Digital computer systems consist of three distinct units. These units are as follows:

Input unit

*Central Processing unit

Output unit

These units are interconnected by electrical cables to permit communication between them. This allows the computer to function as a system.

Input Unit

A computer must receive both data and program statements to function properly and be able to solve problems. The method of feeding data and programs to a computer is accomplished by an input device. Computer input devices read data from a source, such as magnetic disks, and translate that data into electronic impulses for transfer into the CPU. Some typical input devices are a keyboard, a mouse, or a scanner.

Central Processing Unit

The brain of a computer system is the central processing unit (CPU). The CPU processes data transferred to it from one of the various input devices. It then transfers either an intermediate or final result of the CPU to one or more output devices. A central control section and work areas are required to perform calculations or manipulate data. The CPU is the computing center of the system. It consists of a control section, an arithmetic-logic section (fig 3-1), and an internal storage section (main memory). Each section within the CPU serves a specific function and has a particular relationship with the other sections within the CPU.

CONTROL SECTION.-The control section directs the flow of traffic (operations) and data. It also maintains order within the computer. The flow of control is indicated by dotted arrows in figure 3-1 The control section selects one program statement at a time from the program storage area, interprets the statement, and sends the appropriate electronic impulses to the arithmetic-logic and storage sections so they can carry out the instructions. The control section does not perform actual processing operations on the data. The control section instructs the input device on when to start and stop transferring data to the input storage area. It also tells the output device when to start and stop receiving data from the output storage area.

ARITHMETIC-LOGIC SECTION.-The arithmetic-logic section performs arithmetic operations, such as addition, subtraction, multiplication, and division. Through internal logic capability, it tests various conditions encountered during processing and takes action based on the result. As indicated by the solid arrows in figure 3-1 data flows between the arithmetic-logic section and the internal storage section during processing. Specifically, data is transferred as needed from the storage section to the arithmetic-logic section, processed, and returned to internal storage. At no time does processing take place in the storage section. Data maybe transferred back and forth between these two sections several times before processing is completed. The results are then transferred from internal storage to an output device, as indicated by the solid arrow in figure 3-1

INTERNAL STORAGE SECTION.-The internal storage section is sometimes called primary storage, main storage, or main memory, because this section functions similar to our own human memory.

The storage section serves four purposes; three relate to retention (holding) of data during processing. First, as indicated by the solid arrow fig. 3-1 , data is transferred from an input device to the INPUT STORAGE AREA where it remains until the computer is ready to process it. Second, a WORKING STORAGE AREA ("scratch pad" memory) within the storage section holds both the data being processed and the intermediate results of the arithmetic-logic operations. Third, the storage section retains the

Figure 3-1.-Sections of a CPU.

processing results in the OUTPUT STORAGE AREA. From there the processing results can be transferred to an output device. The fourth storage section, the PROGRAM STORAGE AREA, contains the program statements transferred from an input device to process the data. Please note that the four areas (input, working storage, output, and program storage) are NOT fmed in size or location but are determined by individual program requirements.

Output Unit

As program statements and data are received by the CPU from an input device, the results of the processed data are sent from the CPU to an OUTPUT DEVICE. These results are transferred from the output storage area onto an output medium, such as a floppy disk, hard drive, video display, printer, and so on.

By now, you should have an idea of the functions performed by a CPU. It is the CPU that executes stored programs and does all of the processing and manipulating of data. The input and output (I/0) devices simply aid the computer by sending and receiving data and programs.