Jonathan A. Titus

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Lesson 3: Boolean Logic

From "The Digital I/O Handbook"
by Jon Titus and Tom O’Hanlan
Sealevel System, Liberty, SC, 2004

The world of digital electronics allows for only two states, so at a given time, a digital signal can exist in only one of them. Although engineers usually think of these two binary states as logic 0 and logic 1, you can define them as you wish: true and false, black and white, high and low, and so on, as long as everyone else understands your definitions. Basic Boolean logic operations such as AND and OR let electronic circuits make decisions, such as, "If A and B are true, do this..." Figure 1-1 shows the AND and OR logic operations along with the symbol engineers use to represents each operation in a circuit. A truth table shows the output condition produced for each possible set of inputs to the logic circuit, called a "gate." A 2-input gate, for example, has 22, or 4 possible input conditions, while a 5-input gate would allow for 25, or 32 possible binary input combinations.
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by Peter R. Rony, David G. Larsen, and Jonathan A. Titus
"What Is a Logical Instruction"

MOST MICROCOMPUTERS manipulate information eight bits at a time. For example, the 8080A chip can move the eight bits from internal register to internal register, from internal register to memory, and between the accumulator and an external I/O device. It can also perform arithmetic and logical operations, the former including add, subtract, and compare, and the latter including AND, OR, Exclusive-OR, and complement. In this column, we shall be concerned with the logical operations.

The basic rules governing one-bit logic operations are truth tables. A truth table can be defined as a tabulation that shows the relation of all output logic levels of a digital circuit to all possible combinations of input logic levels in such a way as to characterize the circuit functions completely. I The truth tables for the AND, OR, Exclusive-OR, and complement operations are given in Table 1. We call these truth tables one-bit tables because the data words, A and B, each contain only a single bit.
Read More: Adobe Acrobat PDF FileWhat Is A Logical Instruction.pdf [49.2KB]