PC Bus Architectures

In this section, we will introduce some of the main components of the common bus architectures for the IBM PC family of computers.

ISA Bus (Industry Standard Architecture)

This bus architecture is designed for both memory and I/O. Peripheral Devices are attached to the bus and can be programmed, or set to send a signal on an IRQ line. One method of setting is a DIP switch (hard setting), or a configuration setting in a start up program.

• started out as an 8 bit data bus with 62-pin connectors for the 8088.
• There are now 16-bit and 32-bit busses for medium level PC's, but you can still use 8-bit cards.
• current standard is for 24 address lines and 16 data lines, on 2 connector cards.

There are several types of lines on this bus. They have the following functions:

• Address lines: SA0 - SA19, LA17 - LA23
• Data Lines: SD0-SD7, SD8 - SD15
• Utility Lines: Power of various voltages, ground, clock
• Bus Cycle Definition: memory read, memory write, I/O read, I/O write, Address enable
• Bus Control Lines: Memory and I/O chip select
• DMA Lines: DRQs and DACKs.
• Interrupt Request Lines: IRQ3-IRQ15

PCI Bus (Peripheral Component Interonnect)

This architecture has 64 multiplexed address/data lines. You can't use these lines for address and data at the same time. Only 32 bit addresses are used, but 64 bits of data can be transferred in parallel.

It is capable of transferring 1 quadword per clock cycle once a transfer is started. At a bus clock speed of 33 MHz, this bus is capable of 264 MBytes per second

Other Busses used in Personal Computers

MCA - Microchannel adapter

EISA - Extended ISA

PCMCIA - Personal Computer Memory Card Industry Association

• The primary use is for laptops.