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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 803
772
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
a parameter.
Adding a new device type to Linux means adding a new entry to one
of these tables and supplying the corresponding procedures to handle the various
operations on the device.
Some of the operations which may be associated with different character de-
vices are shown in Fig. 10-21. Each row refers to a single I/O device (i.e., a single
driver). The columns represent the functions that all character drivers must sup-
port. Several other functions also exist. When an operation is performed on a char-
acter special file, the system indexes into the hash table of character devices to
select the proper structure, then calls the corresponding function to have the work
performed. Thus each of the file operations contains a pointer to a function con-
tained in the corresponding driver.
Device
Open
Close
Read
Write
Ioctl
Other
Null
null
null
null
null
null
...
Memory
null
null
mem
read
mem
write
null
...
Keyboard
k
open
k
close
k
read
error
k
ioctl
...
Tty
tty
open
tty
close
tty
read
tty
write
tty
ioctl
...
Printer
lp
open
lp
close
error
lp
write
lp
ioctl
...
Figure 10-21.
Some of the file operations supported for typical character devices.
Each driver is split into two parts, both of which are part of the Linux kernel
and both of which run in kernel mode. The top half runs in the context of the caller
and interfaces to the rest of Linux. The bottom half runs in kernel context and
interacts with the device. Drivers are allowed to make calls to kernel procedures
for memory allocation, timer management, DMA control, and other things. The set
of kernel functions that may be called is defined in a document called the
Driver-
Kernel Interface
.
Writing device drivers for Linux is covered in detail in Cooper-
stein (2009) and Corbet et al. (2009).
The I/O system is split into two major components: the handling of block spe-
cial files and the handling of character special files. We will now look at each of
these components in turn.
The goal of the part of the system that does I/O on block special files (e.g.,
disks) is to minimize the number of transfers that must be done.
To accomplish
this goal, Linux has a
cache
between the disk drivers and the file system, as illus-
trated in Fig. 10-22. Prior to the 2.2 kernel, Linux maintained completely separate
page and buffer caches, so a file residing in a disk block could be cached in both
caches. Newer versions of Linux have a unified cache. A
generic block layer
holds
these components together, performs the necessary translations between disk sec-
tors, blocks, buffers and pages of data, and enables the operations on them.
The cache is a table in the kernel for holding thousands of the most recently
used blocks. When a block is needed from a disk for whatever reason (i-node,
directory, or data), a check is first made to see if it is in the cache.
If it is present in
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