ISA is a standard for I/O buses that was set back in 1984. Since IBM was the standard for computers, the bus in the AT became known as Industry Standard Architecture, or ISA. The ISA bus can only handle 16-bit data transfers at a clock speed of 8 MHz. It is also only capable of handling memory under 16 MB.
EISA, or Extended ISA, is an improvement over the ISA bus. It still runs at only 8 MHz, but is capable of 32-bit data transfer and can access all memory in the system.
PCI is a local bus system designed for higher-end computer systems, such as the Pentium. Since the ISA bus can only transfer 16 bits of data at a clock speed of 8 MHz, higher-end applications can be significantly slowed. The PCI bus transfers 32 or 64 bits of data at a clock speed of 33 MHz. PCI is a local bus, meaning it connects the CPU, memory system, and peripherals to a wider, faster data pathway. This allows faster data transfer between the CPU and peripherals. The PCI bus supports 3 to 5 critical peripherals, which are either integrated directly onto the motherboard or added via expansion cards. The PCI bus fully supports cards developed for standard I/O buses.
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