The A20 handler is IBM PC memory manager software controlling access to the High Memory Area. Extended memory managers usually provide this functionality. A20 handlers are named after the 21st address line of the microprocessor, the A20 line. IBM PC (IBM 5150) with keyboard and green screen monochrome monitor (IBM 5151), running MS-DOS 5. ... A memory manager is a part of a computer program which accepts requests from the program to allocate and deallocate chunks of memory. ... The High Memory Area (HMA) is the RAM area consisting of the first 64 kilobytes (KB), minus 16 bytes, of the extended memory on an IBM PC or compatible microcomputer. ... Extended memory refers to memory above the first megabyte of address space in an IBM PC with an 80286 or later processor. ... The A20 or address line 20 is a well known address line in x86 bases systems because it is initially disabled when the system starts. ...

The Intel 8088 CPU used in the original IBM PC was capable of addressing memory using 20 binary digits ("bits"). But the registers in the 8088 were only 16 bits wide. A 20 bit memory address was created by left shifting the 16 bit Segment Address (in effect multiplying it by 16) and adding the 16 bit offset address to it yielding a 20 bit result. An Intel 8088 microprocessor The Intel 8088 is an Intel microprocessor based on the 8086, with 16-bit registers and an 8-bit external data bus. ... CPU redirects here. ... In computer science, a memory address is a unique identifier for a memory location at which a CPU or other device can store a piece of data for later retrieval. ...

For example, using hexadecimal values with a Code Segment (CS) of 1000 and an Instruction Pointer (IP) of 2000, the actual memory address of CS:IP is, as follows: In mathematics and computer science, base-16, hexadecimal, or simply hex, is a numeral system with a radix or base of 16 usually written using the symbols 0–9 and A–F or a–f. ...

 CS : 1000 shifted left by 4 bits gives 10000 So the result is 10000 + 2000 = 12000 

In reality, the result can be 21 bits wide since the total of the two registers can be more than FFFFF hexadecimal, e.g. repeating the above example with CS = FF00 and IP = FFFF. It is possible to generate references to slightly less that 64K bytes that are outside the addressing range of the 8088. When the 80286 CPU which had more than 20 address lines was developed, it became necessary to deal with code that referenced the 64K addressable by ordinary addressing but not present in the 8088. As it turned out, it was sometimes desirable to set the 21st address line, and sometimes desirable not to. A logic gate was added on the motherboard to control the 21st address line. Not surprisingly, this was called the "A20 gate". A logic gate performs a logical operation on one or more logic inputs and produces a single logic output. ... A motherboard, also known as a mainboard, system board, or logic boards on Apple Computers, and sometimes abbreviated as mobo (generally credited to the magazine Maximum PC) is the central or primary circuit board making up a complex electronic system, such as a modern computer. ...

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 65520 (64K - 16) bytes of memory in real mode without dramatic software changes.

See also

• Computer storage
• Computer software

This article or section does not cite its references or sources. ... Software, or program, enables a computer to perform specific tasks, as opposed to the physical components of the system (hardware). ...

References

• Intel 8086
• Intel 80286

It has been suggested that Microprocessor 8086 be merged into this article or section. ... An Intel 80286 Microprocessor AMD 80286 with 12 Mhz. ...

External links

• A20 - a pain from the past

This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL. The Free On-line Dictionary of Computing (FOLDOC) is an online, searchable encyclopedic dictionary of computing subjects. ... GNU logo (similar in appearance to a gnu) The GNU Free Documentation License (GNU FDL or simply GFDL) is a copyleft license for free content, designed by the Free Software Foundation (FSF) for the GNU project. ...