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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 880
SEC. 10.9
SUMMARY
849
Process
State
Importance
system
Core part of operating system
SYSTEM
phone
Always running for telephony stack
PERSISTENT
email
Current foreground application
FOREGROUND
camera
In use by email to load attachment
FOREGROUND
music
Running background service playing music
PERCEPTIBLE
media
In use by music app for accessing user’s music
PERCEPTIBLE
download
Downloading a file for the user
SERVICE
launcher
App launcher not current in use
HOME
maps
Previously used mapping application
CACHED
Figure 10-69.
Typical state of process importance
Process
State
Importance
system
Core part of operating system
SYSTEM
phone
Always running for telephony stack
PERSISTENT
email
Current foreground application
FOREGROUND
music
Running background service playing music
PERCEPTIBLE
media
In-use by music app for accessing user’s music
PERCEPTIBLE
download
Downloading a file for the user
SERVICE
launcher
App launcher not current in use
HOME
camera
Previously used by email
CACHED
maps
Previously used mapping application
CACHED+1
Figure 10-70.
Process state after email stops using camera
10.9 SUMMARY
Linux began its life as an open-source, full-production UNIX clone, and is now
used on machines ranging from smartphones and notebook computers to
supercomputers. Three main interfaces to it exist: the shell, the C library, and the
system calls themselves. In addition, a graphical user interface is often used to sim-
plify user interaction with the system. The shell allows users to type commands for
execution. These may be simple commands, pipelines, or more complex struc-
tures. Input and output may be redirected.
The C library contains the system calls
and also many enhanced calls, such as
printf
for writing formatted output to files.
The actual system call interface is architecture dependent, and on x86 platforms
consists of roughly 250 calls, each of which does what is needed and no more.
The key concepts in Linux include the process, the memory model, I/O, and
the file system. Processes may fork off subprocesses, leading to a tree of processes.
Page 881
850
CASE STUDY 1: UNIX, LINUX, AND ANDROID
CHAP. 10
Process management in Linux is different compared to other UNIX systems in that
Linux views each execution entity—a single-threaded process, or each thread with-
in a multithreaded process or the kernel—as a distinguishable task.
A process, or a
single task in general, is then represented via two key components, the task struc-
ture and the additional information describing the user address space. The former
is always in memory, but the latter data can be paged in and out of memory.
Proc-
ess creation is done by duplicating the process task structure, and then setting the
memory-image information to point to the parent’s memory image. Actual copies
of the memory-image pages are created only if sharing is not allowed and a memo-
ry modification is required. This mechanism is called copy on write. Scheduling is
done using a weighted fair queueing algorithm that uses a red-black tree for the
tasks’ queue management.
The memory model consists of three segments per process: text, data, and
stack. Memory management is done by paging.
An in-memory map keeps track of
the state of each page, and the page daemon uses a modified dual-hand clock algo-
rithm to keep enough free pages around.
I/O devices are accessed using special files, each having a major device num-
ber and a minor device number. Block device I/O uses the main memory to cache
disk blocks and reduce the number of disk accesses. Character I/O can be done in
raw mode, or character streams can be modified via line disciplines. Networking
devices are treated somewhat differently, by associating entire network protocol
modules to process the network packets stream to and from the user process.
The file system is hierarchical with files and directories. All disks are mounted
into a single directory tree starting at a unique root.
Individual files can be linked
into a directory from elsewhere in the file system.
To use a file, it must be first
opened, which yields a file descriptor for use in reading and writing the file. Inter-
nally, the file system uses three main tables: the file descriptor table, the
open-file-description table, and the i-node table. The i-node table is the most im-
portant of these, containing all the administrative information about a file and the
location of its blocks. Directories and devices are also represented as files, along
with other special files.
Protection is based on controlling read, write, and execute access for the
owner, group, and others. For directories, the execute bit means search permission.
Android is a platform for allowing apps to run on mobile devices. It is based
on the Linux kernel, but consists of a large body of software on top of Linux, plus
a small number of changes to the Linux kernel. Most of Android is written in Java.
Apps are also written in Java, then translated to Java bytecode and then to Dalvik
bytecode. Android apps communicate by a form of protected message passing call-
ed transactions. A special Linux kernel model called the
Binder
handles the IPC.
Android packages are self contained and have a manifest desccribing what is in
the package.
Packages contain activities, receivers, content providers, and intents.
The Android security model is different from the Linux model and carefully sand-
boxes each app because all apps are regarded as untrustworthy.
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