Enabling the A20 Line

Okay, this isn't exactly a tutorial. What it is a very well commented example of enabling A20 in assembly.

;;
;; enableA20.s (adapted from Visopsys OS-loader)
;;
;; Copyright (c) 2011, J.Vinoth Kumar
;; You're free to use this code in any manner you like, as long as this
;; notice is included (and you give credit where it is due), and as long
;; as you understand and accept that it comes with NO WARRANTY OF ANY KIND.
;; Contact me at jamesamc@yahoo.com about any bugs or problems.
;;

enableA20:
 ;; This subroutine will enable the A20 address line in the keyboard
 ;; controller.  Takes no arguments.  Returns 0 in EAX on success, 
 ;; -1 on failure.  Written for use in 16-bit code, see lines marked
 ;; with 32-BIT for use in 32-bit code.

 pusha

 ;; Make sure interrupts are disabled
 cli

 ;; Keep a counter so that we can make up to 5 attempts to turn
 ;; on A20 if necessary
 mov CX, 5

 .startAttempt1:  
 ;; Wait for the controller to be ready for a command
 .commandWait1:
 xor AX, AX
 in AL, 64h
 bt AX, 1
 jc .commandWait1

 ;; Tell the controller we want to read the current status.
 ;; Send the command D0h: read output port.
 mov AL, 0D0h
 out 64h, AL

 ;; Wait for the controller to be ready with a byte of data
 .dataWait1:
 xor AX, AX
 in AL, 64h
 bt AX, 0
 jnc .dataWait1

 ;; Read the current port status from port 60h
 xor AX, AX
 in AL, 60h

 ;; Save the current value of (E)AX
 push AX   ; 16-BIT
 ;; push EAX  ; 32-BIT

 ;; Wait for the controller to be ready for a command
 .commandWait2:
 in AL, 64h
 bt AX, 1
 jc .commandWait2

 ;; Tell the controller we want to write the status byte again
 mov AL, 0D1h
 out 64h, AL 

 ;; Wait for the controller to be ready for the data
 .commandWait3:
 xor AX, AX
 in AL, 64h
 bt AX, 1
 jc .commandWait3

 ;; Write the new value to port 60h.  Remember we saved the old
 ;; value on the stack
 pop AX   ; 16-BIT
 ;; pop EAX  ; 32-BIT

 ;; Turn on the A20 enable bit
 or AL, 00000010b
 out 60h, AL

 ;; Finally, we will attempt to read back the A20 status
 ;; to ensure it was enabled.

 ;; Wait for the controller to be ready for a command
 .commandWait4:
 xor AX, AX
 in AL, 64h
 bt AX, 1
 jc .commandWait4

 ;; Send the command D0h: read output port.
 mov AL, 0D0h
 out 64h, AL 

 ;; Wait for the controller to be ready with a byte of data
 .dataWait2:
 xor AX, AX
 in AL, 64h
 bt AX, 0
 jnc .dataWait2

 ;; Read the current port status from port 60h
 xor AX, AX
 in AL, 60h

 ;; Is A20 enabled?
 bt AX, 1

 ;; Check the result.  If carry is on, A20 is on.
 jc .success

 ;; Should we retry the operation?  If the counter value in ECX
 ;; has not reached zero, we will retry
 loop .startAttempt1


 ;; Well, our initial attempt to set A20 has failed.  Now we will
 ;; try a backup method (which is supposedly not supported on many
 ;; chipsets, but which seems to be the only method that works on
 ;; other chipsets).


 ;; Keep a counter so that we can make up to 5 attempts to turn
 ;; on A20 if necessary
 mov CX, 5

 .startAttempt2:
 ;; Wait for the keyboard to be ready for another command
 .commandWait6:
 xor AX, AX
 in AL, 64h
 bt AX, 1
 jc .commandWait6

 ;; Tell the controller we want to turn on A20
 mov AL, 0DFh
 out 64h, AL

 ;; Again, we will attempt to read back the A20 status
 ;; to ensure it was enabled.

 ;; Wait for the controller to be ready for a command
 .commandWait7:
 xor AX, AX
 in AL, 64h
 bt AX, 1
 jc .commandWait7

 ;; Send the command D0h: read output port.
 mov AL, 0D0h
 out 64h, AL 

 ;; Wait for the controller to be ready with a byte of data
 .dataWait3:
 xor AX, AX
 in AL, 64h
 bt AX, 0
 jnc .dataWait3

 ;; Read the current port status from port 60h
 xor AX, AX
 in AL, 60h

 ;; Is A20 enabled?
 bt AX, 1

 ;; Check the result.  If carry is on, A20 is on, but we might warn
 ;; that we had to use this alternate method
 jc .warn

 ;; Should we retry the operation?  If the counter value in ECX
 ;; has not reached zero, we will retry
 loop .startAttempt2


 ;; OK, we weren't able to set the A20 address line.  Do you want
 ;; to put an error message here?
 jmp .fail


 .warn:
 ;; Here you may or may not want to print a warning message about
 ;; the fact that we had to use the nonstandard alternate enabling
 ;; method

 .success:
 sti
 popa
 xor EAX, EAX
 ret

 .fail:
 sti
 popa
 mov EAX, -1
 ret