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What is the difference between a local and an expansion bus?
Originally there were two types of buses: local and expansion.
- The local bus (also called a processor or system bus) is the path directly connected to the CPU. It is actually four buses (data, address, control, and power). Components on the local bus are synchronized with the CPU. This means that they can operate on a 32-bit path at full clock speed (386 processors and above). Generally only the processor�s main memory and controller logic are connected to the local bus.
- The expansion bus (also called an AT Bus) allows the CPU to communicate with controllers and peripheral devices. It connects various peripherals such as the disk and video controllers, serial and parallel cards, and a network interface cards (NIC) Typically these components communicate through 16-bit ISA expansion cards at 8 MHz. In other words, they perform tasks at a much slower speed than the processor. Because of its speed and data path are inferior to the local bus, the expansion bus can become the "bottleneck" in system performance.
The PCI (Peripheral Connection Interface) bus design provides the capability to tap directly into the local bus (32-bit) for faster transfer of data. This allows peripherals like a NIC to bypass the expansion bus. A VLB (VESA Local bus) also attaches directly to the local bus.
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