Bryan Parkoff <BParkoff@satx.rr.com> wrote:

>     Why did Steve Woz, Steve Jobs, and other engineers design Apple IIgs
> motherboard to put seven slots including RAM expansion slot which slot 3
> is not used.

(Steve Jobs wasn't an engineer, and it is Steve "Wozniak" - "Woz" is
just his nickname and signature.)

The IIgs motherboard has eight slots - seven standard slots, plus the
memory expansion slot.  The memory expansion slot is completely
independent of the standard slots, and doesn't get in the way of them
(unless you have a "wide" card in slot 7 or in the memory expansion
slot).

Similarly, the IIe motherboard has eight slots - seven standard slots,
plus the auxiliary slot.  The auxiliary slot has some connection with
slot 3, because it is used for the extended memory required to implement
the built-in 80-column support, which is located in the internal $C300
firmware address space.

In an international IIe, the auxiliary slot is lined up directly in
front of slot 3, and it impossible install a card in slot 3 if there is
a large card in the auxiliary slot.

>     For example, I have to turn off 80-column by selecting 'Your Card'
> instead of 'Built-In' in slot 3 in the Control Panel.  Slot 3 should have an
> ability to operate with any controller card.  Please advise.

How does this differ from any other slot?  In the IIgs, every slot has a
built-in function which you may have to disable if you want to use a
card in the corresponding slot.

Slot 1: Printer port
Slot 2: Modem port
Slot 3: 80-column firmware
Slot 4: Mouse driver
Slot 5: SmartPort (/RAM5 and 3.5" drives)
Slot 6: Disk Port (5.25" drives)
Slot 7: AppleTalk in ROM 00/01 (effectively no conflict in ROM 3)

The key issue is that the design of the 8-bit Apple II system (including
the ProDOS, DOS 3.3 and Apple II Pascal operating systems) is dependent
on the firmware provided by the slots in the $Cn00-$CnFF space (where
'n' is the slot number).

In order to support peripherals in a standard way, the built-in
functions must appear in the slot firmware space, which prevents the use
of most types of I/O card in the same slot.

Most I/O cards are also dependent on the I/O locations in the
$C090-$C0FF area (16 allocated to each slot).  The IIgs's built-in
peripherals only use this area for slot 6 (also shared by slot 5), so
most of the slots are wasting this area.  (It is possible to access this
area for a real card in slot 3 or 4 while the built-in function is
enabled, but for some reason the same doesn't apply to slots 1, 2, 5 or
7.)

Certain types of cards can be installed in any slot without losing the
built-in functionality.  The main examples are accelerators and stereo
output cards.

In some cases, the built-in functionality may still be available to some
extent, even with the slot fully enabled.

For example, on a ROM 3 IIgs, it is  possible to use the ADB mouse in
GS/OS applications even if slot 4 is set to "Your Card" (but you do lose
mouse support in ProDOS-8 applications).

If I remember right, GS/OS also allows access to /RAM5 even if slot 5 is
set to "Your Card", as long as the native GS/OS RAM5 driver is
installed.

In theory, GS/OS could have eventually supported all the built-in
peripherals and all the slots at the same time, as Apple had designed a
"slot arbiter" mechanism.  It was never fully implemented, probably
because of compatibility issues with too much third party software.

-- 
David Empson
dempson@actrix.gen.nz