To handle the "housekeeping" tasks, which in other designs, were assigned to the CPU, Cray included ten other processors, based partly on his earlier computer, the CDC 160-A. These machines, called Peripheral Processors, or PPs, were full computers in their own right, but were tuned to performing I/O tasks and running the operating system. (Substantial parts of the operating system ran on the PP's; thus leaving most of the power of the Central Processor available for user programs.) Only the PPs had access to the I/O channels. One of the PPs (PP0) was in overall control of the machine, including control of the program running on the main CPU, while the others would be dedicated to various I/O tasks; PP9 was dedicated to the system console. When the CP program needed to perform an operating system function, it would put a request in a known location ("Reference Address" + 1) monitored by PP0. If necessary, PP0 would assign another PP to load any necessary code and to handle the request. The PP would then clear RA+1 to inform the CP program that the task was complete.