The CDC 6600 was designed by Seymour Cray as soon as work had completed on the CDC 3600[?], a much smaller computer. With sales of their other machines doing well, CDC allowed Cray as much time as he liked to build his next design.
The basis for the 6600 is what we would today refer to as a RISC system, one in which the processor is tuned to do a small number of instructions as quickly as possible. At the time the CPU's for most mainframe computers included instructions for things like operating the paper-tape machine, and reading punch cards. Instead Cray put only the most basic math and logic instructions in the CPU of the 6600.
To handle the "day to day" tasks which other designs put in the CPU, Cray designs another smaller machine. These machines were full computers in their own right, but in this case tuned only to perform these I/O tasks. When the main CPU needed to perform some sort of I/O, it instead sent (or loaded) a small program into one of these other machines.
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