Initially, the computer was conceived as a device to manipulate numbers and solve arithmetical problems. During its development, it was recognized that a machine capable of manipulating numbers could also be used to manipulate any "symbol" represented in numeric form. An electronic data processing system (EDPS) involves at least three basic elements: the input entering the system, or source data; the orderly processing that takes place within the system; and the output, or end result. The EDPS has four functional units: the input devices; the central processing unit (CPU); the storage, or memory; and the output devices.
The central processing unit (CPU) is the control center of the EDPS, and it has two parts: the "arithmetic/logic unit" (ALU) and the "control unit." The ALU performs operations such as addition, subtraction, multiplication, and division; as well as moving, shifting, and comparing data. The control section of the CPU directs and coordinates all the operations of the computer according to the conditions set forth by the stored program. It selects instructions from the stored program and interprets them. It then generates signals and commands that cause other system units to perform certain operations at appropriate times. It controls the input/output units, the arithmetic-logic operations of the CPU, and the transfer of data to and from storage. It acts as a central nervous system, but performs no actual processing operations on data.
Storage Devices. The main storage of a computer—the memory, or internal storage unit— is basically an electronic filing cabinet where each location is capable of holding data and instructions. The storage unit contains four elements: (1) all data being held for processing, (2) the data being processed, (3) the final result of processing until it is released as output, and (4) all the program instructions while processing is being carried out. Each location in main storage is identified by a particular address. Using this address, the control section can readily locate data and instructions as needed. The size or capacity of main storage determines the amount of data and instructions that can be held within the system at any one time. In summary, the internal memory is a temporary storage and is called "random access memory" (RAM). There is also a second type of memory, called "read-only memory" (ROM). This memory is fixed; meaning it can be read but cannot be written to, changed, or deleted. There are also secondary memory devices or auxiliary storage, sometimes called "sequential access memory," such as diskettes, hard drives, and magnetic tape. Depending on how often the data will be used these auxiliary devices will be chosen. For example, mass storage devices or certain types of tapes may be used for archival purposes of medical records or bank accounts, where certain legal aspects of the data may be required.
Input/Output (I/O) Devices. These are devices that are linked to the computer and can introduce data into the system, and devices that can accept data after it has been processed. Some examples are: disk storage drives, printers, magnetic tape units, display stations, data transmission units, and the old punched card or paper tape. Input devices perform the function of converting the data from a form that is intelligible to the user to a form that is intelligible to the computer. Output, on the other hand, is data that has been processed, (e.g., shown on a display device). In some cases, a printer can readily display the data in an understandable form. In other instances, such as with a magnetic tape drive, the data is carried as input for further processing by another device. In this case, the computer retains the data until further processing takes place. In summary, a digital computer identifies an electronic device capable of manipulating bits of information under the control-sequenced instructions stored within the memory of the device. Some common forms of storing data today include: floppy disks (used mainly for temporary storage); magnetic disks (fixed or removable); and optical disks that can store very large amounts of data. CD-ROM (compact disk— read only memory) devices store the information by means of a finely focused laser beam that detects reflections from the disc. This technology is sometimes referred by the term "write once, read
