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218 CHAPTER 7 / INPUT/OUTPUT

KEY POINTS

◆ The computer system’s I/O architecture is its interface to the outside world.
This architecture provides a systematic means of controlling interaction
with the outside world and provides the operating system with the informa-
tion it needs to manage I/O activity effectively.
◆ The are three principal I/O techniques: programmed I/O, in which I/O oc-
curs under the direct and continuous control of the program requesting the
I/O operation; interrupt-driven I/O, in which a program issues an I/O com-
mand and then continues to execute, until it is interrupted by the I/O hard-
ware to signal the end of the I/O operation; and direct memory access
(DMA), in which a specialized I/O processor takes over control of an I/O
operation to move a large block of data.
◆ Two important examples of external I/O interfaces are FireWire and
Infiniband.

I/O System Design Tool

In addition to the processor and a set of memory modules, the third key element of a
computer system is a set of I/O modules. Each module interfaces to the system bus or
central switch and controls one or more peripheral devices.An I/O module is not sim-
ply a set of mechanical connectors that wire a device into the system bus. Rather, the
I/O module contains logic for performing a communication function between the pe-
ripheral and the bus.
The reader may wonder why one does not connect peripherals directly to the sys-
tem bus.The reasons are as follows:
• There are a wide variety of peripherals with various methods of operation. It
would be impractical to incorporate the necessary logic within the processor
to control a range of devices.
• The data transfer rate of peripherals is often much slower than that of the
memory or processor. Thus, it is impractical to use the high-speed system bus
to communicate directly with a peripheral.
• On the other hand, the data transfer rate of some peripherals is faster than
that of the memory or processor. Again, the mismatch would lead to ineffi-
ciencies if not managed properly.
• Peripherals often use different data formats and word lengths than the com-
puter to which they are attached.