command in the CCW. After the communication blocks have been set up, the CPU enables the
channel attention signal to inform the IOP to start its I/O operation. e CPU then continues
with its own program. e IOP responds to the channel attention signal by placing the address
of the control block into its program counter. e IOP refers to the control block and sets the
busy fl ag. It then checks the operation in the CCW. e PB (parameter block) address and TB
(task block) address are then transferred into internal IOP registers. e IOP starts executing
the program in the task block using the information in the parameter block. e entries in the
parameter block depend on the I/O device. is, in short, is how ...
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