The I-O subsystem and the CPU can function independently.
The CPU, however, has no way of knowing when the I-O subsystem has completed its work, unless the I-O subsystem tells it.
Thus, I-O subsystem sends a flash, a signal to the CPU. The System/360 is fitted with special interrupt hardware.
In the early 1950s, many organizations bought electronic computers such as the IBM 701(known as the Defense Calculator) and IBM 704.
There were only 19 IBM 701s manufactured in the world.
When his time was close, he would be ready with a deck of cards in his hand.
When the previous person finished, the next programmer rushed in, checked the printer board , the punch board, the reader board, hung a magnetic tape, punched his time in a mechanical clock, set the console switches, loaded the card deck, prayed that the first card wouldn't jam, and pressed the load button. There was always unused time between computer sessions.
With access-methods, you don't need to write your own channel programs.