A20 Line
The A20 Address Line is the physical representation of the 21st bit (number 20, counting from 0) of any memory access. When the IBM-AT (Intel 286) was introduced, it was able to access up to sixteen megabytes of memory (instead of the 1 MByte of the 8086). But to remain compatible with the 8086, a quirk in the 8086 architecture (memory wraparound) had to be duplicated in the AT. To achieve this, the A20 line on the address bus was disabled by default.
The wraparound was caused by the fact the 8086 could only access 1 megabyte of memory, but because of the segmented memory model they could effectively address up to 1 megabyte and 64 kilobytes (minus 16 bytes). Because there are twenty address lines on the 8086 (A0 through A19), any address above the 1 megabyte mark wraps around to zero. For some reason a few short-sighted programmers decided to write programs that actually used this wraparound (rather than directly addressing the memory at its normal location at the bottom of memory). Therefore in order to support these 8086-era programs on the new processors, this wraparound had to be emulated on the IBM AT and its compatibles; this was originally achieved by way of a latch that by default set the A20 line to zero. Later the 486 added the logic into the processor and introduced the A20M pin to control it.
For an operating system developer (or Bootloader developer) this means the A20 line has to be enabled so that all memory can be accessed. This started off as a simple hack but as simpler methods were added to do it, it became harder to program code that would definitely enable it and even harder to program code that would definitely disable it.
Keyboard Controller
The traditional method for A20 line enabling is to directly probe the keyboard controller. The reason for this is that Intel's 8042 keyboard controller had a spare pin which they decided to route the A20 line through. This seems foolish now given their unrelated nature, but at the time computers weren't quite so standardized. Keyboard controllers are usually derivatives of the 8042 chip. By programming that chip accurately, you can either enable or disable bit #20 on the address bus.
When your PC boots, the A20 gate is always disabled, but some BIOSes do enable it for you, as do some high-memory managers (HIMEM.SYS) or bootloaders (GRUB).
Testing the A20 line
Before enabling the A20 with any of the methods described below it is better to test whether the A20 address line was already enabled by the BIOS. This can be achieved by comparing, at boot time in real mode, the bootsector identifier (0xAA55) located at address 0000:7DFE with the value 1 MiB higher which is at address FFFF:7E0E. When the two values are different it means that the A20 is already enabled otherwise if the values are identical it must be ruled out that this is not by mere chance. Therefore the bootsector identifier needs to be changed, for instance by rotating it left by 8 bits, and again compared to the 16 bits word at FFFF:7E0E. When they are still the same then the A20 address line is disabled otherwise it is enabled.
The following code performs a check (not like described above -- more directly).
; The following code is public domain licensed
[bits 16]
; Function: check_a20
;
; Purpose: to check the status of the a20 line in a completely self-contained state-preserving way.
; The function can be modified as necessary by removing push's at the beginning and their
; respective pop's at the end if complete self-containment is not required.
;
; Returns: 0 in ax if the a20 line is disabled (memory wraps around)
; 1 in ax if the a20 line is enabled (memory does not wrap around)
check_a20:
pushf
push ds
push es
push di
push si
cli
xor ax, ax ; ax = 0
mov es, ax
not ax ; ax = 0xFFFF
mov ds, ax
mov di, 0x0500
mov si, 0x0510
mov al, byte [es:di]
push ax
mov al, byte [ds:si]
push ax
mov byte [es:di], 0x00
mov byte [ds:si], 0xFF
cmp byte [es:di], 0xFF
pop ax
mov byte [ds:si], al
pop ax
mov byte [es:di], al
mov ax, 0
je check_a20__exit
mov ax, 1
check_a20__exit:
pop si
pop di
pop es
pop ds
popf
ret
Enabling
There are several sources that enable A20, commonly each of the inputs are or'ed together to form the A20 enable signal. This means that using one method (if supported by the chipset) is enough to enable A20. If you want to disable A20, you might have to disable all present sources. Always make sure that the A20 has the requested state by testing the line as described above.
Keyboard Controller
For the original method to enable the A20 line, some hardware IO using the Keyboard Controller chip (8042 chip) is necessary.
void init_A20(void)
{
UCHAR a;
disable_ints();
kyb_wait_until_done();
kyb_send_command(0xAD); // disable keyboard
kyb_wait_until_done();
kyb_send_command(0xD0); // Read from input
kyb_wait_until_done();
a=kyb_get_data();
kyb_wait_until_done();
kyb_send_command(0xD1); // Write to output
kyb_wait_until_done();
kyb_send_data(a|2);
kyb_wait_until_done();
kyb_send_command(0xAE); // enable keyboard
enable_ints();
}
or in assembly
;;
;; NASM 32bit assembler
;;
[bits 32]
[section .text]
enable_A20:
cli
call a20wait
mov al,0xAD
out 0x64,al
call a20wait
mov al,0xD0
out 0x64,al
call a20wait2
in al,0x60
push eax
call a20wait
mov al,0xD1
out 0x64,al
call a20wait
pop eax
or al,2
out 0x60,al
call a20wait
mov al,0xAE
out 0x64,al
call a20wait
sti
ret
a20wait:
in al,0x64
test al,2
jnz a20wait
ret
a20wait2:
in al,0x64
test al,1
jz a20wait2
ret
Fast A20 Gate
On most newer computers starting with the IBM PS/2, the chipset has a FAST A20 option that can quickly enable the A20 line. To enable A20 this way, there is no need for delay loops or polling, just 3 simple instructions.
in al, 0x92
or al, 2
out 0x92, al
However, this is not supported everywhere and there is no reliable way to tell if it will have some effect or not on a given system. Even worse, on some systems, it may actually do something else like blanking the screen, so it should be used only after the BIOS has reported that FAST A20 is available. Code for systems lacking FAST A20 support is also needed, so relying only on this method is discouraged. Also, on some chipsets you might have to enable Fast A20 support in the BIOS configuration screen.