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Taking a big step back, it's important to remember that unlike PC processors, mainframe processors do not do I/O. These big frames have dedicated I/O processors called channels. So the mainframe processors are unencumbered and can run at full speed. They issue CCW's (channel command words) which are fielded by a controller and are routed to the storage device.

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Do you consider talking with the co-processor to not be IO? If not what what counts as IO?

The co-processor runs channel programs autonomously. So "talking" to it is not necessarily bounded by your I/O volume. It's more like passing a quick message to a separate thread to run some operations, and then the other thread can do quite a lot of work.

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