View source for CDC 6600

===Peripheral Processors (PPs)===
To handle the "housekeeping" tasks, which in other designs were assigned to the CPU, Cray included ten other processors, based partly on his earlier{{When|date=May 2022}} computer, the CDC 160-A. These machines, called Peripheral Processors, or PPs, were full computers in their own right, but were tuned to performing [[Input/output|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 [[Channel 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<ref>This description covers early versions of CDC software; later versions used the Central Exchange jump (XJ) instruction to reduce the overhead for functions that could be performed entirely in the CP.</ref> 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.{{clarify|reason=Distinguish between roles assigned by the hardware, of which there is only one, and role assigned by the software.|date=March 2026}} 

The unique role of PP0 in controlling the machine was a potential [[single point of failure]], in that a malfunction here could shut down the whole machine, even if the nine other PPs and the CPU were still functioning properly. Cray fixed this in the design of the successor 7600, when any of the PPs could be the controller, and the CPU could reassign any one to this role.{{When|date=May 2022}}{{disputed inline|PP0?|reason=The fix did not require a hardware redesign, only software.|date=March 2026}}

Each PP included its own memory of 4096 [[12-bit computing|12-bit]] words. This memory served for both for I/O buffering and program storage, but the execution units were shared by ten PPs, in a configuration called the [[Barrel processor|Barrel and slot]]. This meant that the execution units (the "slot") would execute one instruction cycle from the first PP, then one instruction cycle from the second PP, etc. in a round robin fashion. This was done both to reduce costs, and because access to CP memory required 10 PP clock cycles: when a PP accesses CP memory, the data is available next time the PP receives its slot time.

====Central Processor access====
In addition to a conventional instruction set, the PPs have several instructions specifically intended to communicate with the central processor.<ref name=6000Ref />{{rp|pp.4-24–4-27}}
* <code>CRD d</code> - transfers one 60-bit word from central memory at the address specified by the PPs ''A'' register to five consecutive PP words beginning at address ''d''.
* <code>CRM d,m</code> - similar to CRD, but transfers a block of words whose length was previously stored at location ''d'' into PP memory starting at PP address ''m''.
* <code>CWD d</code> - assembles five consecutive PP words beginning at location ''d'', and transfers them to the central memory location specified by register ''A''.
* <code>CWM d,m</code> - transfers a block starting at PP [[memory address]] ''m'' to central memory. The central memory address was stored in register ''A'', and the length was stored at location ''d'' prior to execution.
* <code>RPN</code> - transfers the contents of the central processor's program address register to the PP's ''A'' register.
* <code>EXN</code> - ''Exchange Jump'' transmits an address from the ''A'' register and tells the processor to perform an ''Exchange Jump'' using the address specified. The CP Exchange Jump interrupts the processor, loads its registers from the specified location and stores the previous contents at the same location. This performs a task switch.<ref name="6000Ref">{{cite book |last1=Control Data Corporation |title=Control Data® 6000 Series Computer Systems Reference Manual |id=60100000 |date=July 1965 |url=http://bitsavers.org/pdf/cdc/Tom_Hunter_Scans/6000_Series_Computer_Systems_RefMan_Jul65.pdf |access-date=March 28, 2023}}</ref>{{rp|pp.3-9–3-10}}