Re: Not your father's Applicard

[David Wilson <mcs6502@gmail.com>]

On Jan 12, 1:33 pm, Steven Hirsch <snhir...@gmail.com> wrote:
> I'm always up for a challenge, so why not?  The first order of business is to
> find some architectural information on the 6809 Mill.

Not a problem - the circuit diagram is published in the documentation
that came with it.

> The second is to
> reverse engineer the little piggy-back board that made it possible to run OS-9
> on the Apple.

No problem there either - I built my own to enable switching between
OS-9 and non-OS-9 modes via software instead of needing to take the
lid off and flick the little switch. The card is just a simple 4 bit
in 4 bit out to remap the upper bits of the address bus. Without the
board, the Mill has two address mappings:

0xxx..Fxxx => 0xxx.Fxxx (straight through)
0xxx..7xxx,8xxx..Fxxx => 8xxx..Fxxx,0xxx..7xxx (invert A15)

The daughter board remaps the 16 4KB banks in a similar fashion to the
Z80 card giving OS-9 a larger contiguous address space than either of
the original mappings.

>  My
> CoCo has 512K installed and it's able to use all of that by swapping these
> chunks into the 64k address space as needed by the currently running task.
> Task switching is triggered by a hardware timer interrupt.  Quite
> sophisticated for a little machine.

The Mill OS-9 is only Level 1 so does not support more than 64KB. The
timer interrupt is not available on a standard Apple II so I think it
fakes it somehow.