Editing CDC 6600 (section)

==Peripheral processors ==
The CPU could only execute a limited number of simple instructions. A typical CPU of the era had a [[Complex instruction set computer|complex instruction set]], which included instructions to handle all the normal "housekeeping" tasks, such as memory access and [[input/output]]. Cray instead implemented these instructions in separate, simpler processors dedicated solely to these tasks, leaving the CPU with a much smaller instruction set. This was the first of what later came to be called [[reduced instruction set computer]] (RISC) design.

By allowing the CPU, peripheral processors (PPs) and I/O to operate in parallel, the design considerably improved the performance of the machine. Under normal conditions a machine with several processors would also cost a great deal more. Key to the 6600's design was to make the I/O processors, known as ''peripheral processors'' (PPs), as simple as possible. The PPs were based on the simple 12-bit [[CDC 160 series|CDC 160-A]], which ran much slower than the CPU, gathering up data and transmitting it as [[Burst mode (computing)|bursts]] into main memory at high speed via dedicated hardware.

The 10 PPs were implemented virtually; there was CPU hardware only for a single PP.<ref name=HW6000>{{cite book
|url=http://bitsavers.trailing-edge.com/pdf/cdc/cyber/cyber_70/60100000AL_6000_Series_Computer_Systems_HW_Reference_Aug78.pdf 
|title=Control Data 6000 Series Hardware Reference Manual |date=1978}}</ref>{{rp|pp.4-3 thru 4-4}} This CPU hardware was shared and operated on 10 PP register sets which represented each of the 10 PP ''states'' (similar to modern [[temporal multithreading|multithreading]] processors). The PP ''[[barrel processor|register barrel]]'' would "rotate", with each PP register set presented to the "slot" which the actual PP CPU occupied. The shared CPU would execute all or some portion of a PP's instruction whereupon the barrel would "rotate" again, presenting the next PP's register set (state). Multiple "rotations" of the barrel were needed to complete an instruction. A complete barrel "rotation" occurred in 1000 nanoseconds (100 nanoseconds per PP), and an instruction could take from one to five "rotations" of the barrel to be completed, or more if it was a data transfer instruction.