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Bus

PCs originally had memory on the same bus as everything else, but recent hardware all has a distinct memory bus for increasing memory access speed. Workstations and high-end PCs often have additional busses for I/O.

The memory bus, also known as the external CPU bus, would run best in exactly two cycles: assert address, then read/write data. Unfortunately most RAM is slower than the memory bus, so several wait states, or unused bus cycles, are inserted to give the RAM time to get at its data. Five wait states is quite common.

The ISA and EISA system bus clock frequency can often be increased with a CMOS, or BIOS, setting to some integer fraction (1/8 or 1/10, for example) of the CPU clock speed. If you increase the system bus clock too much, data on the bus will be corrupted and the system will crash, but a moderate increase helps the performance of all cards connected to the bus, such as the disk controller, video controller, and network interface cards.


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