A register is a small unit of very fast memory within the central processing unit (CPU) that holds data temporarily for immediate use by the CPU during arithmetic and logic operations. It helps speed up processing by eliminating the need to retrieve frequently used values from main memory every time the CPU needs them. A register can also be found in embedded systems, where they act as the “brain” or core processing component of smaller computer devices such as mobile phones and household appliances.
There are many different meanings of the word register: as a noun it can refer to an actual book that records information, or the range of sounds a voice can make; as a verb it can mean to sign up for something official, or to record someone’s dissatisfaction with the restaurant in their diary; and as an adjective it can mean formal or impressive. However, the most common sense of the word is that of a small piece of fast memory, with a specific hardware function.
In a computer, a register is an important part of the central processing unit (CPU) that enables it to perform complex and sophisticated operations at high speeds. The CPU uses the register to store inputs, outputs and intermediate results of calculations; this is called locality of reference, since the values stored in the register are immediately accessible by the CPU for immediate processing. Unlike memory, which can be read and written by the CPU, registers are only accessible by the processor itself and cannot be accessed by other hardware components such as the arithmetic logic unit (ALU).
A Register File
In general, register files are pitch-matched to the datapath that they serve. This avoids having busses crossing over the datapath turn corners, wasting area. Also, having each bit line contact the silicon at each transistor in a register file requires less space than having multiple bits share the same wire. In some cases, the register file is pitch-matched to a single datapath, forcing the widest unit to have a bit pitch smaller than other units in the system.
A register can be either read-only or write-only, depending on the design rules for the CPU and programming language. In a typical microprocessor, all instructions start by putting an instruction into one of the registers, and they can be executed only after the value in that register is available to the processor. The instruction can then manipulate the data in that register by using operations such as add, subtract, or shift. In older CPUs, the programmers had to write these operations explicitly in code, but with modern CPUs and compilers, most of these instructions are just things like “increment R0” or “add R0 to R1”. This makes writing programs much easier. This is one of the reasons that embedded systems are so popular, as they eliminate the need to write complicated and time-consuming assembly code to manage the memory and registers.