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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
Showing 60 out of 1137
Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
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 60
SEC. 1.3
COMPUTER HARDWARE REVIEW
29
read sector 11,206 from disk 2.
The controller then has to convert this linear sector
number to a cylinder, sector, and head. This conversion may be complicated by the
fact that outer cylinders have more sectors than inner ones and that some bad sec-
tors have been remapped onto other ones. Then the controller has to determine
which cylinder the disk arm is on and give it a command to move in or out the req-
uisite number of cylinders. It has to wait until the proper sector has rotated under
the head and then start reading and storing the bits as they come off the drive,
removing the preamble and computing the checksum. Finally, it has to assemble
the incoming bits into words and store them in memory.
To do all this work, con-
trollers often contain small embedded computers that are programmed to do their
work.
The other piece is the actual device itself. Devices have fairly simple inter-
faces, both because they cannot do much and to make them standard.
The latter is
needed so that any SAT A disk controller can handle any SAT A disk, for example.
SATA
stands for
Serial ATA
and
AT A
in turn stands for
AT Attachment
. In case
you are curious what AT stands for, this was IBM’s second generation ‘‘Personal
Computer Advanced Technology’’ built around the then-extremely-potent 6-MHz
80286 processor that the company introduced in 1984.
What we learn from this is
that the computer industry has a habit of continuously enhancing existing acro-
nyms with new prefixes and suffixes. We also learned that an adjective like ‘‘ad-
vanced’’ should be used with great care, or you will look silly thirty years down the
line.
SATA is currently the standard type of disk on many computers. Since the ac-
tual device interface is hidden behind the controller, all that the operating system
sees is the interface to the controller, which may be quite different from the inter-
face to the device.
Because each type of controller is different, different software is needed to
control each one. The software that talks to a controller, giving it commands and
accepting responses, is called a
device driver
.
Each controller manufacturer has to
supply a driver for each operating system it supports. Thus a scanner may come
with drivers for OS X, Windows 7, Windows 8, and Linux, for example.
To be used, the driver has to be put into the operating system so it can run in
kernel mode.
Drivers can actually run outside the kernel, and operating systems
like Linux and Windows nowadays do offer some support for doing so.
The vast
majority of the drivers still run below the kernel boundary. Only very few current
systems, such as MINIX 3, run all drivers in user space. Drivers in user space must
be allowed to access the device in a controlled way, which is not straightforward.
There are three ways the driver can be put into the kernel. The first way is to
relink the kernel with the new driver and then reboot the system. Many older UNIX
systems work like this. The second way is to make an entry in an operating system
file telling it that it needs the driver and then reboot the system.
At boot time, the
operating system goes and finds the drivers it needs and loads them.
Windows
works this way. The third way is for the operating system to be able to accept new
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