internal contents rather than binary. Figure 4-2illustrates how binary numbers can be displayed usingthe octal and hexadecimal representations of numbers.You will find this information very useful whenperforming maintenance because many of themaintenance panels and display control units rely onoctal and hexadecimal displays.The binary system is used in computers to representmachine codes used for program instruction andexecution; and for computations (logical andmathematical operations). ^{-}TOPIC 2—COMPUTER LOGICYou know the two digits of the binary numbersystem can be represented by the state or condition ofelectrical or electronic devices. A binary 1 can berepresented by a lamp that is lit or a switch that is on—atrue condition. And the opposite, a binary 0, would berepresented by the same devices in the oppositedirection, the lamp is off or the switch is off-a falsecondition. Boolean algebra, the logic mathematicssystem used with digital equipment, takes the two logiclevels, 1 and 0, and applies them to basic logic gates.Truth tables are frequently used to show the gate outputfor all possible combinations of the inputs. The basiclogic gates, AND, OR, and NOT, are used indifferentvariations and combinations to form the basic buildingblocks used in a computer, the combinational andsequential digital logic circuits. Later in this chapter,we discuss the different uses of these combinational andsequential logic circuits in the computer. In chapter 5,we discuss how the functional areas of the computer usethe combinational and sequential logic circuits toprocess data.TOPIC 3—COMPUTER CIRCUITSThe computer relies on electronic circuitsthroughout; from circuits that convert input power tothe desired requirement to the circuits used for thefunctional areas. Today’s computers rely heavily on theFigure 4-2.—Illustration of how binary numbers can be displayed: A. Octal display using indicator lamps; B. Hexadecimaldisplay using character/digital display.4-4