Front side bus
From Overclocking Wiki
The front side bus (abbreviated FSB) is the primary bus within a computer, linking the CPU with other devices in the system. In modern computers, the front side bus provides a connection between the CPU and northbridge, where many other system busses meet up. Because of the bus's direct connection with the CPU, it has been used in the past as the reference for clock speeds in the rest of the system. For example, the speed of PCI-e slots is commonly a multiple of the front side bus although many newer motherboards have a control that locks the speed to a fixed number.
Contents |
Relationship to CPU Speed
The speed of the CPU is determined in by multiplying the front side bus speed by the CPU's multiplier. For example, with a multiplier of 12 and FSB of 200 the CPU will run at 2400MHz. Any increase in the FSB means a corresponding increase in the CPU speed.
Relationship to RAM Speed
The speed of the RAM within a system also depends on the speed of the front side bus. Before the advent of boards allowing the modification of ratios, RAM always ran 1:1 with the FSB (not taking into account the effect of DDR). Today it is typical to find a control to modify the RAM:FSB ratio within the BIOS. Some BIOSes may display an option for selecting the RAM speed (DDR333, DDR400, DDR533, etc) rather than a ratio -- the motherboard calculates the appropriate ratio in this case.
Typically, the RAM speed is set to its lowest possible settings while overclocking the CPU. For example, running the RAM at half of its rated speed (1:2 ratio, assuming your RAM is rated for 1:1 operation) while overclocking will ensure that the speed of the RAM is not holding back your overclock. And, once the maximum speed of the motherboard and the processor are known, the RAM speed can again be raised to its maximum. Attempting to overclock the RAM, motherboard and CPU all at the same time by ignoring this will greatly increase the difficulty of the overclock.
Exchanges
The FSB originally carried one unit of information per cycle. Newer machines will double-pump or quad-pump the front side bus, which means that two or four units of information are conveyed per each clock cycle. Hence, some memory will show 400 mhz at a 1:2 ratio. Marketing people will call this 800 mhz ram despite its misleading nature.
