Re: PET 4040 disk drives
Date: 2000-09-15 21:37:26

Hallo Bruce,

> I've only just joined this mailing list so apologies if these questions have 
> been asked before.

We are very freindly people indeed. So we'll answer your question for the 3 
zillionth time :)

> The drive uses 2 processors - a 6502 which I assume handles the DOS, and also 
> a 6504. What does this one do?

The 6504 only handles the real disk I/O: reading data from/writing data to the 
floppy. The other one does the communication with the computerer and things 
liking file management.
> It looks as though the CPUs share a common 4K RAM buffer, and as I can't see 
> any other signal control lines, I guess there must be some kind of handshake 
> protocol that's used to communicate between the two. ie. certain byte values 
> are stored in certain memory locations, to instruct whichever processor to 
> carry out a task. Is this the way it works? If it is, does anyone know what 
> byte codes do what?

The 6504 runs on the inverted clock of the 6502. In this way they can run 
parallel and access the same memory. But each processor sees it at another 
address. They communicate with eachother as you already guesed, by exchanging 
bytes. But don't ask me which ones.

Sorry, Hope to answer the rest next time.

Groetjes, Ruud

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