Editing Real mode (section)

==A20 line==
{{Main|A20 line}}

Some programs predating the 80286 were designed to take advantage of the wrap-around (modulo) memory addressing behavior, so the 80286 presented a problem for backward compatibility.  Forcing the 21st address line (the actual logic signal wire coming out of the chip) to a logic low, representing a zero, results in a modulo-2^20 effect to match the earlier processors' address arithmetic, but the 80286 has no internal capability to perform this function. When IBM used the 80286 in their [[IBM Personal Computer/AT|IBM PC/AT]], they solved this problem by including a software-settable gate to enable or disable (force to zero) the A20 address line, between the A20 pin on the 80286 and the system bus; this is known as Gate-A20 (the A20 gate), and it is still implemented in PC chipsets to this day. Most versions of the HIMEM.SYS extended memory driver for IBM-/MS-DOS famously displayed upon loading a message that they had installed an "A20 handler", a piece of software to control Gate-A20 and coordinate it to the needs of programs.  In protected mode the A20 line needs to be enabled, or else physical addressing errors will occur, likely leading to a system crash. Modern legacy boot loaders (such as [[GNU GRUB]]) use A20 line.<ref>{{Cite web|title=A20 Line - OSDev Wiki|url=https://wiki.osdev.org/A20_Line|access-date=2020-09-10|website=wiki.osdev.org}}</ref>