The role of buses

Types of buses

Buses are wires that connect the components of a computer e.g. the processor and main memory
Three buses are needed:
* The address bus
* The data bus and
* The control bus

The address bus

Memory is divided up into groups of bits known as the computer word. Most modern computers have a 32 or 64-bit word size. Each word in memory has its own address.

The width of the address bus i.e. the number of wires determines the number of addresses that can be put on the address bus. 2n memory locations can be addressed where n is the width of the address bus

The address bus is unidirectional, travelling away from the processor. An address is placed on the address bus by the processor e.g. the location in main memory of a word of data it wants to read. The address bus also addresses input output ports.

The data bus

The processor, main memory and disk input/output controller can put data onto the data bus or take it off. The graphics controller receives data from the data bus to display on the screen. Some devices put data on the data bus, some receive data, some both.

The control bus

The control bus is bidirectional. It transmits instructions between components of the computer. Because buses are shared by components control commands are needed to stop different commands from interfering with each other. Some common control lines are:
Memory read: tells the computer to read the data at the addressed location and place the data on the data bus to be sent to the processor
Memory write: tells the computer to write the data from the data bus to the addressed location
Bus request: Indicates a device wants to use the data bus
Bus grant: This is a signal that will be received back after a request to say it is ok to use the data bus
Interrupt request: Indicates that another device wants to use the CPU
Clock signals: The signal on this line is used to synchronize data between the CPU and a device

Example - fetching data from RAMs

To read data from RAM the address of the word to be read is placed on the address bus and the data read control signal sent to tell it to fetch the data from main memory. Next a bus request would be made to say we want to send the data from the location in memory to the processor and when the bus grant is received the data would be sent.

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