As I recalled, you wrote channel instructions, or chains of channel instructions, as "CCW"s (Channel Command Words), which you dispatched to the channels via a "SIO" or "SIOF" instruction (Start I/O, or Start I/O Fast).

Once the operation started, the SIO(F) returned to you and the channel operated asynchronously from the CPU, moving the data directly in or out of memory. I think SIOF returned as soon as the channel received the instruction, whereas SIO waited for the instruction actually began.

So the channel actually added as sort of co-processor with the same access to memory as other CPUS. I'm pretty sure the channel could use virtual addresses, but the memory of course had to be fixed (pagefixed) during the operation.