Re: C-128 and Jeri board
Date: 2001-05-03 06:26:26

On Wed, 2 May 2001, Professor Dredd wrote:

> Symptomatic of the problem inherent in closed and
> semi-closed architectues like the A1200 and C64.
> Eventually you want to expand, but how?
> I think I read somewhere that the expansion port was
> originally intended to house a BASIC enhacement cart,
> because Commodore intended to dump BASIC 2.0
> eventually. The SuperExpander was released, but
> Simon's BASIC was far superior.
> Then, came the REU. Now, how did Commodore expect
> users to write REU software with the new BASIC? You
> can only have one cart plugged in at a time.
> This is where the Apple II really has an advantage
> over the C-64. 7 slots available and each gets its own
> I/O block and seperate variable block. Pretty damn
> slick if you think about it.
> --- Jeri Ellsworth <> wrote:

and all in 4k of space!  The apple][ onboard I/O was in the first 128 bytes
of space at $c000.  This was, mind you, not at all cleanly decoded.  I think
it stepped by $10 bytes for each location ($c000 is read keyboard, $c010 is
clear keyboard strobe, etc).

Then, the 7 slots were provided with the following:

256 bytes of PROM space at $Cx00 (where x is the slot number) for a basic
driver.  The most popular example is the disk controller, which has a
bootstrap rom at that location.

16 bytes of I/O at $C0(9+slot#)0.  This is where the control logic is held
for your card. ;)

The slots also share a 2k shared expansion space at $c800-cfff.  A card's
expansion rom is enabled simply by accessing its prom space, and all roms
are turned off by accessing CFFF.  To get your 2k space, simply access CFFF
(shich turns  all roms off includiing yours), and jump to the rom (and the
reading of the jmp instruction turns the space back on, but for your card
only).  The 80column firmware uses this. ;)

Its a 'mature' way of dealing with expansion, something that gained the
apple][ series a lot of recognition in the 8bitta industry.  The c64, nor
the atari were never cloned by other companies, but the apple ][ was ;)  I
guess they had it down pat.

Going with a proprietary setup like this wouldnt gain us anything unless we
literally used the apple][ implementation.  But then, we'd just have an
apple][ with a c64 sound and video chip ;)  Lots of old and fun hardware
floating about though (!), an in-place standardized port driver structure..
a nice mature DOS..

Ok, I'll stop now.


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