Registers

Processor registers temporarily store data values and serve as input to and output from a variety of instruction operations, including data movement to and from memory, arithmetic, and bit manipulation. Most general-purpose processors include instructions for shifting binary values stored in registers to the left or right and for performing rotation operations in which data bits shifted out one end of the register are inserted at the opposite end. The rotation operation is similar to the ring counter, except the bits in a rotation can hold arbitrary values, while a ring counter typically transfers a single 1 bit through the sequence of locations. Circuits performing these functions are constructed from the low-level gates and flip-flops presented earlier in this chapter.

Registers within a processor are usually written and read in parallel, meaning all the bits are written or read on individual signal lines simultaneously under the control of a common clock edge. The...