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110-520: The Session Manager Subsystem is the first user-mode process started by the kernel. Once started it creates additional paging files with configuration data from HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Memory Management, the environment variables located at the registry entry HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\Environment , and DOS device mappings (e.g. CON: , NUL: , AUX: , COM1: , COM2: , COM3: , COM4: , PRN: , LPT1: , LPT2: , LPT3: , and drive letters) listed at

220-428: A system call to perform a block I/O write operation, then the system call might execute the following instructions: While the writing takes place, the operating system will context switch to other processes as normal. When the device finishes writing, the device will interrupt the currently running process by asserting an interrupt request . The device will also place an integer onto the data bus. Upon accepting

330-483: A 64 KiB page frame in the reserved upper memory area. 80386 and later systems could use a virtual 8086 mode (V86) mode memory manager like EMM386 to create expanded memory from extended memory without the need of an add-on card. The second specification was the Extended Memory Specification (XMS) for 80286 and later systems. This provided a way to copy data to and from extended memory, access to

440-401: A batch file is interpreted as a program to run. Batch files can also make use of internal commands, such as GOTO and conditional statements . The operating system offers an application programming interface that allows development of character-based applications, but not for accessing most of the hardware , such as graphics cards , printers , or mice . This required programmers to access

550-645: A computer even if they are not compatible with the base operating system. A library operating system (libOS) is one in which the services that a typical operating system provides, such as networking, are provided in the form of libraries and composed with a single application and configuration code to construct a unikernel : a specialized (only the absolute necessary pieces of code are extracted from libraries and bound together ), single address space , machine image that can be deployed to cloud or embedded environments. The operating system code and application code are not executed in separated protection domains (there

660-494: A configuration file similar to CONFIG.SYS and AUTOEXEC.BAT. If the MSDOS.SYS BootGUI directive is set to 0 , the boot process will stop with the command processor (typically COMMAND.COM) loaded, instead of executing WIN.COM automatically. DOS uses a filesystem which supports 8.3 filenames : 8 characters for the filename and 3 characters for the extension. Starting with DOS 2 hierarchical directories are supported. Each directory name

770-425: A conversation with fellow United Way National Board Executive Committee member Mary Maxwell Gates , who referred Opel to her son Bill Gates for help with an 8088-compatible build of CP/M. IBM was then sent to Digital Research, and a meeting was set up. However, initial negotiations for the use of CP/M broke down: Digital Research wished to sell CP/M on a royalty basis, while IBM sought a single license, and to change

880-571: A development of MULTICS for a single user. Because UNIX's source code was available, it became the basis of other, incompatible operating systems, of which the most successful were AT&T 's System V and the University of California 's Berkeley Software Distribution (BSD). To increase compatibility, the IEEE released the POSIX standard for operating system application programming interfaces (APIs), which

990-515: A few developers and computer engineers still use it because it is close to the hardware. DOS's structure of accessing hardware directly allows it to be used in embedded devices . The final versions of DR-DOS are still aimed at this market. ROM-DOS is used as operating system for the Canon PowerShot Pro 70. On Linux , it is possible to run DOSEMU , a Linux-native virtual machine for running DOS programs at near native speed. There are

1100-479: A file name with a space, has sometimes been used by viruses or hacking programs to obscure files from users who do not know how to access these locations. DOS was designed for the Intel 8088 processor, which can only directly access a maximum of 1 MiB of RAM. Both IBM and Microsoft chose 640 kibibytes (KiB) as the maximum amount of memory available to programs and reserved the remaining 384 KiB for video memory,

1210-484: A large legal settlement was paid. In the twenty-first century, Windows continues to be popular on personal computers but has less market share of servers. UNIX operating systems, especially Linux, are the most popular on enterprise systems and servers but are also used on mobile devices and many other computer systems. On mobile devices, Symbian OS was dominant at first, being usurped by BlackBerry OS (introduced 2002) and iOS for iPhones (from 2007). Later on,

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1320-442: A library with no protection between applications, such as eCos . A hypervisor is an operating system that runs a virtual machine . The virtual machine is unaware that it is an application and operates as if it had its own hardware. Virtual machines can be paused, saved, and resumed, making them useful for operating systems research, development, and debugging. They also enhance portability by enabling applications to be run on

1430-447: A malformed machine instruction . However, the most common error conditions are division by zero and accessing an invalid memory address . Users can send messages to the kernel to modify the behavior of a currently running process. For example, in the command-line environment , pressing the interrupt character (usually Control-C ) might terminate the currently running process. To generate software interrupts for x86 CPUs,

1540-511: A manifesto proposing the development of an open-source replacement. Within a few weeks, other programmers including Pat Villani and Tim Norman joined the project. A kernel, the COMMAND.COM command line interpreter (shell), and core utilities were created by pooling code they had written or found available. There were several official pre-release distributions of FreeDOS before the FreeDOS 1.0 distribution

1650-658: A more user-friendly environment, numerous software manufacturers wrote file management programs that provided users with WIMP interfaces. Microsoft Windows is a notable example, eventually resulting in Microsoft Windows 9x becoming a self-contained program loader, and replacing DOS as the most-used PC-compatible program loader. Text user interface programs included Norton Commander , DOS Navigator , Volkov Commander , Quarterdesk DESQview , and Sidekick . Graphical user interface programs included Digital Research's GEM (originally written for CP/M) and GEOS . Eventually,

1760-513: A number of other emulators for running DOS on various versions of Unix and Microsoft Windows such as DOSBox . DOSBox is designed for legacy gaming (e.g. King's Quest , Doom ) on modern operating systems. DOSBox includes its own implementation of DOS which is strongly tied to the emulator and cannot run on real hardware, but can also boot MS-DOS, FreeDOS, or other DOS operating systems if needed. MS-DOS and IBM PC DOS related operating systems are commonly associated with machines using

1870-455: A particular application's memory is stored, or even whether or not it has been allocated yet. In modern operating systems, memory which is accessed less frequently can be temporarily stored on a disk or other media to make that space available for use by other programs. This is called swapping , as an area of memory can be used by multiple programs, and what that memory area contains can be swapped or exchanged on demand. Virtual memory provides

1980-503: A program does not interfere with memory already in use by another program. Since programs time share, each program must have independent access to memory. Cooperative memory management, used by many early operating systems, assumes that all programs make voluntary use of the kernel 's memory manager, and do not exceed their allocated memory. This system of memory management is almost never seen any more, since programs often contain bugs which can cause them to exceed their allocated memory. If

2090-408: A program fails, it may cause memory used by one or more other programs to be affected or overwritten. Malicious programs or viruses may purposefully alter another program's memory, or may affect the operation of the operating system itself. With cooperative memory management, it takes only one misbehaved program to crash the system. Memory protection enables the kernel to limit a process' access to

2200-413: A program run from one floppy while accessing its data on another. Hard drives were originally assigned the letters "C" and "D". DOS could only support one active partition per drive. As support for more hard drives became available, this developed into first assigning a drive letter to each drive's active primary partition , then making a second pass over the drives to allocate letters to logical drives in

2310-440: A program tries to access memory that is not accessible memory, but nonetheless has been allocated to it, the kernel is interrupted (see § Memory management ) . This kind of interrupt is typically a page fault . When the kernel detects a page fault it generally adjusts the virtual memory range of the program which triggered it, granting it access to the memory requested. This gives the kernel discretionary power over where

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2420-501: A retail version of MS-DOS, starting with MS-DOS 5.0. In the mid-1980s, Microsoft developed a multitasking version of DOS . This version of DOS is generally referred to as "European MS-DOS 4" because it was developed for ICL and licensed to several European companies. This version of DOS supports preemptive multitasking, shared memory, device helper services and New Executable ("NE") format executables. None of these features were used in later versions of DOS, but they were used to form

2530-726: A second Client/Server Runtime Subsystem ( csrss.exe ) for Session 0, a session dedicated to system processes. From here, the Windows Startup Application starts the Service Control Manager ( services.exe ), which starts all the Windows services that are set to "Auto-Start". The application also starts the Local Security Authority Subsystem Service ( lsass.exe ). Before Windows Vista, these processes where started by Windows Logon instead of

2640-566: A series of disagreements over two successor operating systems to DOS, OS/2 and Windows. They split development of their DOS systems as a result. The last retail version of MS-DOS was MS-DOS 6.22; after this, MS-DOS became part of Windows 95, 98 and Me. The last retail version of PC DOS was PC DOS 2000 (also called PC DOS 7 revision 1), though IBM did later develop PC DOS 7.10 for OEMs and internal use. The FreeDOS project began on 26 June 1994, when Microsoft announced it would no longer sell or support MS-DOS. Jim Hall then posted

2750-467: A significant amount of CPU time. Direct memory access (DMA) is an architecture feature to allow devices to bypass the CPU and access main memory directly. (Separate from the architecture, a device may perform direct memory access to and from main memory either directly or via a bus.) When a computer user types a key on the keyboard, typically the character appears immediately on the screen. Likewise, when

2860-402: A specific moment in time. Hard real-time systems require exact timing and are common in manufacturing , avionics , military, and other similar uses. With soft real-time systems, the occasional missed event is acceptable; this category often includes audio or multimedia systems, as well as smartphones. In order for hard real-time systems be sufficiently exact in their timing, often they are just

2970-415: A time can use them, and DOS itself has no functionality to allow more than one program to execute at a time. The DOS kernel provides various functions for programs (an application program interface ), like character I/O, file management, memory management, program loading and termination. DOS provides the ability for shell scripting via batch files (with the filename extension .BAT ). Each line of

3080-417: A user moves a mouse , the cursor immediately moves across the screen. Each keystroke and mouse movement generates an interrupt called Interrupt-driven I/O . An interrupt-driven I/O occurs when a process causes an interrupt for every character or word transmitted. Devices such as hard disk drives , solid-state drives , and magnetic tape drives can transfer data at a rate high enough that interrupting

3190-453: A variation of the classic reader/writer problem . The writer receives a pipe from the shell for its output to be sent to the reader's input stream. The command-line syntax is alpha | bravo . alpha will write to the pipe when its computation is ready and then sleep in the wait queue. bravo will then be moved to the ready queue and soon will read from its input stream. The kernel will generate software interrupts to coordinate

3300-561: A year Microsoft licensed MS-DOS to over 70 other companies, which supplied the operating system for their own hardware, sometimes under their own names. Microsoft later required the use of the MS-DOS name, with the exception of the IBM variant. IBM continued to develop their version, PC DOS , for the IBM PC. Digital Research became aware that an operating system similar to CP/M was being sold by IBM (under

3410-418: Is interrupted by it. Operating systems are found on many devices that contain a computer – from cellular phones and video game consoles to web servers and supercomputers . In the personal computer market, as of September 2024 , Microsoft Windows holds a dominant market share of around 73%. macOS by Apple Inc. is in second place (15%), Linux is in third place (5%), and ChromeOS

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3520-562: Is remote direct memory access , which enables each CPU to access memory belonging to other CPUs. Multicomputer operating systems often support remote procedure calls where a CPU can call a procedure on another CPU, or distributed shared memory , in which the operating system uses virtualization to generate shared memory that does not physically exist. A distributed system is a group of distinct, networked computers—each of which might have their own operating system and file system. Unlike multicomputers, they may be dispersed anywhere in

3630-484: Is a change away from the currently running process. Similarly, both hardware and software interrupts execute an interrupt service routine . Software interrupts may be normally occurring events. It is expected that a time slice will occur, so the kernel will have to perform a context switch . A computer program may set a timer to go off after a few seconds in case too much data causes an algorithm to take too long. Software interrupts may be error conditions, such as

3740-432: Is a family of disk-based operating systems for IBM PC compatible computers. The DOS family primarily consists of IBM PC DOS and a rebranded version, Microsoft 's MS-DOS , both of which were introduced in 1981. Later compatible systems from other manufacturers include DR-DOS (1988), ROM-DOS (1989), PTS-DOS (1993), and FreeDOS (1998). MS-DOS dominated the IBM PC compatible market between 1981 and 1995. Although

3850-530: Is a real DOS, like MS-DOS 6.22 or PC DOS 5.00. One makes a bootable floppy disk of the DOS, adds a number of drivers from OS/2, and then creates a special image. The DOS booted this way has full access to the system, but provides its own drivers for hardware. One can use such a disk to access cdrom drives for which there is no OS/2 driver. In all 32-bit (IA-32) editions of the Windows NT family since 1993, DOS emulation

3960-705: Is also 8.3 format but the maximum directory path length is 64 characters due to the internal current directory structure (CDS) tables that DOS maintains. Including the drive name, the maximum length of a fully qualified filename that DOS supports is 80 characters using the format drive:\path\filename.ext followed by a null byte. DOS uses the File Allocation Table (FAT) filesystem. This was originally FAT12 which supported up to 4078 clusters per drive. DOS 3.0 added support for FAT16 which used 16-bit allocation entries and supported up to 65518 clusters per drive. Compaq MS-DOS 3.31 added support for FAT16B which removed

4070-431: Is an optional built-in driver for a fourth line printer supported in some versions of DR-DOS since 7.02. CONFIG$ constitutes the real mode PnP manager in MS-DOS 7.0–8.0. AUX typically defaults to COM1 , and PRN to LPT1 ( LST ), but these defaults can be changed in some versions of DOS to point to other serial or parallel devices. The PLT device (present only in some HP OEM versions of MS-DOS)

4180-520: Is available in COMMAND.COM. Programs like the Microsoft CD-ROM Extensions (MSCDEX) provide access to files on CD-ROM disks. Some TSRs can even perform a rudimentary form of task switching. For example, the shareware program Back and Forth (1990) has a hotkey to save the state of the currently-running program to disk, load another program, and switch to it, making it possible to switch "back and forth" between programs (albeit slowly, due to

4290-408: Is based upon DOS 5. Although there is a default configuration (config.sys and autoexec.bat), one can use alternate files on a session-by-session basis. It is possible to load drivers in these files to access the host system, although these are typically third-party. Under OS/2 2.x and later, the DOS emulation is provided by DOSKRNL. This is a file that represents the combined IBMBIO.COM and IBMDOS.COM,

4400-422: Is difficult to define, but has been called "the layer of software that manages a computer's resources for its users and their applications ". Operating systems include the software that is always running, called a kernel —but can include other software as well. The two other types of programs that can run on a computer are system programs —which are associated with the operating system, but may not be part of

4510-896: Is in fourth place (2%). In the mobile sector (including smartphones and tablets ), as of September 2023 , Android's share is 68.92%, followed by Apple's iOS and iPadOS with 30.42%, and other operating systems with .66%. Linux distributions are dominant in the server and supercomputing sectors. Other specialized classes of operating systems (special-purpose operating systems), such as embedded and real-time systems, exist for many applications. Security-focused operating systems also exist. Some operating systems have low system requirements (e.g. light-weight Linux distribution ). Others may have higher system requirements. Some operating systems require installation or may come pre-installed with purchased computers ( OEM -installation), whereas others may run directly from media (i.e. live CD ) or flash memory (i.e. USB stick). An operating system

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4620-502: Is invalid." These names (except for NUL) have continued to be supported in all versions of MS-DOS, PC DOS and DR-DOS ever since. LST was also available in some OEM versions of MS-DOS 1.25, whereas other OEM versions of MS-DOS 1.25 already used LPT1 (first line printer ) and COM1 (first serial communication device ) instead, as introduced with PC DOS. In addition to LPT1 and LPT2 as well as COM1 to COM3 , Hewlett-Packard's OEM version of MS-DOS 2.11 for

4730-413: Is likely to render the media unbootable. It is, however, possible to replace the shell at will, a method that can be used to start the execution of dedicated applications faster. This limitation does not apply to any version of DR DOS, where the system files can be located anywhere in the root directory and do not need to be contiguous. Therefore, system files can be simply copied to a disk provided that

4840-443: Is only a single application running, at least conceptually, so there is no need to prevent interference between applications) and OS services are accessed via simple library calls (potentially inlining them based on compiler thresholds), without the usual overhead of context switches , in a way similarly to embedded and real-time OSes. Note that this overhead is not negligible: to the direct cost of mode switching it's necessary to add

4950-504: Is provided by way of a virtual DOS machine (NTVDM). 64-bit (IA-64 and x86-64) versions of Windows do not support NTVDM and cannot run 16-bit DOS applications directly; third-party emulators such as DOSbox can be used to run DOS programs on those machines. DOS systems use a command-line interface . A program is started by entering its filename at the command prompt. DOS systems include utility programs and provide internal commands that do not correspond to programs. In an attempt to provide

5060-499: Is supported by most UNIX systems. MINIX was a stripped-down version of UNIX, developed in 1987 for educational uses, that inspired the commercially available, free software Linux . Since 2008, MINIX is used in controllers of most Intel microchips , while Linux is widespread in data centers and Android smartphones. The invention of large scale integration enabled the production of personal computers (initially called microcomputers ) from around 1980. For around five years,

5170-473: Is that they do not load user-installed software. Consequently, they do not need protection between different applications, enabling simpler designs. Very small operating systems might run in less than 10 kilobytes , and the smallest are for smart cards . Examples include Embedded Linux , QNX , VxWorks , and the extra-small systems RIOT and TinyOS . A real-time operating system is an operating system that guarantees to process events or data by or at

5280-435: Is the part of the operating system that provides protection between different applications and users. This protection is key to improving reliability by keeping errors isolated to one program, as well as security by limiting the power of malicious software and protecting private data, and ensuring that one program cannot monopolize the computer's resources. Most operating systems have two modes of operation: in user mode ,

5390-608: The HKLM\System\CurrentControlSet\Control\Session Manager\DOS Devices registry key. This can be used to create permanent subst drives. The manager is responsible for starting the kernel and user modes of the Win32 subsystem. This subsystem includes win32k.sys (kernel-mode), winsrv.dll (user-mode), and csrss.exe (user-mode). Any other subsystems listed in the Required value of

5500-648: The HKLM\System\CurrentControlSet\Control\Session Manager\SubSystems Registry key are also started. The manager is also responsible for doing any operations that are requested to be done at the start of a session. Commands listed in HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\BootExecute , such as autochk and convert , are executed. These commands are run before services are loaded by later steps of

5610-566: The Asynchronous Local Inter-Process Communication (ALPC) port SmSsWinStationApiPort to start new sessions. Each time a user logs onto the system, the initial Session Manager creates a new instance of itself to configure a new session. This new process starts a Win32 subsystem and Winlogon process for the new session. This allows for multiple users to logon at the same time on Windows Server systems. DOS DOS ( / d ɒ s / , / d ɔː s / )

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5720-535: The CP/M (Control Program for Microcomputers) was the most popular operating system for microcomputers. Later, IBM bought the DOS (Disk Operating System) from Microsoft . After modifications requested by IBM, the resulting system was called MS-DOS (MicroSoft Disk Operating System) and was widely used on IBM microcomputers. Later versions increased their sophistication, in part by borrowing features from UNIX. Apple 's Macintosh

5830-686: The HP Portable Plus also supported LST as alias for LPT2 and 82164A as alias for COM2 ; it also supported PLT for plotters . Otherwise, COM2 , LPT2 , LPT3 and the CLOCK$ (still named CLOCK in some issues of MS-DOS 2.11 ) clock device were introduced with DOS 2.0, and COM3 and COM4 were added with DOS 3.3. Only the multitasking MS-DOS 4 supported KEYBD$ and SCREEN$ . DR DOS 5.0 and higher and Multiuser DOS support an $ IDLE$ device for dynamic idle detection to saving power and improve multitasking. LPT4

5940-496: The INT assembly language instruction is available. The syntax is INT X , where X is the offset number (in hexadecimal format) to the interrupt vector table . To generate software interrupts in Unix-like operating systems, the kill(pid,signum) system call will send a signal to another process. pid is the process identifier of the receiving process. signum is

6050-631: The Intel x86 or compatible CPUs , mainly IBM PC compatibles . Machine-dependent versions of MS-DOS were produced for many non-IBM-compatible x86 -based machines, with variations from relabelling of the Microsoft distribution under the manufacturer's name, to versions specifically designed to work with non-IBM-PC-compatible hardware. As long as application programs used DOS APIs instead of direct hardware access, they could run on both IBM-PC-compatible and incompatible machines. The original FreeDOS kernel, DOS-C ,

6160-490: The extended partition , then a third pass to give any other non-active primary partitions their names (where such additional partitions existed and contained a DOS-supported file system). Lastly, DOS allocates letters for optical disc drives , RAM disks , and other hardware. Letter assignments usually occur in the order the drivers are loaded, but the drivers can instruct DOS to assign a different letter; drivers for network drives, for example, typically assign letters nearer to

6270-509: The read-only memory of adapters on some video and network peripherals, and the system's BIOS. By 1985, some DOS applications were already hitting the memory limit, while much of reserved was unused, depending on the machine's specifications. Specifications were developed to allow access to additional memory. The first was the Expanded Memory Specification (EMS) was designed to allow memory on an add-on card to be accessed via

6380-420: The transistor in the mid-1950s, mainframes began to be built. These still needed professional operators who manually do what a modern operating system would do, such as scheduling programs to run, but mainframes still had rudimentary operating systems such as Fortran Monitor System (FMS) and IBSYS . In the 1960s, IBM introduced the first series of intercompatible computers ( System/360 ). All of them ran

6490-494: The 32‑ MiB drive limit and could support up to 512 MiB. Finally MS-DOS 7.1 (the DOS component of Windows 9x) added support for FAT32 which used 32-bit allocation entries and could support hard drives up to 137 GiB and beyond. Starting with DOS 3.1, file redirector support was added to DOS. This was initially used to support networking but was later used to support CD-ROM drives with MSCDEX . IBM PC DOS 4.0 also had preliminary installable file system (IFS) support but this

6600-572: The 65,520-byte high memory area directly above the first megabyte of memory and the upper memory block area. Generally XMS support was provided by HIMEM.SYS or a V86 mode memory manager like QEMM or 386MAX which also supported EMS. Starting with DOS 5, DOS could directly take advantage of the HMA by loading its kernel code and disk buffers there via the DOS=HIGH statement in CONFIG.SYS. DOS 5+ also allowed

6710-410: The CPU for every byte or word transferred, and having the CPU transfer the byte or word between the device and memory, would require too much CPU time. Data is, instead, transferred between the device and memory independently of the CPU by hardware such as a channel or a direct memory access controller; an interrupt is delivered only when all the data is transferred. If a computer program executes

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6820-474: The CPU to re-enter supervisor mode , placing the kernel in charge. This is called a segmentation violation or Seg-V for short, and since it is both difficult to assign a meaningful result to such an operation, and because it is usually a sign of a misbehaving program, the kernel generally resorts to terminating the offending program, and reports the error. Windows versions 3.1 through ME had some level of memory protection, but programs could easily circumvent

6930-541: The DOS virtual machine is provided by WINOLDAP. WinOldAp creates a virtual machine based on the program's PIF file, and the system state when Windows was loaded. The DOS graphics mode, both character and graphic, can be captured and run in the window. DOS applications can use the Windows clipboard by accessing extra published calls in WinOldAp, and one can paste text through the WinOldAp graphics. The emulated DOS in OS/2 and Windows NT

7040-603: The Windows Startup Application. Once the session is configured, the Session Manager Subsystem starts Winlogon (Windows Logon Application), which is responsible for handling interactive logons to a Windows system, either local or remote. After the boot process is finished, the program resides in memory and can be seen running in the Windows Task Manager . It then waits for either winlogon.exe or csrss.exe to end, at which point Windows will shut down. If

7150-534: The application program, which then interacts with the user and with hardware devices. However, in some systems an application can request that the operating system execute another application within the same process, either as a subroutine or in a separate thread, e.g., the LINK and ATTACH facilities of OS/360 and successors . An interrupt (also known as an abort , exception , fault , signal , or trap ) provides an efficient way for most operating systems to react to

7260-536: The basis of the OS/2 1.0 kernel. This version of DOS is distinct from the widely released PC DOS 4.0 which was developed by IBM and based upon DOS 3.3. Digital Research attempted to regain the market lost from CP/M-86, initially with Concurrent DOS , FlexOS and DOS Plus (both compatible with both MS-DOS and CP/M-86 software), later with Multiuser DOS (compatible with both MS-DOS and CP/M-86 software) and DR DOS (compatible with MS-DOS software). Digital Research

7370-411: The boot sector is DR DOS compatible already. In PC DOS and DR DOS 5.0 and above, the DOS system files are named IBMBIO.COM instead of IO.SYS and IBMDOS.COM instead of MSDOS.SYS . Older versions of DR DOS used DRBIOS.SYS and DRBDOS.SYS instead. Starting with MS-DOS 7.0 the binary system files IO.SYS and MSDOS.SYS were combined into a single file IO.SYS whilst MSDOS.SYS became

7480-404: The booting process. Any rename operations queued at HKLM\SYSTEM\CurrentControlSet\Control\Session Manager\PendingFileRenameOperations . This is used to allow previously in-use files (e.g. drivers) to be replaced as part of a reboot. Starting with Windows Vista, the Session Manager Subsystem creates a temporary instance of itself that launches the Windows Startup Application ( wininit.exe ) and

7590-453: The computer's memory. Various methods of memory protection exist, including memory segmentation and paging . All methods require some level of hardware support (such as the 80286 MMU), which does not exist in all computers. In both segmentation and paging, certain protected mode registers specify to the CPU what memory address it should allow a running program to access. Attempts to access other addresses trigger an interrupt, which causes

7700-404: The corresponding load drive whenever an application starts. There are reserved device names in DOS that cannot be used as filenames regardless of extension as they are occupied by built-in character devices. These restrictions also affect several Windows versions, in some cases causing crashes and security vulnerabilities. The reserved names are: In Windows 95 and Windows 98 , typing in

7810-499: The default OS kernel , though the MS-DOS component remained for compatibility. With Windows 95 and 98, but not ME, the MS-DOS component could be run without starting Windows. With DOS no longer required to use Windows, the majority of users stopped using it directly. As of 2024 , available compatible systems are FreeDOS , ROM-DOS , PTS-DOS , RxDOS and REAL/32 . Some computer manufacturers, including Dell and HP , sell computers with FreeDOS as an OEM operating system. And

7920-471: The details of how interrupt service routines behave vary from operating system to operating system. However, several interrupt functions are common. The architecture and operating system must: A software interrupt is a message to a process that an event has occurred. This contrasts with a hardware interrupt — which is a message to the central processing unit (CPU) that an event has occurred. Software interrupts are similar to hardware interrupts — there

8030-406: The disk access required). Back and Forth could not enable background processing however; that needed DESQview (on at least a 386 ). Operating system An operating system ( OS ) is system software that manages computer hardware and software resources, and provides common services for computer programs . Time-sharing operating systems schedule tasks for efficient use of

8140-408: The end of the alphabet. Because DOS applications use these drive letters directly (unlike the /dev directory in Unix-like systems), they can be disrupted by adding new hardware that needs a drive letter. An example is the addition of a new hard drive having a primary partition where a pre-existing hard drive contains logical drives in extended partitions; the new drive will be assigned a letter that

8250-422: The environment. Interrupts cause the central processing unit (CPU) to have a control flow change away from the currently running program to an interrupt handler , also known as an interrupt service routine (ISR). An interrupt service routine may cause the central processing unit (CPU) to have a context switch . The details of how a computer processes an interrupt vary from architecture to architecture, and

8360-410: The hardware checks that the software is only executing legal instructions, whereas the kernel has unrestricted powers and is not subject to these checks. The kernel also manages memory for other processes and controls access to input/output devices. The operating system provides an interface between an application program and the computer hardware, so that an application program can interact with

8470-400: The hardware directly, usually resulting in each application having its own set of device drivers for each hardware peripheral. Hardware manufacturers would release specifications to ensure device drivers for popular applications were available. The DOS system files loaded by the boot sector must be contiguous and be the first two directory entries . As such, removing and adding this file

8580-493: The hardware only by obeying rules and procedures programmed into the operating system. The operating system is also a set of services which simplify development and execution of application programs. Executing an application program typically involves the creation of a process by the operating system kernel , which assigns memory space and other resources, establishes a priority for the process in multi-tasking systems, loads program binary code into memory, and initiates execution of

8690-418: The indirect pollution of important processor structures (like CPU caches , the instruction pipeline , and so on) which affects both user-mode and kernel-mode performance. The first computers in the late 1940s and 1950s were directly programmed either with plugboards or with machine code inputted on media such as punch cards , without programming languages or operating systems. After the introduction of

8800-404: The interrupt request, the operating system will: When the writing process has its time slice expired, the operating system will: With the program counter now reset, the interrupted process will resume its time slice. Among other things, a multiprogramming operating system kernel must be responsible for managing all system memory which is currently in use by the programs. This ensures that

8910-575: The introduction of Xenix . The company planned to improve MS-DOS over time, so it would be almost indistinguishable from single-user Xenix, or XEDOS , which would also run on the Motorola 68000 , Zilog Z-8000 , and LSI-11 ; they would be upwardly compatible with Xenix, which BYTE in 1983 described as "the multi-user MS-DOS of the future". IBM, however, did not want to replace DOS. After AT&T began selling Unix, Microsoft and IBM began developing OS/2 as an alternative. The two companies later had

9020-431: The kernel—and applications—all other software. There are three main purposes that an operating system fulfills: With multiprocessors multiple CPUs share memory. A multicomputer or cluster computer has multiple CPUs, each of which has its own memory . Multicomputers were developed because large multiprocessors are difficult to engineer and prohibitively expensive; they are universal in cloud computing because of

9130-458: The location of the reserved name (such as CON/CON, AUX/AUX, or PRN/PRN) crashes the operating system, of which Microsoft has provided a security fix for the issue. In Windows XP , the name of the file or folder using a reserved name silently reverts to its previous name, with no notification or error message. In Windows Vista and later, attempting to use a reserved name for a file or folder brings up an error message saying "The specified device name

9240-399: The manufacturers of major DOS systems began to include their own environment managers. MS-DOS/IBM DOS 4 included DOS Shell ; DR DOS 5.0, released the following year, included ViewMAX , based upon GEM. Although DOS is not a multitasking operating system, it does provide a terminate-and-stay-resident (TSR) function which allows programs to remain resident in memory. These programs can hook

9350-400: The memory allocated to a different one. Around the same time, teleprinters began to be used as terminals so multiple users could access the computer simultaneously. The operating system MULTICS was intended to allow hundreds of users to access a large computer. Despite its limited adoption, it can be considered the precursor to cloud computing . The UNIX operating system originated as

9460-431: The name has come to be identified specifically with this particular family of operating systems, DOS is a platform-independent acronym for disk operating system , whose use predates the IBM PC. Dozens of other operating systems also use the acronym, beginning with the mainframe DOS/360 from 1966. Others include Apple DOS , Apple ProDOS , Atari DOS , Commodore DOS , TRSDOS , and AmigaDOS . IBM PC DOS (and

9570-668: The name to "PC DOS". Digital Research founder Gary Kildall refused, and IBM withdrew. IBM again approached Bill Gates. Gates in turn approached Seattle Computer Products . There, programmer Tim Paterson had developed a variant of CP/M-80 , intended as an internal product for testing SCP's new 16-bit Intel 8086 CPU card for the S-100 bus . The system was initially named QDOS (Quick and Dirty Operating System), before being made commercially available as 86-DOS . Microsoft purchased 86-DOS, allegedly for US$ 50,000. This became Microsoft Disk Operating System, MS-DOS, introduced in 1981. Within

9680-408: The need to use it. A general protection fault would be produced, indicating a segmentation violation had occurred; however, the system would often crash anyway. The use of virtual memory addressing (such as paging or segmentation) means that the kernel can choose what memory each program may use at any given time, allowing the operating system to use the same memory locations for multiple tasks. If

9790-408: The open-source Android operating system (introduced 2008), with a Linux kernel and a C library ( Bionic ) partially based on BSD code, became most popular. The components of an operating system are designed to ensure that various parts of a computer function cohesively. With the de facto obsoletion of DOS , all user software must interact with the operating system to access hardware. The kernel

9900-421: The piping. Signals may be classified into 7 categories. The categories are: Input/output (I/O) devices are slower than the CPU. Therefore, it would slow down the computer if the CPU had to wait for each I/O to finish. Instead, a computer may implement interrupts for I/O completion, avoiding the need for polling or busy waiting. Some computers require an interrupt for each character or word, costing

10010-479: The processes do not end in an expected fashion, smss.exe may hang the system, or a bugcheck will occur. It also initiates new user sessions when needed. In some versions of Windows, by using special tools, the critical process status on smss.exe can be removed, after that, it can be terminated without a bluescreen, but any functions that use smss.exe stop working until next reboot The Local Session Manager Service ( lsm.exe ) sends requests to SMSS through

10120-501: The same name that IBM insisted upon for CP/M), and threatened legal action. IBM responded by offering an agreement: they would give PC consumers a choice of PC DOS or CP/M-86 , Kildall's 8086 version. Side-by-side, CP/M cost US$ 200 more than PC DOS, and sales were low. CP/M faded, with MS-DOS and PC DOS becoming the marketed operating system for PCs and PC compatibles. Microsoft originally sold MS-DOS only to original equipment manufacturers (OEMs). One major reason for this

10230-418: The same operating system— OS/360 —which consisted of millions of lines of assembly language that had thousands of bugs . The OS/360 also was the first popular operating system to support multiprogramming , such that the CPU could be put to use on one job while another was waiting on input/output (I/O). Holding multiple jobs in memory necessitated memory partitioning and safeguards against one job accessing

10340-501: The separately sold MS-DOS ) and its predecessor, 86-DOS , ran on Intel 8086 16-bit processors. It was developed to be similar to Digital Research 's CP/M —the dominant disk operating system for 8-bit Intel 8080 and Zilog Z80 microcomputers—in order to simplify porting CP/M applications to MS-DOS. When IBM introduced the IBM PC , built with the Intel 8088 microprocessor, they needed an operating system. Chairman John Opel had

10450-400: The signal number (in mnemonic format) to be sent. (The abrasive name of kill was chosen because early implementations only terminated the process.) In Unix-like operating systems, signals inform processes of the occurrence of asynchronous events. To communicate asynchronously, interrupts are required. One reason a process needs to asynchronously communicate to another process solves

10560-400: The size of the machine needed. The different CPUs often need to send and receive messages to each other; to ensure good performance, the operating systems for these machines need to minimize this copying of packets . Newer systems are often multiqueue —separating groups of users into separate queues —to reduce the need for packet copying and support more concurrent users. Another technique

10670-442: The system and may also include accounting software for cost allocation of processor time , mass storage , peripherals, and other resources. For hardware functions such as input and output and memory allocation , the operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and frequently makes system calls to an OS function or

10780-506: The system calls are passed through to the OS/2 windowing services. DOS programs run in their own environment, the bulk of the DOS utilities are provided by bound DOS / OS2 applications in the \OS2 directory. OS/2 can run Windows 3.1 applications by using a modified copy of Windows (Win-OS/2). The modifications allow Windows 3.1 programs to run seamlessly on the OS/2 desktop, or one can start a WinOS/2 desktop, similar to starting Windows from DOS. OS/2 allows for 'DOS from Drive A:', (VMDISK). This

10890-606: The system timer or keyboard interrupts to allow themselves to run tasks in the background or to be invoked at any time, preempting the current running program and effectively implementing a simple form of multitasking on a program-specific basis. The DOS PRINT command does this to implement background print spooling. Borland Sidekick , a popup personal information manager (PIM), also uses this technique. Terminate-and-stay-resident programs are also used to provide additional features not available by default. Programs like CED and DOSKEY provide command-line editing facilities beyond what

11000-685: The use of available upper memory blocks via the DOS=UMB statement in CONFIG.SYS. The DOS emulation in OS/2 and Windows runs in much the same way as native applications do. They can access all of the drives and services, and can even use the host's clipboard services. Because the drivers for file systems and such forth reside in the host system, the DOS emulation needs only provide a DOS API translation layer which converts DOS calls to OS/2 or Windows system calls. The translation layer generally also converts BIOS calls and virtualizes common I/O port accesses which many DOS programs commonly use. In Windows 3.1 and 9x,

11110-417: The user changes them. Under DOS, this problem can be worked around by defining a SUBST drive and installing the DOS program into this logical drive. The assignment of this drive would then be changed in a batch job whenever the application starts. Under some versions of Concurrent DOS , as well as under Multiuser DOS , System Manager and REAL/32 , the reserved drive letter L: will automatically be assigned to

11220-473: The world. Middleware , an additional software layer between the operating system and applications, is often used to improve consistency. Although it functions similarly to an operating system, it is not a true operating system. Embedded operating systems are designed to be used in embedded computer systems , whether they are internet of things objects or not connected to a network. Embedded systems include many household appliances. The distinguishing factor

11330-416: Was bought by Novell , and DR DOS became PalmDOS and Novell DOS ; later, it was part of Caldera (under the names OpenDOS and DR-DOS 7.02 / 7.03 ), Lineo , and DeviceLogics . Gordon Letwin wrote in 1995 that "DOS was, when we first wrote it, a one-time throw-away product intended to keep IBM happy so that they'd buy our languages." Microsoft expected that it would be an interim solution before

11440-508: Was derived from DOS/NT for the Motorola 68000 series of CPUs in the early 1990s. While these systems loosely resembled the DOS architecture, applications were not binary compatible due to the incompatible instruction sets of these non-x86-CPUs. However, applications written in high-level languages could be ported easily. DOS is a single-user, single-tasking operating system with basic kernel functions that are non-reentrant : only one program at

11550-461: Was previously assigned to one of the extended partition logical drives. Moreover, even adding a new hard drive having only logical drives in an extended partition would still disrupt the letters of RAM disks and optical drives. This problem persisted through Microsoft's DOS-based 9x versions of Windows until they were replaced by versions based on the NT line, which preserves the letters of existing drives until

11660-467: Was reconfigurable as well. Filenames ended with a colon ( : ) such as NUL: conventionally indicate device names, but the colon is not actually a part of the name of the built-in device drivers. Colons are not necessary to be typed in some cases, for example: It is still possible to create files or directories using these reserved device names, such as through direct editing of directory data structures in disk sectors. Such naming, such as starting

11770-532: Was released on 3 September 2006. Made available under the GNU General Public License (GPL), FreeDOS does not require license fees or royalties. Early versions of Microsoft Windows ran on MS-DOS. By the early 1990s, the Windows graphical shell saw heavy use on new DOS systems. In 1995, Windows 95 was bundled as a standalone operating system that did not require a separate DOS license. Windows 95 (and Windows 98 and ME, that followed it) took over as

11880-415: Was that not all early PCs were 100% IBM PC compatible . DOS was structured such that there was a separation between the system specific device driver code ( IO.SYS ) and the DOS kernel ( MSDOS.SYS ). Microsoft provided an OEM Adaptation Kit (OAK) which allowed OEMs to customize the device driver code to their particular system. By the early 1990s, most PCs adhered to IBM PC standards so Microsoft began selling

11990-406: Was the first popular computer to use a graphical user interface (GUI). The GUI proved much more user friendly than the text-only command-line interface earlier operating systems had used. Following the success of Macintosh, MS-DOS was updated with a GUI overlay called Windows . Windows later was rewritten as a stand-alone operating system, borrowing so many features from another ( VAX VMS ) that

12100-518: Was unused and removed in DOS 5.0. DOS also supported Block Devices ("Disk Drive" devices) loaded from CONFIG.SYS that could be used under the DOS file system to support network devices. In DOS, drives are referred to by identifying letters. Standard practice is to reserve "A" and "B" for floppy drives . On systems with only one floppy drive DOS assigns both letters to the drive, prompting the user to swap disks as programs alternate access between them. This facilitates copying from floppy to floppy or having

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