The address line in an x86-based PC that points beyond one megabyte. When IBM introduced the 286-based PC AT in 1984, its 24 address lines could reach 16MB of RAM, whereas the previous 8088 CPU could only reach 1MB and had the oddity of wrapping addresses beyond 1MB back to zero. In order to provide compatibility with the 8088, IBM made the A20 line switchable so that it could be disabled when running in Real Mode (within 1MB), and only lines A0 through A19 would be operable. Thus, earlier applications that depended on the wrap-around quirk would continue to run.
A Legacy Example
The A20/memory wrap-around is an example of an early function that continues to be supported for years to come. PC operating systems simulate the wrap-around when running in Virtual 8086 Mode, and the PC BIOSes still have an option to turn the A20 line on and off as required. See
Virtual 8086 Mode,
Real Mode,
address bus and
binary values.