Editing Computer program (section)

===Operating system===
{{See also|Operating system}}
[[File:Concepts- Program vs. Process vs. Thread.jpg|thumb|Program vs. [[Process (computing)|Process]] vs. [[Thread (computing)|Thread]] <br/>[[Scheduling (computing)|Scheduling]], [[Preemption (computing)|Preemption]], [[Context switch|Context Switching]]|upright=1.8]]
An [[operating system]] is the low-level software that supports a computer's basic functions, such as [[Scheduling (computing)|scheduling]] [[Process (computing)|processes]] and controlling [[peripheral]]s.<ref name="osc-overview"/>

In the 1950s, the programmer, who was also the operator, would write a program and run it. After the program finished executing, the output may have been printed, or it may have been punched onto paper tape or cards for later processing.<ref name="osc-ch1-p6"/> More often than not the program did not work. The programmer then looked at the console lights and fiddled with the console switches. If less fortunate, a memory printout was made for further study. In the 1960s, programmers reduced the amount of wasted time by automating the operator's job. A program called an ''operating system'' was kept in the computer at all times.<ref name="sco-ch1-p11">{{cite book
 |url=https://archive.org/details/structuredcomput00tane/page/11
 |title=Structured Computer Organization, Third Edition
 |last=Tanenbaum
 |first=Andrew S.
 |publisher=Prentice Hall
 |year=1990
 |isbn=978-0-13-854662-5
 |page=[https://archive.org/details/structuredcomput00tane/page/11 11]}}</ref>

The term ''operating system'' may refer to two levels of software.<ref name="lpi-ch2-p21">{{cite book
 |title=The Linux Programming Interface
 |last=Kerrisk
 |first=Michael
 |publisher=No Starch Press
 |year=2010
 |isbn=978-1-59327-220-3
 |page=21}}</ref> The operating system may refer to the [[Kernel (operating system)|kernel program]] that manages the [[Process (computing)|processes]], [[Computer memory|memory]], and [[Peripheral|devices]]. More broadly, the operating system may refer to the entire package of the central software. The package includes a kernel program, [[Command-line interface|command-line interpreter]], [[graphical user interface]], [[Utility software|utility programs]], and [[Source-code editor|editor]].<ref name="lpi-ch2-p21"/>

====Kernel Program====
[[File:Kernel Layout.svg|thumb|A kernel connects the application software to the hardware of a computer.]]
The kernel's main purpose is to manage the limited resources of a computer:
* The kernel program should perform [[process scheduling]],<ref name="lpi-ch2-p22">{{cite book
 |title=The Linux Programming Interface
 |last=Kerrisk
 |first=Michael
 |publisher=No Starch Press
 |year=2010
 |isbn=978-1-59327-220-3
 |page=22}}</ref> which is also known as a [[context switch]]. The kernel creates a [[process control block]] when a ''computer program'' is [[Loader (computing)|selected for execution]]. However, an executing program gets exclusive access to the [[central processing unit]] only for a [[Preemption (computing)#time slice|time slice]]. To provide each user with the [[Time-sharing|appearance of continuous access]], the kernel quickly [[Preemption (computing)|preempts]] each process control block to execute another one. The goal for [[Systems programming|system developers]] is to minimize [[dispatch latency]].
[[File:Virtual memory.svg|thumb|250px|Physical memory is scattered around RAM and the hard disk. Virtual memory is one continuous block.]]
* The kernel program should perform [[memory management]].
:* When the kernel initially [[Loader (computing)|loads]] an executable into memory, it divides the address space logically into [[Region-based memory management|regions]].<ref name="duos-ch6-p152">{{cite book
  | last = Bach
  | first = Maurice J.
  | title = The Design of the UNIX Operating System
  | publisher = Prentice-Hall, Inc.
  | year = 1986
  | page = 152
  | isbn = 0-13-201799-7
  }}</ref> The kernel maintains a master-region table and many per-process-region (pregion) tables—one for each running [[Process (computing)|process]].<ref name="duos-ch6-p152"/> These tables constitute the [[virtual address space]]. The master-region table is used to determine where its contents are located in [[physical memory]]. The pregion tables allow each process to have its own program (text) pregion, data pregion, and stack pregion.
:*The program pregion stores machine instructions. Since machine instructions do not change, the program pregion may be shared by many processes of the same executable.<ref name="duos-ch6-p152"/>
:* To save time and memory, the kernel may load only blocks of execution instructions from the disk drive, not the entire execution file completely.<ref name="lpi-ch2-p22"/>
:*The kernel is responsible for translating virtual addresses into [[physical address]]es. The kernel may request data from the [[memory controller]] and, instead, receive a [[page fault]].<ref name="sco6th-ch6-p443">{{cite book
  | last = Tanenbaum
  | first = Andrew S.
  | title = Structured Computer Organization, Sixth Edition
  | publisher = Pearson
  | year = 2013
  | page = 443
  | isbn = 978-0-13-291652-3
  }}</ref> If so, the kernel accesses the [[memory management unit]] to populate the physical data region and translate the address.<ref name="esa-ch1-p8">{{cite book
  | last = Lacamera
  | first = Daniele
  | title = Embedded Systems Architecture
  | publisher = Packt
  | year = 2018
  | page = 8
  | isbn = 978-1-78883-250-2
  }}</ref>
:* The kernel allocates memory from the ''heap'' upon request by a process.<ref name="cpl-ch8-p187"/> When the process is finished with the memory, the process may request for it to be [[Manual memory management|freed]]. If the process exits without requesting all allocated memory to be freed, then the kernel performs [[Garbage collection (computer science)|garbage collection]] to free the memory.
:* The kernel also ensures that a process only accesses its own memory, and not that of the kernel or other processes.<ref name="lpi-ch2-p22"/>
* The kernel program should perform [[File system|file system management]].<ref name="lpi-ch2-p22"/> The kernel has instructions to create, retrieve, update, and delete files.
* The kernel program should perform [[Peripheral|device management]].<ref name="lpi-ch2-p22"/> The kernel provides programs to standardize and simplify the interface to the mouse, keyboard, disk drives, printers, and other devices. Moreover, the kernel should arbitrate access to a device if two processes request it at the same time.
* The kernel program should perform [[network management]].<ref name="lpi-ch2-p23">{{cite book
 |title=The Linux Programming Interface
 |last=Kerrisk
 |first=Michael
 |publisher=No Starch Press
 |year=2010
 |isbn=978-1-59327-220-3
 |page=23}}</ref> The kernel transmits and receives [[Network packet|packets]] on behalf of processes. One key service is to find an efficient [[Routing table|route]] to the target system.
* The kernel program should provide [[system calls|system level functions]] for programmers to use.<ref name="upe-ch7-p201">{{cite book
 |title=The Unix Programming Environment
 |last=Kernighan
 |first=Brian W.
 |publisher=Prentice Hall
 |year=1984
 |isbn=0-13-937699-2
 |page=201}}</ref>
** Programmers access files through a relatively simple interface that in turn executes a relatively complicated low-level I/O interface. The low-level interface includes file creation, [[file descriptor]]s, file seeking, physical reading, and physical writing.
** Programmers create processes through a relatively simple interface that in turn executes a relatively complicated low-level interface.
** Programmers perform date/time arithmetic through a relatively simple interface that in turn executes a relatively complicated low-level time interface.<ref name="lpi-ch10-p187">{{cite book
 |title=The Linux Programming Interface
 |last=Kerrisk
 |first=Michael
 |publisher=No Starch Press
 |year=2010
 |isbn=978-1-59327-220-3
 |page=187}}</ref>
* The kernel program should provide a [[Inter-process communication|communication channel]] between executing processes.<ref name="usp-ch6-p121">{{cite book
 |title=Unix System Programming
 |last=Haviland
 |first=Keith
 |publisher=Addison-Wesley Publishing Company
 |year=1987
 |isbn=0-201-12919-1
 |page=121}}</ref> For a large software system, it may be desirable to [[Software engineering|engineer]] the system into smaller processes. Processes may communicate with one another by sending and receiving [[Signal (IPC)|signals]].

Originally, operating systems were programmed in [[assembly language|assembly]]; however, modern operating systems are typically written in higher-level languages like [[C (programming language)|C]], [[Objective-C]], and [[Swift (programming language)|Swift]].{{efn|The [[UNIX]] operating system was written in C, [[macOS]] was written in Objective-C, and Swift replaced Objective-C.}}