A20 line: Meaning (information, definition, explanation, facts)

In the x86 architecture (Intel 80286 CPU and above), the A20 line is a control line (typically stored in the keyboard controller) that, when enabled, turns off memory wrap-around above the 1-megabyte mark.

When disabled, the memory locations 0000:0000 (the first byte of memory), and ffff:0010 (the first byte above the 1MB mark) refer to the same byte of memory. This is switched off by default in order to maintain backwards compatibility with the XT architecture, which has a dependence on this memory wrap around feature. Thus, without special measures to enable this line, the processor mimics the wrap-around these programs expect.

Enabling the A20 line is one of the first steps an operating system does in the bootup process, often before control has been passed onto the kernel from the bootstrap (in the case of Linux, for example).

Letter "A" in the name A20 refers to "Above", while "20" refers to the 20th power of 2. (2²⁰bytes = 1Mbyte)