In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible

central processing unit A central processing unit (CPU), also called a central processor, main processor or just processor, is the electronic circuitry that executes instructions comprising a computer program. The CPU performs basic arithmetic, logic, controlling, a ...

s (CPUs). It allows system software to use features such as

virtual memory In computing, virtual memory, or virtual storage is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very ...

, paging and safe multi-tasking designed to increase an operating system's control over

application software Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a ...

. When a processor that supports x86 protected mode is powered on, it begins executing instructions in

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 is characterized by a 20- bit ...

, in order to maintain

backward compatibility Backward compatibility (sometimes known as backwards compatibility) is a property of an operating system, product, or technology that allows for interoperability with an older legacy system, or with input designed for such a system, especiall ...

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 A control register is a processor register which changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, ...

0 (CR0). Protected mode was first added to the x86 architecture in 1982, with the release of

Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...

's 80286 (286) processor, and later extended with the release of the

80386 The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors 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 architecture, although many of those enhancements, such as added instructions and new registers, also brought benefits to the real mode.

History

The Intel 8086, the predecessor to the 286, was originally designed with a 20- bit

address bus In computer architecture, a bus (shortened form of the Latin '' omnibus'', and historically also called data highway or databus) is a communication system that transfers data between components inside a computer, or between computers. This e ...

for its

memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...

. This allowed the processor to access 2²⁰

byte The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...

s of memory, equivalent to 1

megabyte The megabyte is a multiple of the unit byte for digital information. Its recommended unit symbol is MB. The unit prefix ''mega'' is a multiplier of (106) in the International System of Units (SI). Therefore, one megabyte is one million bytes o ...

. At the time, 1 megabyte was considered a relatively large amount of memory, so the designers of the

IBM Personal Computer The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a tea ...

reserved the first 640

kilobyte The kilobyte is a multiple of the unit byte for digital information. The International System of Units (SI) defines the prefix '' kilo'' as 1000 (103); per this definition, one kilobyte is 1000 bytes.International Standard IEC 80000-13 Quant ...

s for use by applications and the operating system and the remaining 384 kilobytes for 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.

intended to solve this limitation along with others with the release of the 286.

The 286

The initial protected mode, released with the 286, was not widely used; for example, it was used by

Coherent Coherence, coherency, or coherent may refer to the following: Physics * Coherence (physics), an ideal property of waves that enables stationary (i.e. temporally and spatially constant) interference * Coherence (units of measurement), a deriv ...

(from 1982), Microsoft

Xenix Xenix is a discontinued version of the Unix operating system for various microcomputer platforms, licensed by Microsoft from AT&T Corporation in the late 1970s. The Santa Cruz Operation (SCO) later acquired exclusive rights to the software, ...

(around 1984) and Minix. Several shortcomings such as the inability to access the BIOS or 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 16 bit segments via each of four segment registers, meaning only 4*2¹⁶ bytes, equivalent to 256

s, could be accessed at a time. Because changing 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; therefore, the strategy of computing segment addresses on-the-fly in order to access data structures larger than 128

s (the combined size of the two data segments) became impractical, even for those few programmers who had mastered it on the 8086/8088. The 286 maintained backwards compatibility with its precursor the 8086 by initially entering

on power up. Real mode functioned virtually identically to the 8086, allowing the vast majority of existing 8086

software Software is a set of computer programs and associated documentation and data. This is in contrast to hardware, from which the system is built and which actually performs the work. At the lowest programming level, executable code consist ...

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 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 2²⁴ bytes of memory, equivalent to 16

The 386

With 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 2³² 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 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 system An operating system (OS) is system software that manages computer hardware, software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for ef ...

s which run on the x86 architecture, such as

Microsoft Windows Windows is a group of several proprietary graphical operating system families developed and marketed by Microsoft. Each family caters to a certain sector of the computing industry. For example, Windows NT for consumers, Windows Server for ...

Linux Linux ( or ) is a family of open-source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991, by Linus Torvalds. Linux is typically packaged as a Linux distribution, whi ...

, and many others. Furthermore, learning from the failures of the 286 protected mode to satisfy 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.

386 additions to protected mode

With the release of the 386, the following additional features were added to protected mode: * Paging *

32-bit In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in 32- bit units. Compared to smaller bit widths, 32-bit computers can perform large calculati ...

physical and virtual

address space In computing, an address space defines a range of discrete addresses, each of which may correspond to a network host, peripheral device, disk sector, a memory cell or other logical or physical entity. For software programs to save and retrieve s ...

(The 32-bit physical address space is not present on the

80386SX The Intel 386, originally released as 80386 and later renamed i386, is a 32-bit microprocessor introduced in 1985. The first versions had 275,000 transistors) * 32-bit segment offsets * Ability to switch back to real mode without resetting * Virtual 8086 mode

Entering and exiting protected mode

Until the release of the 386, protected mode did not offer a direct method to switch back into real mode once protected mode was entered. IBM devised a workaround (implemented in the

IBM AT The IBM Personal Computer/AT (model 5170, abbreviated as IBM AT or PC/AT) was released in 1984 as the fourth model in the IBM Personal Computer line, following the IBM PC/XT and its IBM Portable PC variant. It was designed around the Intel 802 ...

) 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 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, 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 The IBM Personal Computer (model 5150, commonly known as the IBM PC) is the first microcomputer released in the IBM PC model line and the basis for the IBM PC compatible de facto standard. Released on August 12, 1981, it was created by a team ...

and PC/XT models). After performing those two steps, the PE bit must be set in the CR0 register and a far jump must be made to clear the prefetch input queue. ; MASM program ; enter protected mode (set PE bit) mov EBX, CR0 ; save control register 0 (CR0) to EBX or EBX, PE_BIT ; set PE bit by ORing, save to EBX mov CR0, EBX ; save EBX back to CR0 ; clear prefetch queue; (using far jump instruction jmp) jmp CLEAR_LABEL CLEAR_LABEL: 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.

Features

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 the operating system to function in a way that would be significantly more difficult or even impossible without proper hardware support.

Privilege levels

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, the operating system and some device drivers run in ring 0 and applications run in ring 3.

Real mode application compatibility

According to the ''Intel 80286 Programmer's Reference Manual'', For the most part, the binary compatibility with real-mode code, the ability to access up to 16 MB of physical memory, and 1 GB of

, 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: * Segment arithmetic * Privileged instructions * Direct hardware access * Writing to a code segment * Executing data * Overlapping segments * Use of BIOS functions, due to the BIOS interrupts being reserved by Intel In reality, almost all DOS application programs violated these rules. Due to these limitations, virtual 8086 mode was introduced with 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 Windows 2.0 is a major release of Microsoft Windows, a family of graphical operating systems for personal computers developed by Microsoft. It was released to manufacturing on December 9, 1987, as a successor to Windows 1.0. The product i ...

and Windows 2.1x) applications in protected mode, which ran in real mode in Windows 2.x.

Virtual 8086 mode

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 the 8086 to run unmodified and concurrently with other tasks, without compromising security or system stability. Virtual 8086 mode, however, is not completely backwards 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 a trap with the use of instructions that involve

input/output In computing, input/output (I/O, or informally io or IO) is the communication between an information processing system, such as a computer, and the outside world, possibly a human or another information processing system. Inputs are the signals ...

(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 backwards compatibility when dealing with

legacy software In computing, a legacy system is an old method, technology, computer system, or application program, "of, relating to, or being a previous or outdated computer system", yet still in use. Often referencing a system as "legacy" means that it paved ...

. An example of such a compromise can be seen with the release of

Windows NT Windows NT is a proprietary graphical operating system produced by Microsoft, the first version of which was released on July 27, 1993. It is a processor-independent, multiprocessing and multi-user operating system. The first version of Win ...

, which dropped backwards compatibility for "ill-behaved" DOS applications.

Segment addressing

Real mode

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, ..., 2²⁰-16. The offset part of 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), respectively (segment_part × 16 + offset) mod 2²⁰ (if A20 is off) Every segment has a size of 2¹⁶ bytes.

Protected mode

In protected mode, the is replaced by a 16-bit ''selector'', in which the 13 upper bits (bit 3 to bit 15) contain 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 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).

286

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 the descriptor table entry has a length of 16 bits so segment length can be between 1 byte and 2¹⁶ byte. The calculated linear address equals the physical memory address.

386

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 a granularity flag (G-bit, for short): * If G-bit is zero limit has a granularity of 1 byte, i.e. segment size may be 1, 2, ..., 2²⁰ bytes. * If G-bit is one limit has a granularity of 2¹² bytes, i.e. segment size may be 1 × 2¹², 2 × 2¹², ..., 2²⁰ × 2¹² bytes. If paging is off, the calculated linear address equals the physical memory address. If paging is on, the calculated linear address is used as input of paging. 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.

Structure of segment descriptor entry

Where: *''A'' is the ''Accessed'' bit; *''R'' is the ''Readable'' bit; *''C'' (Bit 42) depends on ''X'': **if ''X'' = 1 then ''C'' is the ''Conforming'' bit, and determines which privilege levels can far-jump to this segment (without changing privilege level): ***if ''C'' = 0 then only code with the same privilege level as ''DPL'' may jump here; ***if ''C'' = 1 then code with the same or a lower privilege level relative to ''DPL'' may jump here. **if ''X'' = 0 then ''C'' is the ''direction'' bit: ***if ''C'' = 0 then the segment grows ''up''; ***if ''C'' = 1 then the segment grows ''down''. *''X'' is the ''Executable'' bit: **if ''X'' = 1 then the segment is a code segment; **if ''X'' = 0 then the segment is a data segment. *''S'' is the ''Segment type'' bit, which should generally be cleared for system segments; *''DPL'' is the ''Descriptor Privilege Level''; *''P'' is the ''Present'' bit; *''D'' is the ''Default operand size''; *''G'' is the ''Granularity'' bit; *Bit 52 of the 80386 descriptor is not used by the hardware.

Paging

Virtual address space and physical address space relationship

In addition to adding virtual 8086 mode, 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 Computer data storage is a technology consisting of computer components and recording media that are used to retain digital data. It is a core function and fundamental component of computers. The central processing unit (CPU) of a compute ...

, such as a

hard disk drive A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating platters coated with mag ...

. 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 table A page table is the data structure used by a virtual memory system in a computer operating system to store the mapping between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing Process ( ...

s. 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 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.

Multitasking

Through the use of the rings, privileged call gates, and the

Task State Segment The task state segment (TSS) is a structure on x86-based computers which holds information about a task. It is used by the operating system kernel for task management. Specifically, the following information is stored in the TSS: * Processor reg ...

(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, 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

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 Crash or CRASH may refer to: Common meanings * Collision, an impact between two or more objects * Crash (computing), a condition where a program ceases to respond * Cardiac arrest, a medical condition in which the heart stops beating * Couch ...

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 Unix (; trademarked as UNIX) is a family of multitasking, multiuser computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, ...

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 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 Windows 9x is a generic term referring to a series of Microsoft Windows computer operating systems produced from 1995 to 2000, which were based on the Windows 95 kernel and its underlying foundation of MS-DOS, both of which were updated in sub ...

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 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 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 A DOS extender is a computer software program running under DOS that enables software to run in a protected mode environment even though the host operating system is only capable of operating in real mode. DOS extenders were initially developed ...

programs (through virtual DOS machines) or Windows 3.x applications (through the

Windows on Windows In computing, Windows on Windows (commonly referred to as WOW), was a compatibility layer of 32-bit versions of the Windows NT family of operating systems since 1993 with the release of Windows NT 3.1, which extends NTVDM to provide limited ...

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

, all under control of 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.

References

External links

Protected Mode Basics

* ttps://web.archive.org/web/20140303113814/http://www.turboirc.com/asm/ TurboIRC.COM tutorial to enter protected mode from DOS
Protected Mode Overview and Tutorial

Code Project Protected Mode Tutorial

Akernelloader switching from real mode to protected mode
{{Memory management Programming language implementation X86 operating modes