INPUT/OUTPUT 91
was to design a new and faster bus for the next model. Because nobody ever
moved I/O devices from an old model to a new one, this approached worked fine.
However, in the PC world, people often upgraded their CPU but wanted to
move their printer, scanner, and modem to the new system. Also, a huge industry
had grown up around providing a vast range of I/O devices for the IBM PC bus,
and this industry had exceedingly little interest in throwing out its entire investment
and starting over. IBM learned this the hard way when it brought out the
successor to the IBM PC, the PS/2 range. The PS/2 had a new, and faster bus, but
most clone makers continued to use the old PC bus, now called the ISA (Industry
Standard Architecture) bus. Most disk and I/O device makers also continued to
make controllers for it, so IBM found itself in the peculiar situation of being the
only PC maker that was no longer IBM compatible. Eventually, it was forced
back to supporting the ISA bus. As an aside, please note that ISA stands for
Instruction Set Architecture in the context of machine levels whereas it stands for
Industry Standard Architecture in the context of buses.
Nevertheless, despite the market pressure not to change anything, the old bus
really was too slow, so something had to be done. This situation led to other companies
developing machines with multiple buses, one of which was the old ISA
bus, or its backward-compatible successor, the EISA (Extended ISA) bus. The
most popular of these now is the PCI (Peripheral Component Interconnect)
bus. It was designed by Intel, but Intel decided to put all the patents in the public
domain, to encourage the entire industry (including its competitors) to adopt it.

The PCI bus can be used in many configurations, but a typical one is illustrated
in Fig. 2-3. Here the CPU talks to a memory controller over a dedicated
high-speed connection. The controller talks to the memory and to the PCI bus
directly, so CPU-memory traffic does not go over the PCI bus. However, highbandwidth
(i.e., high data rate) peripherals, such as SCSI disks, can connect to the
PCI bus directly. In addition, the PCI bus has a bridge to the ISA bus, so that ISA
controllers and their devices can still be used. A machine of this design would
typically contain three or four empty PCI slots and another three or four empty
ISA slots, to allow customers to plug in both old ISA I/O cards (usually for slow
devices) and new PCI I/O cards (usually for fast devices).
Many kinds of I/O devices are available today. A few of the more common
ones are discussed below.

2.4.2 Terminals

Computer terminals consist of two parts: a keyboard and a monitor. In the
mainframe world, these parts are often integrated into a single device and attached
to the main computer by a serial line or over a telephone line. In the airline reservation,
banking, and other mainframe-oriented industries, these devices are still in
widespread use. In the personal computer world, the keyboard and monitor are
independent devices. Either way, the technology of the two parts is the