Page 37 - UNI 101 Computer Science Handout.

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includes the necessary connections to allow DMA, interrupt, and control capability. Except for issues of

performance, it is not important to understand the details of a particular bus. What is important to note

is how similar buses are, in essence, differing mostly in details and implementation. Each bus provides
the necessary data and address lines, lines for interrupts, and lines to control timing, reads and writes,

and soon. The major differences, other than specifications, actual pin assignments, and physical

differences, lie in the way the control signals are implemented. For example, the ISA bus is defined by
its data width as a ‘‘16-bit’’bus. The PCI bus can be used to support either a 32-bit or 64-bit data width.

The ISA bus has separate data and address lines. As noted earlier, addresses and data are multiplexed

on the PCI bus. The PCI bus is designed to transfer several pieces of data in a rapid sequence called a

burst once a starting address is established. For most current personal computer motherboard designs,

the PCI-Express has assumed predominance as the master bus of choice. PCI-Express is gradually
replacing an older bus, called simply PCI. PCI-Express is a serial I/O bus that was designed to replace the

parallel PCI bus that long dominated backplane design. PCI-Express is designed to be data and signal

compatible with components designed for the PCI bus. The original PCI bus is a ‘‘32- or 64-bit’’ (meaning
32 or 64 bits of data at a time) backplane bus that provides plug-in capability for various I/O modules

that control external serial and parallel ports, sound cards, network cards, and the like. The PCI bus

provides 32 or, optionally, 64 lines that are used for both addresses and data, labeled AD00 through

AD31 or AD63, plus various control and power lines. The power lines provide required power for the

plug-in peripheral interface cards. The control lines control timing, handle interrupts, arbitrate between
different devices seeking to use the bus, and perform other similar functions. All lines, other than the

power lines, carry digital signals. A connection diagram of the PCI bus, which is still provided as an I/O

interface in most PCs and many other computers, is shown in Figure 11.8. Unlike the parallel PCI bus,
however, the PCI-Express is made up of a bundle of thirty-two serial, bidirectional point-to-point buses.

Each bus consists of two simplex lines that carry data, addresses, and control signals simultaneously in

both directions at a current maximum rate of 1 GB per second in each direction. Each two-way bus is

called a lane.

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