I'd like to code a detection routine into my boot program so that the a20 line is enabled if necessary, however, since my bios does it automatically I have no way of checking it.

to get to the point, does anyone know (and could tell me) which bit people are referring to by A20? i.e., with or without zero-based addressing: bit 0, bit 1, ... , bit 19 = A20? for instance:

when the A20 line is disabled, two of these lines should give the same result.

Thanks.

EDIT: I have no helpful answer so,
heres some pages about A20
http://www.win.tue.nl/~aeb/linux/kbd/A20.html
http://wiki.osdev.org/A20_Line
http://www.osdever.net/tutorials/a20.php

mov eax,[1 shl 20] is the correct answer.

In electronics the address buss does start from A0, A1, A2 ... A19, A20, A21 ... A31 for 32 bits.

IF you would have done a simple calculation THEN you would have noticed that (1 shl 20) is in fact exactly 1 Mega byte.

bogdanontanu,

Not exactly, to be compliant to SI…

Yes, I know but I am not compliant to the SI in terms of binary data units, for me 1 Mega byte is 2^20 (aka 1024Kilo bytes, aka 1024*1024 bytes) and not 10^6.

The reason for this is exactly the physical address buses that go in powers of 2 and not in powers of 10.

Please excuse

Wrong. BIOS does what ? No "work" is needed to get A20 working if it is not crippled in the hardware.



Bit 20 of course ... physical address 1 MiB or $0010'0000



As when line is enabled ...

The safe way is to CLI, peek value from address 0, save it, INC it, write it to address $0010'0000, recheck address 0, draw the conclusion, and restore original value in address 0, STI.