Second Sight.)

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