![IBM PC Memory areas](https://upload.wikimedia.org/wikipedia/commons/4/4d/IBM_PC_Memory_areas.svg)
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 (log
2(4 GB) = 2
32), 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 2
20.
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 2
20 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 ...
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 ...
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
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 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 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 ...
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.
See also
*
Bug compatibility
Computer hardware or software is said to be bug compatible if it exactly replicates even an undesirable feature of a previous version. The phrase is found in the Jargon File.
An aspect of maintaining backward compatibility with an older system i ...
*
Computer storage
*
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)
*
LOADFIX (CONFIG.SYS directive) (PTS-DOS)
*
Incomplete address decoding
*
Boot loaders
In computing, booting is the process of starting a computer as initiated via hardware such as a button or by a software command. After it is switched on, a computer's central processing unit (CPU) has no software in its main memory, so so ...
References
Further reading
*
*
*
*
* {{cite web , title=x86 architecture legacy stuff: KBC, PS/2, and A20M# , author-first=Christian , author-last=Ludloff , date=2011 , work=sandpile.org , url=https://sandpile.org/x86/legacy.htm , access-date=2022-01-02 , url-status=live , archive-url=https://web.archive.org/web/20210815001552/https://sandpile.org/x86/legacy.htm , archive-date=2021-08-15
X86 memory management
Memory expansion
X86 architecture
IBM PC compatibles