A20 Line

The A20 Line or Address Line 20 is used to transmit the 21st bit on an x86 CPUs address bus.

The Intel 80186 processor, and many before, had 20 address lines. With these, the processor could access up to 220 bytes or 1 MiB, but the internal registers of the 186 only had 16 bits. To access the full 1 MiB of address space, a memory reference also included a segment number which made the address calculation look like this: segment × 16 + offset. Because of this there are many ways to reference the same address. For example F000:FFFF, FFFF:000F and F800:7FFF all reference the byte at 0x000FFFFF. If referenced in the last way and the offset was incremented with one it would translate to the physical address 0x00100000, but because the processor didn’t have enough address lines it would flip over to 0x00000000. Some programs relied on this behavior and would fail without it.

When IBM designed the PC AT machine in 1984 they decided to use the new 80286 which could address up to 16 MiB of memory in protected mode. However, the processor was supposed to emulate the 80186’s behavior in real mode, so it could run operating systems and programs that were not written for the new protected mode. The issue was that the 286 didn’t force the A20 line to be zero so addresses would no longer flip over.

The problem then needed to be fixed and IBM chose to put a logic gate on the A20 line between the processor and address bus. The BIOS then disables it before transferring control to the operating system. This was named the A20 gate.

A20 Gate

The A20 gate is the logic gate controlling the A20 line. It is route through the 8042 keyboard controller because it had a spare pin. Today this looks like an odd choice bit back then, but at the time computers weren’t as standardized compared to today.

Enabling the A20 Line

To enable the A20 line you have to use port I/O to communicate with the 8042. Here is the flowchart for enabling it:

  1. Disable interrupts
  2. Wait until the keyboard controller buffer is empty
  3. Disable the keyboard controller
  4. Wait until the keyboard controller buffer is empty
  5. Request status of the keyboard controllers output pins
  6. Wait until the keyboard controller buffer is empty
  7. Read the byte from the data port and save it
  8. Wait until the keyboard controller buffer is empty
  9. Request to write to the output pin status
  10. Wait until the keyboard controller buffer is empty
  11. Send the saved byte ORed by 2 (the A20 gate bit)
  12. Wait until the keyboard controller buffer is empty
  13. Enable the keyboard controller
  14. Enable interrupts (optional)

The data port is 0x60, the command port is 0x64, the disable command is 0xAD, the enable command is 0xAE, the “read control output port” command is 0xD0 and the “write control output port” command is 0xD1.

Sources: