The memory bus is used to transfer information between the CPU and main memorythe RAM in your system. This bus is usually connected to the motherboard chipset North Bridge or Memory Controller Hub chip. Depending on the type of memory your chipset (and therefore motherboard) is designed to handle, the North Bridge runs the memory bus at various speeds. The best solution is if the memory bus runs at the same speed as the processor bus. Systems that use PC133 SDRAM have a memory bandwidth of 1066MBps, which is the same as the 133MHz CPU bus. In another example, Athlon systems running a 266MHz processor bus also run PC2100 DDR-SDRAM, which has a bandwidth of 2133MBpsexactly the same as the processor bus in those systems. Systems running a Pentium 4 with its 400MHz processor bus also use dual-channel RDRAM memory, which runs 1600MBps for each channel, or a combined bandwidth (both memory channels run simultaneously) of 3200MBps, which is exactly the same as the Pentium 4 CPU bus. Pentium 4 systems with the 533MHz bus run dual-channel DDR PC2100 or PC2700 modules, which match or exceed the throughput of the 4266MBps processor bus.

Running memory at the same speed as the processor bus negates the need for having cache memory on the motherboard. That is why when the L2 cache moved into the processor, nobody added an L3 cache to the motherboard. Some very high-end processors, such as the Itanium and Itanium 2 and the Intel Pentium 4 Extreme Edition, have integrated 2MB4MB of full-core speed L3 cache into the CPU. However, the most recent high-performance chips, such as the new Pentium Extreme Edition, use only L1 and L2 cache. Thus, it appears that L2 cache will continue to be the most common type of secondary cache for the foreseeable future.

The Need for Expansion Slots

The I/O bus or expansion slots enable your CPU to communicate with peripheral devices. The bus and its associated expansion slots are needed because basic systems can't possibly satisfy all the needs of all the people who buy them. The I/O bus enables you to add devices to your computer to expand its capabilities. The most basic computer components, such as sound cards and video cards, can be plugged into expansion slots; you also can plug in more specialized devices, such as network interface cards, SCSI host adapters, and others.