the variety of tasks they can perform by the following factors: l The design of their central processors . The variety of input/output devices used l The programmer’s capability to develop a numerical method for representing and solving the problem There are two basic types of digital computers— the special-purpose and the general-purpose computer. Special-Purpose Digital Computers Special-purpose digital computers are designed to follow a specific set of instruction sequences that are fixed at the time they are manufactured. To change the operation of this type of computer, the actual construction of the machine has to be altered. General-Purpose Digital Computers General-purpose digital computers follow instruction sequences that are read into and stored in memory prior to the calculation performance. This type of computer operation can be altered by inputting a different set of instructions. Since the operation of general-purpose digital computers can be changed with relative ease, as compared to special-purpose computers, they provide a far greater usage flexibility. DIGITAL COMPUTER OPERATION Learning Objective: Recognize the operating principles of a digital computer. Each major section of the digital computer is comprised of various electrical circuits. These circuits include flip-flops (bistable devices), amplifiers, gates (such as AND and OR gates), and passive memory elements. These elements are organized into registers, counters, and gates. Registers are a series of electronic devices for temporary storage of a binary word. Counters are a series of electronic devices that progress through a specific binary sequence. The gates are used to set a flip-flop or generate a times condition signal. The computer manipulates binary numbers representing numerical values or conditions. Devices to retain these binary figures comprise the majority of the computer registers, and each register has a distinct purpose or function. Many operations require that the binary word or data be transferred from one register to another. It is possible for several different words to be transferred simultaneously. Gates are used to control the transfer of data words from one register to another. These gates consist of diode and resistor networks. The gate circuit generates a signal to transfer the contents of one register to another at a particular time if certain conditions are met. For example, if the instruction being executed is an add, and if one of the numbers being added is a negative number, then the gate will generate a command signal. If these conditions are not met, the gate will not generate the command signal. Several gates in the computer are active only during specific instructions, such as divide or multiply, and then only during that particular instruction. On the other hand, some gates are active during several instructions, generating command signals. In the design of a computer, each instruction that the computer is to perform is very methodically analyzed, and for each signal required, a gate is designated to generate the signal. The size of the registers determines the general size of the computer. Not all registers in the computer have the same word length. Some are determined by the accuracy required, while others are determined by the instruction word, number of addresses in the memory, and various other parameters. DIGITAL DATA PROCESSOR Learning Objective: Referring to various schematic and block diagrams, recognize the components of a digital data processor and the function(s) of each. Figure 8-1 is a functional block diagram of a digital data processing set. Of the processes that take place within a computer, the manipulation of data is Figure 8-1.-Digital data processor block diagram. 8-3