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8502 and Z80A

I know that the C128 has two CPUs. Is it possible to run the 8502 and the Z80A in parallel? If yes, how would you do that?

Not at the same time, they

Not at the same time, they are on the exact same address and data bus.  Well now that I have said that there might be a way... :)  If you turned off the VIC chip and ran the Z80 during the VIC times (PHI Low) and the 6502 during Phi High.    The Z80 is somewhat PHI level agnostic with the exception that there is some special logic done in jungle gates that may have to be told it resides on the low instead of high level PHI. 
The IO routines are only implemented in 6502, I.E. the Z80 makes kernal calls for disk.

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