In a digital computer there are two types of memory units namely, operational units and storage units. The name that is commonly associated with an operational unit is a register. A register is used for the temporary storage and the manipulation of information.
Some of the most important registers are contained in the central processing unit (CPU) of the computer. The CPU contains registers (sometimes called accumulators) which hold the arguments of arithmetic computations.
Some very complex integrated circuitry allows the information which is stored in the accumulator to be added, subtracted, multiplied, and divided. In addition, certain bits of the accumulator can be tested to determine if the normal sequence of control in a program should be altered. Besides storing operands and the results from arithmetic operations, registers are also used to temporarily store program instructions and control information concerning which instruction is to be executed next.
Because of their highly specialized nature, registers have a great deal of combinational logic associated with them. This makes them expensive relative to storage-type memory units in a computer. Consequently, registers are only used to store information temporarily.
The storage-type memory unit is designed to store information which is more permanent in nature. A particular storage unit or set of storage units is associated with a particular variable in a program.
However, before arithmetic computations involving a variable are performed, the value of the variable, as stored in the memory unit, must be transferred to a register unit. The transfer must take place because memory units do not have the necessary logic associated with them to execute arithmetic operations. If the result of a computation is to be assigned to a variable, the resultant value must be transferred from an arithmetic register back to the memory unit associated with the variable.
When a program is executed, its instructions and data generally reside in storage units. The entire set of storage units in the main frame or main part of a computer is often called main memory. In some instances programs can also reside in storage units which do not belong to main memory. Examples of such storage unit devices are magnetic disks and drums.
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The most commonly used type of main memory storage is semiconductor. A semiconductor unit holds one bit of information. Information is input to a storage unit by the presence of a low (0) or high voltage (1) vice versa.
