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103-491: When a processor that supports x86 protected mode is powered on, it begins executing instructions in real mode , in order to maintain backward compatibility with earlier x86 processors. Protected mode may only be entered after the system software sets up one descriptor table and enables the Protection Enable (PE) bit in the control register 0 (CR0). Protected mode was first added to the x86 architecture in 1982, with

206-399: A hard disk drive . This allows for more memory to be used than physically available in primary storage. The x86 architecture allows control of pages through two arrays : page directories and page tables . Originally, a page directory was the size of one page, four kilobytes, and contained 1,024 page directory entries (PDE), although subsequent enhancements to the x86 architecture have added

309-428: 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

412-418: A trap with the use of instructions that involve input/output (I/O), which can negatively impact performance. Due to these limitations, some programs originally designed to run on the 8086 cannot be run in virtual 8086 mode. As a result, system software is forced to either compromise system security or backward compatibility when dealing with legacy software . An example of such a compromise can be seen with

515-468: A 16-bit address offset; the maximum sum occurs when both the segment and offset are 0xFFFF, yielding 0xFFFF0 + 0xFFFF = 0x10FFEF. On the 8086, 8088, and 80186, the result of an effective address that overflows 20 bits is that the address "wraps around" to the zero end of the address range, i.e. it is taken modulo 2^20 (2^20 = 1048576 = 0x100000). However, the 80286 has 24 address bits and computes effective addresses to 24 bits even in real mode. Therefore, for

618-400: A 32-bit kernel. However, 64-bit operating systems (which run in long mode ) no longer use this, since virtual 8086 mode has been removed from long mode. Real mode Real mode , also called real address mode , is an operating mode of all x86 -compatible CPUs . The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode

721-645: 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

824-403: A computer. OS/2 1.x defines restrictive programming rules allowing a Family API or bound program to run in either real or protected mode. Some early Unix operating systems, OS/2  1.x, and Windows used this mode. Windows 3.0 was able to run real mode programs in 16-bit protected mode; when switching to protected mode, it decided to preserve the single privilege level model that

927-571: 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

1030-543: A direct method to switch back into real mode once protected mode was entered. IBM devised a workaround (implemented in the IBM AT ) which involved resetting the CPU via the keyboard controller and saving the system registers, stack pointer and often the interrupt mask in the real-time clock chip's RAM. This allowed the BIOS to restore the CPU to a similar state and begin executing code before

1133-651: A far jump must be made to clear the prefetch input queue . With the release of the 386, protected mode could be exited by loading the segment registers with real mode values, disabling the A20 line and clearing the PE bit in the CR0 register, without the need to perform the initial setup steps required with the 286. Protected mode has a number of features designed to enhance an operating system's control over application software, in order to increase security and system stability. These additions allow

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1236-499: A granularity flag (G-bit, for short): The 386 processor also uses 32 bit values for the address offset. For maintaining compatibility with 286 protected mode a new default flag (D-bit, for short) was added. If the D-bit of a code segment is off (0) all commands inside this segment will be interpreted as 16-bit commands by default; if it is on (1), they will be interpreted as 32-bit commands. Where: In addition to adding virtual 8086 mode,

1339-484: 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,

1442-442: 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

1545-447: 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,

1648-455: 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

1751-503: 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

1854-408: 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

1957-440: 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

2060-486: A segment register in protected mode caused a 6-byte segment descriptor to be loaded into the CPU from memory, the segment register load instruction took many tens of processor cycles, making it much slower than on the 8086 and 8088; therefore, the strategy of computing segment addresses on-the-fly in order to access data structures larger than 128  kilobytes (the combined size of the two data segments) became impractical, even for those few programmers who had mastered it on

2163-421: 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

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2266-402: 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

2369-538: A system crash. Modern legacy boot loaders (such as GNU GRUB ) use A20 line. Intel introduced protected mode into the x86 family with the intention that operating systems which used it would run entirely in the new mode and that all programs running under a protected mode operating system would run in protected mode as well. Because of the substantial differences between real mode and even the rather limited 286 protected mode, programs written for real mode cannot run in protected mode without being rewritten. Therefore, with

2472-417: 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

2575-453: 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

2678-454: A wide base of existing real mode applications which users depended on, abandoning real mode posed problems for the industry, and programmers sought a way to switch between the modes at will. However, Intel, consistent with their intentions for the processor's usage, provided an easy way to switch into protected mode on the 80286 but no easy way to switch back to real mode. Before the 386 the only way to switch from protected mode back to real mode

2781-562: 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

2884-428: 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 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 ,

2987-484: 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

3090-503: Is also the one used by DPMI (under real, not emulated, DOS) and DOS extenders like DOS/4GW to allow protected mode programs to run under DOS; the DPMI system or DOS extender switches to real mode to invoke DOS or BIOS calls, then switches back to return to the application program which runs in protected mode. The changing towards the NT kernel resulted in the operating system not needing DOS to boot

3193-518: Is characterized by a 20- bit segmented memory address space (giving 1 MB of addressable memory) and unlimited direct software access to all addressable memory, I/O addresses and peripheral hardware. Real mode provides no support for memory protection, multitasking, or code privilege levels. Before the introduction of protected mode with the release of the 80286 , real mode was the only available mode for x86 CPUs; and for backward compatibility , all x86 CPUs start in real mode when reset, though it

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3296-637: Is dangerous; programs should only keep handles to memory blocks when not running. Starting an old program while Windows 3.0 is running in protected mode triggers a warning dialog, suggesting to either run Windows in real mode or to obtain an updated version of the application. Updating well-behaved programs using the MARK utility with the MEMORY parameter avoids this dialog. It is not possible to have some GUI programs running in 16-bit protected mode and other GUI programs running in real mode. In Windows 3.1 , real mode

3399-422: 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

3502-443: 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

3605-479: Is possible to emulate real mode on other systems when starting in other modes. The 80286 architecture introduced protected mode , allowing for (among other things) hardware-level memory protection. Using these new features, however, required a new operating system that was specifically designed for protected mode. Since a primary design specification of x86 microprocessors is that they are fully backward compatible with software written for all x86 chips before them,

3708-499: 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,

3811-473: 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

3914-400: Is the mode modern 32-bit x86 operating systems run in. The 8086, 8088, and 80186 have a 20-bit address bus, but the unusual segmented addressing scheme Intel chose for these processors actually produces effective addresses which can have 21 significant bits. This scheme shifts a 16-bit segment number left four bits (making a 20-bit number with four least-significant zeros) before adding to it

4017-435: 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 ,

4120-405: Is the same as the 16-bit selector, provided the lower three bits are zeroed. The descriptor table entry defines the real linear address of the segment, a limit value for the segment size, and some attribute bits (flags). The segment address inside the descriptor table entry has a length of 24 bits so every byte of the physical memory can be defined as bound of the segment. The limit value inside

4223-494: The A20 line (21st address line) also must be enabled to allow the use of all the address lines so that the CPU can access beyond 1 megabyte of memory (Only the first 20 are allowed to be used after power-up, to guarantee compatibility with older software written for the Intel 8088-based IBM PC and PC/XT models). After performing those two steps, the PE bit must be set in the CR0 register and

Protected mode - Misplaced Pages Continue

4326-516: The BIOS (Basic Input/Output System) and memory for add-on devices . As the cost of memory decreased and memory use increased, the 1 MB limitation became a significant problem. Intel intended to solve this limitation along with others with the release of the 286. The initial protected mode, released with the 286, was not widely used; for example, it was used by Coherent (from 1982), Microsoft Xenix (around 1984) and Minix . Several shortcomings such as

4429-535: 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

4532-496: 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

4635-530: The Intel 8086 , had a 20- bit address bus for its memory , as did its Intel 8088 variant. This allowed them to access 2 bytes of memory, equivalent to 1 megabyte . At the time, 1 megabyte was considered a relatively large amount of memory, so the designers of the IBM Personal Computer reserved the first 640 kilobytes for use by applications and the operating system and the remaining 384 kilobytes for

4738-460: The Task State Segment (TSS), introduced with the 286, preemptive multitasking was made possible on the x86 architecture. The TSS allows general-purpose registers, segment selector fields, and stacks to all be modified without affecting those of another task. The TSS also allows a task's privilege level, and I/O port permissions to be independent of another task's. In many operating systems,

4841-608: The X86S specification. The PC BIOS which IBM introduced operates in real mode, as do the DOS operating systems ( MS-DOS , DR-DOS , etc.). Early versions of Microsoft Windows ran in real mode. Windows/386 made it possible to make some use of protected mode, and this was more fully realized in Windows 3.0 , which could run in either real mode or make use of protected mode in the manner of Windows/386. Windows 3.0 actually had several modes: "real mode", "standard mode" and "386-enhanced mode";

4944-498: 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 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

5047-420: 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

5150-459: The 286 chip was made to start in 'real mode' – that is, in a mode which turned off the new memory protection features, so that it could run operating systems written for the 8086 and the 8088 . As of 2018, current x86 CPUs (including x86-64 CPUs) are able to boot real mode operating systems and can run software written for almost any previous x86 chip without emulation or virtualization. In 2023 Intel proposed to drop real mode from future CPUs in

5253-446: The 386 also added paging to protected mode. Through paging, system software can restrict and control a task's access to pages, which are sections of memory. In many operating systems, paging is used to create an independent virtual address space for each task, preventing one task from manipulating the memory of another. Paging also allows for pages to be moved out of primary storage and onto a slower and larger secondary storage , such as

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5356-485: The 386. Despite such potential setbacks, Windows 3.0 and its successors can take advantage of the binary compatibility with real mode to run many Windows 2.x ( Windows 2.0 and Windows 2.1x ) applications in protected mode, which ran in real mode in Windows 2.x. With the release of the 386, protected mode offers what the Intel manuals call virtual 8086 mode . Virtual 8086 mode is designed to allow code previously written for

5459-441: The 8086 and 8088. The 286 maintained backward compatibility with the 8086 and 8088 by initially entering real mode on power up. Real mode functioned virtually identically to the 8086 and 8088, allowing the vast majority of existing software for those processors to run unmodified on the newer 286. Real mode also served as a more basic mode in which protected mode could be set up, solving a sort of chicken-and-egg problem. To access

5562-459: The 8086 to run unmodified and concurrently with other tasks, without compromising security or system stability. Virtual 8086 mode, however, is not completely backward compatible with all programs. Programs that require segment manipulation, privileged instructions, direct hardware access, or use self-modifying code will generate an exception that must be served by the operating system. In addition, applications running in virtual 8086 mode generate

5665-410: 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

5768-483: The CPU into protected mode at startup, never return to real mode and provide all of the benefits of protected mode all of the time. 64-bit operating systems use real mode only at startup stage, and the OS kernel will switch the CPU into long mode . Notably, the protected mode of the 80286 is considerably more primitive than the improved protected mode introduced with the 80386; the latter is sometimes called 386 protected mode, and

5871-474: 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

5974-407: The ability to access up to 16 MB of physical memory, and 1 GB of virtual memory , were the most apparent changes to application programmers. This was not without its limitations. If an application utilized or relied on any of the techniques below, it would not run: In reality, almost all DOS application programs violated these rules. Due to these limitations, virtual 8086 mode was introduced with

6077-437: The ability to use larger page sizes. Each PDE contained a pointer to a page table. A page table was also originally four kilobytes in size and contained 1,024 page table entries (PTE). Each PTE contained a pointer to the actual page's physical address and are only used when the four-kilobyte pages are used. At any given time, only one page directory may be in active use. Through the use of the rings, privileged call gates , and

6180-534: 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

6283-403: The chip) to a logic low, representing a zero, results in a modulo-2^20 effect to match the earlier processors' address arithmetic, but the 80286 has no internal capability to perform this function. When IBM used the 80286 in their IBM PC/AT , they solved this problem by including a software-settable gate to enable or disable (force to zero) the A20 address line, between the A20 pin on the 80286 and

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6386-477: The computer as well as being unable to use it. The need to restart the computer in real mode MS-DOS declined after Windows 3.1x until it was no longer supported in Windows ME . The only way of currently running DOS applications that require real mode from within newer versions of Windows is by using emulators such as DOSBox or x86 virtualization products. Operating system An operating system ( OS )

6489-453: 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

6592-416: The descriptor table entry has a length of 16 bits so segment length can be between 1 byte and 2 byte. The calculated linear address equals the physical memory address. The segment address inside the descriptor table entry is expanded to 32 bits so every byte of the physical memory can be defined as bound of the segment. The limit value inside the descriptor table entry is expanded to 20 bits and completed with

6695-471: 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

6798-422: 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

6901-407: The extended functionality of the 286, the operating system would set up some tables in memory that controlled memory access in protected mode, set the addresses of those tables into some special registers of the processor, and then set the processor into protected mode. This enabled 24-bit addressing, which allowed the processor to access 2 bytes of memory, equivalent to 16  megabytes . With

7004-477: The full features of the TSS are not used. This is commonly due to portability concerns or due to the performance issues created with hardware task switches. As a result, many operating systems use both hardware and software to create a multitasking system. Operating systems like OS/2 1.x try to switch the processor between protected and real modes. This is both slow and unsafe, because a real mode program can easily crash

7107-410: 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

7210-493: 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

7313-411: The inability to make BIOS and DOS calls due to inability to switch back to real mode without resetting the processor prevented widespread usage. Acceptance was additionally hampered by the fact that the 286 only allowed memory access in 64  kilobyte segments, addressed by its four segment registers, meaning that only 4 × 64 KB , equivalent to 256 KB, could be accessed at a time. Because changing

7416-404: The increased size of the address bus and segment registers, many other new features were added with the intention of increasing operational security and stability. Protected mode is now used in virtually all modern operating systems which run on the x86 architecture, such as Microsoft Windows , Linux , and many others. Furthermore, learning from the failures of the 286 protected mode to satisfy

7519-499: The index of an entry inside a descriptor table . The next bit (bit 2) specifies whether the operation is used with the GDT or the LDT. The lowest two bits (bit 1 and bit 0) of the selector are combined to define the privilege of the request, where the values of 0 and 3 represent the highest and the lowest privilege, respectively. This means that the byte offset of descriptors in the descriptor table

7622-418: 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

7725-404: 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

7828-422: The interrupt vector table.) So, the actual amount of memory addressable by the 80286 and later x86 CPUs in real mode is 1 MB + 64 KB – 16 B = 1,114,096 B. Some programs predating the 80286 were designed to take advantage of the wrap-around (modulo) memory addressing behavior, so the 80286 presented a problem for backward compatibility. Forcing the 21st address line (the actual logic signal wire coming out of

7931-431: 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

8034-476: The latter required some of the virtualization features of the 80386 processor, and thus would not run on an 80286. Windows 3.1 removed support for real mode, and it was the first mainstream operating environment which required at least an 80286 processor. None of these versions could be considered a modern x86 operating system, since they switched to protected mode only for certain functions. Unix , Linux , OS/2 , Windows NT are considered modern OS's as they switch

8137-416: The logical address contains an offset inside the segment, i.e. the physical address can be calculated as physical_address = segment_part × 16 + offset, if the address line A20 is enabled, or (segment_part × 16 + offset) mod 2, if A20 is off. Every segment has a size of 2 bytes. In protected mode, the segment_part is replaced by a 16-bit selector , in which the 13 upper bits (bit 3 to bit 15) contain

8240-400: 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

8343-408: 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

8446-434: The needs for multiuser DOS , Intel added a separate virtual 8086 mode , which allowed multiple virtualized 8086 processors to be emulated on the 386. Hardware x86 virtualization required for virtualizing the protected mode itself, however, had to wait for another 20 years. With the release of the 386, the following additional features were added to protected mode: Until the release of the 386, protected mode did not offer

8549-408: 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

8652-420: 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 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

8755-479: The operating system and some device drivers run in ring 0 and applications run in ring 3. According to the Intel 80286 Programmer's Reference Manual , the 80286 remains upwardly compatible with most 8086 and 80186 application programs. Most 8086 application programs can be re-compiled or re-assembled and executed on the 80286 in Protected Mode. For the most part, the binary compatibility with real-mode code,

8858-440: The operating system to function in a way that would be significantly more difficult or even impossible without proper hardware support. In protected mode, there are four privilege levels or rings , numbered from 0 to 3, with ring 0 being the most privileged and 3 being the least. The use of rings allows for system software to restrict tasks from accessing data, call gates or executing privileged instructions. In most environments,

8961-421: 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

9064-438: The release of Intel 's 80286 (286) processor, and later extended with the release of the 80386 (386) in 1985. Due to the enhancements added by protected mode, it has become widely adopted and has become the foundation for all subsequent enhancements to the x86 (IA-32) architecture, although many of those enhancements, such as added instructions and new registers, also brought benefits to the real mode. The first x86 processor,

9167-452: The release of Windows NT , which dropped backward compatibility for "ill-behaved" DOS applications. In real mode each logical address points directly into a physical memory location, every logical address consists of two 16-bit parts: The segment part of the logical address contains the base address of a segment with a granularity of 16 bytes, i.e. a segment may start at physical address 0, 16, 32, ..., 2 − 16. The offset part of

9270-449: The release of the 386 in 1985, many of the issues preventing widespread adoption of the previous protected mode were addressed. The 386 was released with an address bus size of 32 bits, which allows for 2 bytes of memory accessing, equivalent to 4 gigabytes . The segment sizes were also increased to 32 bits, meaning that the full address space of 4 gigabytes could be accessed without the need to switch between multiple segments. In addition to

9373-522: The reset. Later, a triple fault was used to reset the 286 CPU, which was a lot faster and cleaner than the keyboard controller method (and does not depend on IBM AT-compatible hardware, but will work on any 80286 CPU in any system). To enter protected mode, the Global Descriptor Table (GDT) must first be created with a minimum of three entries: a null descriptor, a code segment descriptor and data segment descriptor. In an IBM-compatible machine,

9476-418: 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

9579-432: The saved state from memory. It can then run other real mode code until the program is ready to switch back to protected mode. The switch to real mode is costly in terms of time, but this technique allows protected mode programs to use services such as BIOS, which runs entirely in real mode (having been designed originally for the 8088 -based IBM Personal Computer model (machine type) 5150). This mode-switching technique

9682-416: The segment 0xFFFF and offset greater than 0x000F, the 80286 would actually make an access into the beginning of the second megabyte of memory, whereas the 80186 and earlier would access an address equal to [offset]-0x10, which is at the beginning of the first megabyte. (Note that on the 80186 and earlier, the first kilobyte of the address space, starting at address 0, is the permanent, immovable location of

9785-619: 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

9888-400: 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

9991-400: 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

10094-562: The system bus; this is known as Gate-A20 (the A20 gate), and it is still implemented in PC chipsets to this day. Most versions of the HIMEM.SYS extended memory driver for IBM-/MS-DOS famously displayed upon loading a message that they had installed an "A20 handler", a piece of software to control Gate-A20 and coordinate it to the needs of programs. In protected mode the A20 line needs to be enabled, or else physical addressing errors will occur, likely leading to

10197-473: 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

10300-536: Was no longer supported and could not be accessed. In modern 32-bit operating systems, virtual 8086 mode is still used for running applications, e.g. DPMI compatible DOS extender programs (through virtual DOS machines ) or Windows 3.x applications (through the Windows on Windows subsystem) and certain classes of device drivers (e.g. for changing the screen-resolution using BIOS functionality) in OS/2 2.0 (and later OS/2) and 32-bit Windows NT , all under control of

10403-406: 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

10506-411: Was to reset the processor; after a reset it always starts up in real mode to be compatible with earlier x86 CPUs back to the 8086. Resetting the processor does not clear the system's RAM, so this, while awkward and inefficient, is actually feasible. From protected mode, the processor's state is saved in memory, then the processor is reset, restarts in real mode, and executes some real mode code to restore

10609-521: Was used in real mode, which is why Windows applications and DLLs can hook interrupts and do direct hardware access. That lasted through the Windows 9x series. If a Windows 1.x or 2.x program is written properly and avoids segment arithmetic, it will run the same way in both real and protected modes. Windows programs generally avoid segment arithmetic because Windows implements a software virtual memory scheme, moving program code and data in memory when programs are not running, so manipulating absolute addresses

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