The A20, or address line 20, is one of the electrical lines that make up the system bus of an x86-based computer system. The A20 line in particular is used to transmit the 21st bit on the address bus.

A microprocessor typically has a number of address lines equal to the base-two logarithm of the number of words in its physical address space. For example, a processor with 4 GB of byte-addressable physical space requires 32 lines (log₂(4 GB) = log₂(2³² B) = 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 2²⁰.

The Intel 8086, Intel 8088, 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 to the left 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):

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 (1984) machine, it decided to use the new higher-performance Intel 80286 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 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 software cared only about the area slightly above 1 MB, so the Gate-A20 line was enough.