2-8.
STORAGE
a. Overview. It would be impossible for a computer to operate without the
capability to remember, or store, instructions and facts and figures for retrieval when
needed. Computer storage, often called "memory," is an electronic file which retains
instructions and data as long as they are required. When data are placed in storage
through an input unit, they remain there until called for by the control unit of the CPU.
The amount of data required by a program or set of programs generally exceeds the
capacity of the CPU's main or primary storage.
main storage: the internal storage of a computer from which instructions are
executed; the fastest storage of a computer.
(1) In such cases the data are stored in auxiliary storage (also known as
secondary or peripheral storage). The most common types of peripheral storage are
magnetic tape and magnetic disk. (Other media include punch cards, mass storage,
and magnetic drums). These auxiliary storage media cost much less than primary
storage and thus make storage of large volumes of data more economical.
auxiliary storage: a supplement to the main storage; usually supplied by
magnetic disks, magnetic drums, magnetic tape, or magnetic cards.
(2) Secondary storage media are connected to the CPU. Once data has
been placed in there, they can be retrieved for processing. Retrieving items from
secondary storage takes longer than retrieving from main storage. After processing has
been completed the results can be written back onto the auxiliary storage.
b. Main vs Auxiliary Storage. As stated earlier, main storage (also known as
internal or primary storage) is an integral part of the central processor. All data to be
processed by the computer must pass through the main storage. Main storage is used
to store both instructions and data. The main storage unit must have sufficient storage
capacity to hold the program being used and the data needed for the problem. When
additional storage is needed, the computer system may be supplemented with auxiliary
storage.
c. Direct vs Sequential Access. There are two type of auxiliary storage:
direct (or random) access and sequential access. Direct access devices, such as
magnetic disk, give immediate access to a particular item of stored data. Sequential
access devices consist of tape units (magnetic tape or paper tape), whose stored data
must be read from the beginning in order to read or write a particular item of data.
Auxiliary storage may be used to store both instructions and data. Before these
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