Chapter 1 - Logic Gates
1.1 Logic Gates
Logic gates are the building blocks used in
designing a CPU.
In a microchip, a logic is made up of a specific arrangement of transistors
created on a silicon wafer.
Each terminal of a logic gate is either in high state (binary 1) with +5 volts or low state (binary 0) with no voltage. Below are some of the basic logic gates.
Logic gates are combined to create the circuitry in microchips. The SR Latch below can function as one bit of memory - it stores a 1 when you press set (S) and 0 when you press reset (R). The 2 Bit Half Adder adds two binary digits.
A decoder has n inputs and selects one of 2n outputs. It can be used for selecting a memory address. For example, if the memory address bus is 8-bits, then it can address 256 different memory location. The decoder takes the 8-bit binary digit and sends a signal along one of 256 different wires to select a memory address. Below is a 2-bit decoder circuit.
A multiplexer has multiple inputs and selects one
of them for the output. It is also known as a data selector.
An example would be selecting which data input goes to a communication line.