The A20, or address line 20, is one of the

electrical Electricity is the set of physical phenomena associated with the presence and motion of matter that has a property of electric charge. Electricity is related to magnetism, both being part of the phenomenon of electromagnetism, as described ...

lines that make up the system bus of an

x86 x86 (also known as 80x86 or the 8086 family) is a family of complex instruction set computer (CISC) instruction set architectures initially developed by Intel based on the Intel 8086 microprocessor and its 8088 variant. The 8086 was intr ...

-based computer system. The A20 line in particular is used to transmit the 21st bit on the

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 ...

. A microprocessor typically has a number of

address line 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 ...

s equal to the base-two

logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 of ...

of the number of

words A word is a basic element of language that carries an objective or practical meaning, can be used on its own, and is uninterruptible. Despite the fact that language speakers often have an intuitive grasp of what a word is, there is no conse ...

in its physical address space. For example, a processor with 4 GB of byte-addressable physical space requires 32 lines (log₂(4 GB) = 2³²), which are named A0 through A31. The lines are named after the zero-based number of the bit in the address that they are transmitting. The least significant bit is first and is therefore numbered bit 0 and signaled on line A0. A20 transmits bit 20 (the 21st bit) and becomes active once addresses reach 1 MB, or 2²⁰.

Overview

The Intel 8086,

Intel 8088 The Intel 8088 ("''eighty-eighty-eight''", also called iAPX 88) microprocessor is a variant of the Intel 8086. Introduced on June 1, 1979, the 8088 has an eight-bit external data bus instead of the 16-bit bus of the 8086. The 16-bit registers and ...

, and Intel 80186 processors had 20 address lines, numbered A0 to A19; with these, the processor can access 2²⁰ bytes, or 1 MB. Internal address registers of such processors only had 16 bits. To access a 20-bit address space, an external memory reference was made up of a 16-bit ''offset'' address added to a 16-bit ''segment'' number, shifted 4 bits so as to produce a 20-bit physical address. The resulting address is equal to ''segment'' × 16 + ''offset''. There are many combinations of segment and offset that produce the same 20-bit physical address. Therefore, there were various ways to address the same byte in memory. For example, here are four of the 4096 different segment:offset combinations, all referencing the byte whose physical address is 0x000FFFFF (the last byte in 1 MB-memory space): : F000:FFFF : FFFF:000F : F555:AAAF : F800:7FFF Referenced the last way, an increase of one in the offset yields F800:8000, which is a proper address for the processor, but since it translates to the physical address 0x00100000 (the first byte over 1 MB), the processor would need another address line for actual access to that byte. Since there is no such line on the 8086 line of processors, the 21st bit above, while set, gets dropped, causing the address F800:8000 to "wrap around" and to actually point to the physical address 0x00000000. When IBM designed the

IBM PC 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 80 ...

(1984) machine, it decided to use the new higher-performance

Intel 80286 The Intel 80286 (also marketed as the iAPX 286 and often called Intel 286) is a 16-bit microprocessor that was introduced on February 1, 1982. It was the first 8086-based CPU with separate, non- multiplexed address and data buses and also the ...

microprocessor. The 80286 could address up to 16 MB of system memory in protected mode. However, the CPU was supposed to emulate an 8086's behavior in real mode, its startup mode, so that it could run operating systems and programs that were not written for protected mode. The 80286 did not force the A20 line to zero in real mode, however. Therefore, the combination F800:8000 would no longer point to the physical address 0x00000000, but to the address 0x00100000. As a result, programs relying on the address wrap around would no longer work. To remain compatible with such programs, IBM decided to correct the problem on the motherboard. That was accomplished by inserting a logic gate on the A20 line between the processor and system bus, which got named ''Gate-A20''. Gate-A20 can be enabled or disabled by software to allow or prevent the address bus from receiving a signal from A20. It is set to non-passing for the execution of older programs that rely on the wrap-around. At boot time, the BIOS first enables Gate-A20 when it counts and tests all of the system memory, and then disables it before transferring control to the operating system. Originally, the logic gate was a gate connected to the

Intel 8042 The MCS-48 microcontroller series, Intel's first microcontroller, was originally released in 1976. Its first members were 8048, 8035 and 8748. The 8048 is probably the most prominent member of the family. Initially, this family was produced u ...

keyboard controller. Controlling it was a relatively slow process. Other methods have since been added to allow more efficient multitasking of programs that require this wrap-around with programs that access all of the system memory. There are multiple methods to control the A20 line. Disconnecting A20 would not wrap ''all'' memory accesses above 1 MB, just those in the 1–2 MB, 3–4 MB, 5–6 MB, etc. ranges.

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 s ...

software cared only about the area slightly above 1 MB, so the Gate-A20 line was enough. Enabling the Gate-A20 line is one of the first steps that a

protected-mode In computing, protected mode, also called protected virtual address mode, is an operational mode of x86-compatible central processing units (CPUs). It allows system software to use features such as virtual memory, paging and safe multi-tasking d ...

operating system An operating system (OS) is system software that manages computer hardware, software resources, and provides common services for computer programs. Time-sharing operating systems schedule tasks for efficient use of the system and may also i ...

does in the bootup process, often before control has been passed to the kernel from the bootstrap (in the case of Linux, for example).

Virtual 8086 mode In the 80386 microprocessor and later, virtual 8086 mode (also called virtual real mode, V86-mode, or VM86) allows the execution of real mode applications that are incapable of running directly in protected mode while the processor is running a ...

, introduced with the

Intel 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 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 ...

facilities of the processor; physical memory may be mapped to multiple virtual addresses. Thus, the memory mapped at the first megabyte of virtual memory may be mapped again in the second megabyte of virtual memory. The operating system may intercept changes to Gate A20 and make corresponding changes to the virtual-memory address space, which also makes irrelevant the efficiency of Gate-A20 line toggling.

A20 gate

Controlling the A20 line was an important feature at one stage in the growth of the IBM PC architecture, as it added access to an additional 65,520 bytes (64 KB − 16 bytes) of memory in real mode, without significant software changes. In what was arguably a "hack", the A20 gate was originally part of the keyboard controller on the motherboard, which could open or close it depending on what behavior was desired. The A20 gate is still present on many modern PCs , and the gate is initially closed right after boot. Modern

operating systems typically open the A20 gate early during the boot process and never close it again. Such operating systems do not have the compatibility reasons for keeping it closed, and they gain access to the full range of physical addresses available by opening it. The Intel 80486 and

Pentium Pentium is a brand used for a series of x86 architecture-compatible microprocessors produced by Intel. The original Pentium processor from which the brand took its name was first released on March 22, 1993. After that, the Pentium II and P ...

added a special pin named ''A20M#'', which when asserted low forces bit 20 of the physical address to be zero for all on-chip cache- or external-memory accesses. It was necessary, since the 80486 introduced an on-chip cache and so masking this bit in external logic was no longer possible. Software still needs to manipulate the gate and must still deal with external peripherals (the

chipset In a computer system, a chipset is a set of electronic components in one or more integrated circuits known as a "Data Flow Management System" that manages the data flow between the processor, memory and peripherals. It is usually found on the mo ...

) for that. Support for the A20 gate was changed in the

Nehalem microarchitecture Nehalem is the codename for Intel's 45 nm microarchitecture released in November 2008. It was used in the first-generation of the Intel Core i5 and i7 processors, and succeeds the older Core microarchitecture used on Core 2 processors. The ...

(some sources incorrectly claim that A20 support was removed). Rather than the CPU having a dedicated A20M# pin that receives the signal whether or not to mask the A20 bit, it has been virtualized so that the information is sent from the peripheral hardware to the CPU using special bus cycles. From a software point of view, the mechanism works exactly as before, and an operating system must still program external hardware (which in-turn sends the aforementioned bus cycles to the CPU) to disable the A20 masking. Intel no longer supports the A20 gate, starting with Haswell. Page 271 of the Intel System Programmers Manual Vol. 3A from June 2013 states: "The functionality of A20M# is used primarily by older operating systems and not used by modern operating systems. On newer Intel 64 processors, A20M# may be absent."

A20 handler

The A20 handler is IBM PC memory manager software that controls access to the

high memory area In DOS memory management, the high memory area (HMA) is the RAM area consisting of the first 65520 bytes above the one megabyte in an IBM AT or compatible computer. In real mode, the segmentation architecture of the Intel 8086 and subsequen ...

(HMA). Extended-memory managers usually provide this functionality. A20 handlers are named after the 21st address line of the microprocessor, the A20 line. In

DOS DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems. DOS may also refer to: Computing * Data over signalling (DoS), multiplexing data onto a signalling channel * Denial-of-service attack (DoS), an attack on a communicat ...

, HMA managers such as

HIMEM.SYS HIMEM.SYS is a DOS device driver which allows DOS programs to store data in extended memory according to the Extended Memory Specification (XMS). The memory beyond the first 1 MB of address space is required by Windows 9x/ Me in order to load ...

have the "extra task" of managing A20. HIMEM.SYS provided an

API An application programming interface (API) is a way for two or more computer programs to communicate with each other. It is a type of software interface, offering a service to other pieces of software. A document or standard that describes how ...

for opening/closing A20. DOS itself could use the area for some of its storage needs, thereby freeing up more conventional memory for programs. That functionality was enabled by the

DOS 

 DOS is shorthand for the MS-DOS and IBM PC DOS family of operating systems.

DOS may also refer to:

 Computing
*  Data over signalling (DoS), multiplexing data onto a signalling channel
* Denial-of-service attack (DoS), an attack on a communicat ...
=HIGH

or HIDOS=ON directives in the CONFIG.SYS configuration file.

Affected programs

Since 1980, the address wrap was internally used by 86-DOS and

MS-DOS MS-DOS ( ; acronym for Microsoft Disk Operating System, also known as Microsoft DOS) is an operating system for x86-based personal computers mostly developed by Microsoft. Collectively, MS-DOS, its rebranding as IBM PC DOS, and a few ope ...

to implement the DOS CALL 5 entry point at offset +5 to +9 (which emulates the CP/M-80-style CALL 5 BDOS

entry point at offset +5 to +7) in the

Program Segment Prefix The Program Segment Prefix (PSP) is a data structure used in DOS systems to store the state of a program. It resembles the Zero Page in the CP/M operating system. The PSP has the following structure: The PSP is most often used to get the com ...

(PSP) (which partially resembles CP/M-80's

zero page The zero page or base page is the block of memory at the very beginning of a computer's address space; that is, the page whose starting address is zero. The size of a page depends on the context, and the significance of zero page memory versus h ...

). This was, in particular, utilized by programs machine-translated from CP/M-80 through assembly language translators like Seattle Computer Products' TRANS86. The CALL 5 handler this entry point refers to resides at the machine's physical address 0x000000C0 (thereby overlapping the four bytes of the

interrupt service routine In computer systems programming, an interrupt handler, also known as an interrupt service routine or ISR, is a special block of code associated with a specific interrupt condition. Interrupt handlers are initiated by hardware interrupts, softwar ...

entry point reserved for INT 30h and the first byte of INT 31h in the x86 real mode

interrupt vector table An interrupt vector table (IVT) is a data structure that associates a list of interrupt handlers with a list of interrupt requests in a table of interrupt vectors. Each entry of the interrupt vector table, called an interrupt vector, is the add ...

). However, by the design of CP/M-80, which loaded the operating system immediately above the memory available for the application program to run in, the

8080 The Intel 8080 (''"eighty-eighty"'') is the second 8-bit microprocessor designed and manufactured by Intel. It first appeared in April 1974 and is an extended and enhanced variant of the earlier 8008 design, although without binary compatibil ...

Z80 The Z80 is an 8-bit microprocessor introduced by Zilog as the startup company's first product. The Z80 was conceived by Federico Faggin in late 1974 and developed by him and his 11 employees starting in early 1975. The first working samples were ...

16-bit target address stored at offset +6 to +7 in the zero page could deliberately also be interpreted as the size of the first memory segment. In order to emulate this in DOS with its 8086 segment:offset addressing scheme, the far call entry point's 16-bit offset had to match this segment size (i.e. 0xFEF0), which is stored at offset +6 to +7 in the PSP, overlapping parts of the CALL 5. The only way to reconcile these requirements was to choose a segment value that, when added to 0xFEF0, results in an address of 0x001000C0, which, on an 8086, wraps around to 0x000000C0. A20 had to be disabled for the wraparound to occur and DOS programs using this interface to work. Newer DOS versions which can relocate parts of themselves into the HMA, typically craft a copy of the entry point at FFFF:00D0 in the HMA (which again resolves to physical 0x001000C0), so that the interface can work without regard to the state of A20. One program known to use the CALL 5 interface is the DOS version of the

Small-C Small-C is both a subset of the C programming language, suitable for resource-limited microcomputers and embedded systems, and an implementation of that subset. Originally valuable as an early compiler for microcomputer systems available during ...

compiler. Also, the SPELL utility in Microsoft's Word 3.0 (1987) is one of the programs depending on the CALL 5 interface to be set up correspondingly. Sun Microsystems' PC-NFS (1993) requires the CALL 5 fix-up as well. Also, to save program space, a trick was used by some BIOS and DOS programmers, for example, to have one segment that has access to program data (such as from F800:0000 to F800:7FFF, pointing to the physical addresses 0x000F8000–0x000FFFFF), as well as the I/O data (such as the keyboard buffer) that was located in the first memory segment (with addresses F800:8000 to F800:FFFF pointing to the physical addresses 0x00000000 to 0x00007FFF). This trick works for as long as the code isn't executed in

low memory In DOS memory management, conventional memory, also called base memory, is the first 640 kilobytes of the memory on IBM PC or compatible systems. It is the read-write memory directly addressable by the processor for use by the operating system ...

, the first 64 KB of RAM, a condition that was always true in older DOS versions without load-high capabilities. With the DOS kernel relocated into higher memory areas, low memory increasingly became available for programs, causing those depending on the wraparound to fail. The executable loaders in newer versions of DOS attempt to detect some common types of affected programs and either patch them on-the-fly to function also in low memory or load them above the first 64 KB before passing execution on to them. For programs, which are not detected automatically, LOADFIX or MEMMAX -L can be used to force programs to be loaded above the first 64 KB. The trick was utilized by IBM/Microsoft Pascal itself as well as by programs compiled with it, including Microsoft's

MASM The Microsoft Macro Assembler (MASM) is an x86 assembler that uses the Intel syntax for MS-DOS and Microsoft Windows. Beginning with MASM 8.0, there are two versions of the assembler: One for 16-bit & 32-bit assembly sources, and another (ML64) ...

. Other commonly used development utilities using this were executable compressors like Realia's Spacemaker (written by Robert B. K. Dewar in 1982 and used to compress early versions of the

Norton Utilities Norton Utilities is a utility software suite designed to help analyze, configure, optimize and maintain a computer. The latest version of the original series of Norton Utilities is Norton Utilities 16 for Windows XP/Vista/7/8 was released 26 Oc ...

) and Microsoft's EXEPACK (written by Reuben Borman in 1985) as well as the equivalent /E EPACKoption in Microsoft's LINK 3.02 and higher. Programs processed with EXEPACK would display a "Packed file is corrupt" error message. Various third-party utilities exist to modify compressed executables either replacing the problematic uncompression routine(s) through restubbing, or attempting to expand and restore the original file. Modern Legacy BIOS boot loaders (such as

GNU GRUB GNU GRUB (short for GNU GRand Unified Bootloader, commonly referred to as GRUB) is a boot loader package from the GNU Project. GRUB is the reference implementation of the Free Software Foundation's Multiboot Specification, which provides a u ...

) use the A20 line. UEFI boot loaders use 32-bit protected mode or 64-bit long mode.

Overview

A20 gate

A20 handler

Affected programs

See also

References

Further reading