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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 35
providing application programmers (and application programs, naturally) a clean
abstract set of resources instead of the messy hardware ones and managing these
hardware resources. Depending on who is doing the talking, you might hear mostly
about one function or the other.
Let us now look at both.
1.1.1 The Operating System as an Extended Machine
(instruction set, memory organization, I/O, and bus struc-
ture) of most computers at the machine-language level is primitive and awkward to
program, especially for input/output.
To make this point more concrete, consider
Serial ATA
) hard disks used on most computers.
A book (Ander-
son, 2007) describing an early version of the interface to the disk—what a pro-
grammer would have to know to use the disk—ran over 450 pages. Since then, the
interface has been revised multiple times and is more complicated than it was in
2007. Clearly, no sane programmer would want to deal with this disk at the hard-
ware level. Instead, a piece of software, called a
disk driver
, deals with the hard-
ware and provides an interface to read and write disk blocks, without getting into
the details.
Operating systems contain many drivers for controlling I/O devices.
But even this level is much too low for most applications. For this reason, all
operating systems provide yet another layer of abstraction for using disks: files.
Using this abstraction, programs can create, write, and read files, without having to
deal with the messy details of how the hardware actually works.
This abstraction is the key to managing all this complexity. Good abstractions
turn a nearly impossible task into two manageable ones. The first is defining and
implementing the abstractions.
The second is using these abstractions to solve the
problem at hand. One abstraction that almost every computer user understands is
the file, as mentioned above.
It is a useful piece of information, such as a digital
photo, saved email message, song, or Web page. It is much easier to deal with pho-
tos, emails, songs, and Web pages than with the details of SATA (or other) disks.
The job of the operating system is to create good abstractions and then implement
and manage the abstract objects thus created.
In this book, we will talk a lot about
abstractions. They are one of the keys to understanding operating systems.
This point is so important that it is worth repeating in different words. With all
due respect to the industrial engineers who so carefully designed the Macintosh,
hardware is ugly. Real processors, memories, disks, and other devices are very
complicated and present difficult, awkward, idiosyncratic, and inconsistent inter-
faces to the people who have to write software to use them.
Sometimes this is due
to the need for backward compatibility with older hardware. Other times it is an
attempt to save money. Often, however, the hardware designers do not realize (or
care) how much trouble they are causing for the software. One of the major tasks
of the operating system is to hide the hardware and present programs (and their
programmers) with nice, clean, elegant, consistent, abstractions to work with in-
stead. Operating systems turn the ugly into the beautiful, as shown in Fig. 1-2.

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