In this setup, any register can be either of the ALU's two inputs, so address arithmetic can be done directly on the contents of the S register. One "register" is hard-wired to always contain the bit pattern for +1. There are no real flip-flops for this pseudo-register: connections to constant 1 and constant 0 provide the needed bit pattern and it is impossible to store anything back into it. Some designs also have a hard-wired "register" that stores 0, and perhaps another that stores 111....1.

The CSC-2 has eight registers, not 6 or 14, because it is convenient to have the number of registers be a multiple of two. Other designs may have 16, 32 or many hundreds.

The heavy lines coming out of and going into the registers represent 16 data wires. These are the buses. Multiplexors are used to connect the registers to the buses.