There are busses both within the CPU
and connecting it to external memory
devices. The data bus, address bus and control signals, despite their names, really constitute a single bus since each is useless without the others.
The width of the data bus is usually specified in bits
and is the number of parallel connectors. This and the clock rate
determine the bus's data rate (the number of bytes
per second which it can carry). This is one of the factors limiting a computer's performance. Most current microprocessors
have 32-bit busses both internally and externally. 100 or 133 megahertz
bus clock rates are common. The bus clock is typically slower than the processor clock.
Some processors have internal busses which are wider than their external busses (usually twice the width) since the width of the internal bus affects the speed of all operations and has less effect on the overall system cost than the width of the external bus.
Various bus designs have been used in the PC
, including ISA
, Micro Channel, VL-bus and PCI
. Other peripheral busses are NuBus, TURBOchannel, VMEbus, MULTIBUS and STD bus.