Why does the motherboard sometimes support more than one bus speed?

Answer 1

<Original Answer> because some devices on the board are slower than others and do not require or cannot support the same bus speeds.

<Update> The book that this question comes from has another answer: that it is required for backwards compatibility. This is the answer for the test, but, I do not like it.

My answer is that having a high speed bus to some devices is a waste of CPU effort that could be better used on RAM or other high-speed items. There is not point to putting a modem or printer on a high speed bus that the CPU has to monitor only to find out that the printer or modem is done with the last burst of data sent.

Answer 2

This started with the limitations of the earlier PCs. The 8088 CPU in the original IBM PC-XT ran at 4.77 MHz, and at the time, so did the 8-bit ISA bus, and the PC-AT operated at 6-8 MHz. Anyway, the peripherals available were designed to operate at those speeds, and everything was tied to the system clock and changing it to change the CPU speed would have consequences on everything.

Then came the early PC clones. They sped up the CPU and might have used faster memory, but kept the peripheral bus and other timings at the original speeds. That way, existing peripherals on the market and existing memory could be used. So they sped up what they could and still have compatibility, and kept other components like the peripheral bus, keyboard, and so on at the same speed for compatibility.

Then as time went on, the CPU kept getting faster than the motherboard could handle and keep all the timings unified, so that was where multipliers and all were introduced. So a 66 MHz CPU was still clocking the motherboard at 33 MHz, and and peripheral bus at 16 or 33 MHz depending on the type of interface.

As chipsets got faster, they still could not keep up with the CPU. So an Athlon-64 CPU may be clocked at 2-3 Ghz, but the communication from the CPU to the Northbridge of the chipset can only go at about 1 GHz. The buses got faster with time, but still cannot keep up with the CPU, and now, CPUs are multi-core, hyperthreaded, and so on.

Answer 3

because some devices on the board are slower then others , and do not require or can not support the same bus speeds

Answer 4

To support a number of processors and memory that differ in bus speed.

Answer 5

to support a number of processors and memory..

that differ in bus speed