Re: Apple IIx

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

Jeff Blakeney <CUTblakeney@home.com> wrote:

> On Tue, 29 Jun 1999 18:02:49 -0500, Rubywand <rubywand@swbell.net>
> wrote:
> 
> >     Another bad decision was sticking with the old Slots. A pure GS could
> >have had Slots with complete addressing and some useful interface signals
> >which would have made graphics expansion via a Slot card  feasible.
> 
> Don't the slots already have all the addressing stuff?

No.  I/O cards in slots cannot provide any more memory-mapped I/O than
in an 8-bit Apple II (16 bytes plus 256 bytes plus 2KB).  By mirroring
main and auxiliary memory (requires the M2B0 signal), something like the
Video Overlay Card becomes possible.

This is still too limiting for more advanced video memory
implementations.

The speed is another major problem.  If the IIgs supported fast cycles
on the slots then I/O bandwidth could have been improved significantly.

Doing something like the IBM AT's extension of the XT's slot would have
been useful.  The extra pins could provide access to fast memory cycles
and a larger address space (by supplying the bank address in some form).

For example, the memory space between banks $80 and $DF could have been
mapped to six slots, giving a one megabyte address space for each slot
to use.  (I'm not sure which slot should miss out.)

Similarly, banks $E2 through $EF could have been made available for
slots to implement (128KB each).

This would be ideal for additional video buffers.

> After all, the 65816 only has 16 address lines and 8 data lines.  The data
> lines double as both the high byte of the address and as the data bus.

The bank information is not available to the standard slots.  The slot
bus conforms to the 8-bit machines, which means that the data bus is not
driven during the first half of each cycle (hiding the video data from
I/O cards).

In addition, the slot addressing signals are only valid while the
processor (via the FPI) is doing a slow (1 MHz) cycle.

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