Busses

Address Bus

The address bus carries the address of the piece of memory or I/O device to be read from or written to.  It is a unidirectional bus, meaning that data travels only one way: from the CPU to memory.

The number of lines on the bus determines the number of addressable memory elements. For example an 8 bit bus can represent 2 to the power of 8 unique addresses. This equates to 256 unique memory addresses. A 16 bit bus can address 65536 unique addresses and so on.

Data Bus

The data bus carries the data that is to be written or has been read from memory.  It is a bidirectional bus as it can carry data to or from memory.

The width of the data bus is directly related to the largest number that the bus can carry. For example an 8 bit bus can represent 2 to the power of 8 unique values. This equates to the numbers 0 to 255. A 16 bit bus can carry the values 0 to 65535 and so on.

Control Bus

The control bus carries signals that control the actions of the computer.  For example one line of the control bus may be the read/write line. If the wire is low (no electricity flowing) then the memory is read. If the wire is high (with electricity flowing) then the memory is written.

 Next: Memory