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Comment by pkroll | Hacker News Reader

The 6502's two-phase clock throws a wrench in direct clock speed comparisons, no?

>The 6502's two-phase clock throws a wrench in direct clock speed comparisons, no?

Not really? The two phases are more or less just inverted versions of each other[1], but 1 clock cycle, for the purposes of counting cycles-per-instruction, is the same time (I think) whether you're measuring phase-1-high to phase-1-high or phase-2-low to phase-2-low.

[1]http://lateblt.livejournal.com/88105.html

The "two-phase clock" is just the alternate halves of one square wave. The 6502 accesses memory during one half and processes during the other. Only one clock signla is needed, and one instruction will be completed at the end of every complete clock cycle.

Fun fact: you can run a dual core 6502 system by hooking up the two processors to the same memory bus, one using an inverted clock. (Slightly more complicated than that, but only slightly.)

The Commodore Pet disk drives used this trick; one processor was the application processor, which listened for IEEE488 commands and handled the disk format; the other processor handled the low-level stuff (we'd call it a DSP today).

Yes, the Pet disk drive did have twice the number of processors as the computer it was attached to.

http://www.6502.org/users/andre/petindex/drives/arch/index.h...

And then you got the Amiga that apparently took that a step further by having the chipset act independently of the cpu, with a but of circuitry sitting in the middle to make sure they didn't both access the shared ram at the same time.

It's immensely charming that you are keeping lit the flame of this three decade old argument.

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