There are three registers of 15 bit length which deal exclusively with addresses. The design allows for expansion to 18 bits. These registers are:
Memory Addressing
The Memory Address Register holds the number of the memory location that is currently being interrogated. It receives this number from the Program Counter, the Index Adder or the Memory Buffer.
Program Counter
The Program Counter holds the memory location of the next instruction to be executed.
Index Adder
The Index Adder is a 15 bit ring accumulator. The sum of an instruction base address, Y, and the contents of an index register, C(x), are formed in this register. This register holds the previous content of the Program Counter in the "jump and save Program Counter," jps, instruction. The Index Adder also serves as the step counter in shift, multiply, and divide.