SUPPLEMENT
The CPU reads in data or control signals through the input ports and sends data or control signals to the outside world through the output ports.
System inputloutput devices may also be called peripherals. Many different types of peripherals exist: some peripheral devices can do limited processing on the data given to them by the CPU.
In a typical
In addition to reading input characters and displaying them on the screen, the CPU may also scroll character lines up the screen and perform special functions such as instructing the displayed characters to blink or to be highlighted.
In this CRT application, as with others, the CPU provides the real intelligence in the microcomputer system and relies on memory and 110 devices for support.
WHAT ARE DATA, ADDRESS AND CONTROL BUSSES?
The CPU is physically connected to the memory and 110 devices bY,the bus interface which is a connection of parallel wires (some- times called "lines") that perform a similar function. As Figure I shows, there are three different busses that interface a CPU to other system components. They are the data bus, the address bus, and the control bus.
The data bus, as the name implies, is the set of wires over which data passes between the CPU and the memory and 1/0. The data can either be instructions for the CPU, or infor- mation the CPU is passing to or from 1/0 ports.
The CPU uses the address bus to select the desired memory or 110 device by providing a unique address that corresponds to one of the many memory or 110 elements in the system.
The control bus contains control lines for signals to the memory and 110 devices and specifies whether data is to go into or out of the CPU and exactly when the data is being transferred.
From one microcomputer to another, the number of bus lines may vary. A microcom- puter is called an
Also, the number of address bus lines varies from one microcomputer to another. Some smaller machines, like the Intel 8008 have only 14 lines in the address bus, providing unique addressability of about 16,000 pieces of information. (All the signals emanating from a microprocessor are interpreted in terms of voltage levels (high or low) on the bus lines. The signals on the address bus represent a binary number: HIGH voltages are I's, LOW voltage are O's. Thus, a
In an
The Intel 8088, described herein, actually has 20 lines in its address bus, providing the binary addressability for over 1 million bytes of information.
