Re: Emulating a Z80, and, So that's why CP/M 3.1 is so slow...

[mjmahon@aol.com (Michael J. Mahon)]

Dosius wrote:

>We're working on emulating the Z80 in EMU][, which has come a lot
>farther than my attempts in Dapple.  Well, we have CP/M 3.1 booting,
>but not yet 2.2 (I have to figure how to implement some ops myself,
>possibly, and the code is very different from any of the other Z80
>cores I have).  Right now, I'm trying to get the "CP" opcode to work.
>
>Well, interestingly enough, while running CP/M-80 3.1 on EMU][, there
>was visibly a lot of alternating between the 6502 and the Z80, and, as
>with Apple Oasis, it ran like molasses.
>
>That would explain it...the frequent CPU switching probably slowed the
>system down to a crawl.  No wonder it dragged.

It's good to hear that your emulation is coming along.

But in a real system, switching CPUs is not a very expensive
operation, since, at least in the SoftCard model, each CPU is
just waiting to be "tagged" while the other CPU is running.

Although the SoftCard scheme was speed-limited by sharing
the Apple's memory, it at least had the advantage that no data
copying had to be done.  (I'm kidding, of course--having most
of the Apple's memory as an I/O buffer is sweet, as in the
AppliCard approach.)

-michael

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