Re: second processor on a II:How?

[david@zircon.cs.uow.edu.au (David E A Wilson)]

dpscholn@mtu.edu (Dan P. Scholnik) writes:
>	I am thinking about making a 6809 interface card to hook up to a II+
>or IIe (basically to save me the trouble of building a whole system) and I
>was wondering how that was typically done, ala Z80 card and those 6502
>accelerators.

It has already been done (twice). I have the Stellation Mill card. It uses
a 6809E processor and puts the 65(c)02 to sleep with the DMA line whenever
it needs to access the bus. When it doesn't (VMA = deasserted) the 65(c)02 will
get the bus for a cycle or more. The end result is the 65(c)02 runs at approx
20% full speed and the 6809 runs at full speed.

>From the manual I see that pulling DMA low is the only way
>(on a II+, anyway) to take the busses away from the 6502. How could another
>processor take over the memory and leave the video pretty much as is?

All coprocessors only access the bus during phase 2 of the clock.

>I assume that the video can just be left alone to refresh the ram, but I don't
>think the 6809 sits idle for half a cycle like the 6502. Any suggestions
>from the many experts among you?

Yes, the 6809 has a similar bus cycle to the 6502 (which was based on the 6800).
The Q signal can be ignored from the point of view of peripherals & memory.

>	I might also just make it a separate system that can communicate to
>the apple via the slot. I just want a 6809 system to play with, plus the
>fun of designing it.

If I were designing a coprocessor card I would copy the idea of the Premium
z80 card for the //e. Make the card plug into the AUX slot and give it its
own memory so that the 65(c)02 can run at the same time and do the i/o.