The Central Processor of PDP-3 contains the Control Element, the Memory
Buffer Register, the Arithmetic Element, and the Memory Addressing
Element. The Control Element governs the complete operation of the
computer including memory timing, instruction performance, and the
initiation of input-output commands. The Arithmetic Element, which
includes the Accumulator, the In-Out Register, and the Carry Storage
Register, performs the arithmetic operations. The Memory Addressing
Element which includes the Index Adder, the Program Counter, and the
Memory Address Register, performs address bookkeeping and modification.