Computer bus

From ArticleWorld

A simple microcomputer consists of three major parts, namely, the central processing unit (CPU), the memory and the input and output circuitry (I/O), all of which are interconnected by three sets of lines running parallel to each other. These lines each constitute what is known as a computer bus. Buses can be of three kinds: address bus, data bus and control bus.

The address bus

The address bus in a computer may consist of 16, 20, 24 or 32 parallel signal lines. The address lines are used by the CPU to send out the address of the memory location or port that is to be written to or read from. The number of address lines determines the exact number of memory locations which the CPU can address. If, for instance, there are N address lines, the CPU can address 2N locations in the memory.

Data bus

This bus can comprise of 8, 16 or 32 lines. Unlike the address bus lines, these lines are bidirectional. The CPU can obtain data from memory or a port, as well as send data to the memory or port. A device connected to the data bus must have its outputs disabled in order to allow data to be put on the bus.

Control bus

This consists of parallel signal lines between 4 and 10 in number. These lines are used by the CPU to send out signals to enable the outputs of addressed memory locations in memory chips or port devices. In order for the CPU to read a data byte from somewhere in the memory, the CPU sends out the memory address of the particular byte onto the address lines, after which a ‘memory read’ signal on the control bus is sent. In this way, the addressed memory device is enabled so as to output a data word to the data bus. The data bus is thus used by the data word to travel to the CPU from the memory.