Modern Operating Systems by Herbert Bos ...
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Modern Operating Systems by Herbert Bos and Andrew...
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Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 368
In addition to providing abstractions such as processes, address spaces, and
files, an operating system also controls all the computer’s I/O (Input/Output) de-
vices. It must issue commands to the devices, catch interrupts, and handle errors.
It should also provide an interface between the devices and the rest of the system
that is simple and easy to use.
To the extent possible, the interface should be the
same for all devices (device independence).
The I/O code represents a significant
fraction of the total operating system. How the operating system manages I/O is
the subject of this chapter.
This chapter is organized as follows. We will look first at some of the prin-
ciples of I/O hardware and then at I/O software in general.
I/O software can be
structured in layers, with each having a well-defined task.
We will look at these
layers to see what they do and how they fit together.
Next, we will look at several I/O devices in detail: disks, clocks, keyboards,
and displays. For each device we will look at its hardware and software. Finally,
we will consider power management.
Different people look at I/O hardware in different ways. Electrical engineers
look at it in terms of chips, wires, power supplies, motors, and all the other physi-
cal components that comprise the hardware. Programmers look at the interface

Page 369
presented to the software—the commands the hardware accepts, the functions it
carries out, and the errors that can be reported back.
In this book we are concerned
with programming I/O devices, not designing, building, or maimtaining them, so
our interest is in how the hardware is programmed, not how it works inside. Never-
theless, the programming of many I/O devices is often intimately connected with
their internal operation.
In the next three sections we will provide a little general
background on I/O hardware as it relates to programming.
It may be regarded as a
review and expansion of the introductory material in Sec. 1.3.
5.1.1 I/O Devices
I/O devices can be roughly divided into two categories:
block devices
character devices
A block device is one that stores information in fixed-size
blocks, each one with its own address. Common block sizes range from 512 to
65,536 bytes.
All transfers are in units of one or more entire (consecutive) blocks.
The essential property of a block device is that it is possible to read or write each
block independently of all the other ones. Hard disks, Blu-ray discs, and USB
sticks are common block devices.
If you look very closely, the boundary between devices that are block address-
able and those that are not is not well defined. Everyone agrees that a disk is a
block addressable device because no matter where the arm currently is, it is always
possible to seek to another cylinder and then wait for the required block to rotate
under the head. Now consider an old-fashioned tape drive still used, sometimes, for
making disk backups (because tapes are cheap).
Tapes contain a sequence of
blocks. If the tape drive is given a command to read block
, it can always rewind
the tape and go forward until it comes to block
This operation is analogous to a
disk doing a seek, except that it takes much longer. Also, it may or may not be pos-
sible to rewrite one block in the middle of a tape. Even if it were possible to use
tapes as random access block devices, that is stretching the point somewhat: they
are normally not used that way.
The other type of I/O device is the character device. A character device deliv-
ers or accepts a stream of characters, without regard to any block structure.
It is
not addressable and does not have any seek operation.
Printers, network interfaces,
mice (for pointing), rats (for psychology lab experiments), and most other devices
that are not disk-like can be seen as character devices.
This classification scheme is not perfect. Some devices do not fit in.
for example, are not block addressable. Nor do they generate or accept character
streams. All they do is cause interrupts at well-defined intervals. Memory-mapped
screens do not fit the model well either. Nor do touch screens, for that matter.
the model of block and character devices is general enough that it can be used as a
basis for making some of the operating system software dealing with I/O device in-
dependent. The file system, for example, deals just with abstract block devices and
leaves the device-dependent part to lower-level software.

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