The 8250 (the equivalent disk drive for the PET) had two 6502s inside! In today's language, one was the application processor running DOS and the other was a D

The 8250 (the equivalent disk drive for the PET) had two 6502s inside! In today's language, one was the application processor running DOS and the other was a DSP that did the low level hardware monkeying; they were connected up to a shared data bus. Because the 6502 only does bus accesses on one phase of the clock, by inverting the clock for one processor, it was safe to connect them both up to the bus simultaneously with no loss of speed...