A20 Line

In the x86 architecture (Intel 80286 CPU and above), 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 wrap-around is a feature of older processors that some programs came to rely on for normal operation, and thus the designers of the 286 processor retained backwards compatibility by creating the A20 line, and disabling it by default. 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).