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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
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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 382
SEC. 5.1
PRINCIPLES OF I/O HARDWARE
351
point are allowed to have any noticeable effect on the machine state. Here the price
is paid not in time, but in chip area and in complexity of the design.
If precise in-
terrupts were not required for backward compatibility purposes, this chip area
would be available for larger on-chip caches, making the CPU faster.
On the other
hand, imprecise interrupts make the operating system far more complicated and
slower, so it is hard to tell which approach is really better.
5.2 PRINCIPLES OF I/O SOFTWARE
Let us now turn away from the I/O hardware and look at the I/O software. First
we will look at its goals and then at the different ways I/O can be done from the
point of view of the operating system.
5.2.1 Goals of the I/O Software
A key concept in the design of I/O software is known as
device independence
.
What it means is that we should be able to write programs that can access any I/O
device without having to specify the device in advance. For example, a program
that reads a file as input should be able to read a file on a hard disk, a DVD, or on a
USB stick without having to be modified for each different device. Similarly, one
should be able to type a command such as
sort <input >output
and have it work with input coming from any kind of disk or the keyboard and the
output going to any kind of disk or the screen.
It is up to the operating system to
take care of the problems caused by the fact that these devices really are different
and require very different command sequences to read or write.
Closely related to device independence is the goal of
uniform naming
. The
name of a file or a device should simply be a string or an integer and not depend on
the device in any way.
In UNIX, all disks can be integrated in the file-system hier-
archy in arbitrary ways so the user need not be aware of which name corresponds
to which device. For example, a USB stick can be
mounted
on top of the directory
/usr/ast/backup
so that copying a file to
/usr/ast/backup/monday
copies the file to
the USB stick.
In this way, all files and devices are addressed the same way: by a
path name.
Another important issue for I/O software is
error handling
.
In general, errors
should be handled as close to the hardware as possible.
If the controller discovers
a read error, it should try to correct the error itself if it can.
If it cannot, then the
device driver should handle it, perhaps by just trying to read the block again. Many
errors are transient, such as read errors caused by specks of dust on the read head,
and will frequently go away if the operation is repeated. Only if the lower layers
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