Each PP included its own memory of 4096 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 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.