OSDev.org

Hello,

Sorry me English is not very well.

I don't understand why we must activate A20 line before switch to protected mode. Actually i don't make this and my program work fine.

With A0 to A19 address line we can acces from 0 to FFFFF memory (1Mo).
With A20 activated, we can acces from 0 to 1FFFFF memory (2Mo).

In protected mode adresse are coded on 32 bits, from 0 to FFFFFFFF (4Go).
What's the role of A20 line here ? To acces to memory from 1 to 2Mo if i return to real mode ?

Can you explain me ?
Thank you very much !

Hi,

It's mainly a compatibility thing. On some machines you will find that if A20 is not activated, you will only be able to access every other MiB of address space in PMode. It may be that emulators/modern PC's don't do this - but better to be safe.

Cheers,
Adam

so i might not need to enable a20 at all? how do i investigate that?

just try if its enabled or not

Ah, c'mon. It's only 30 or 40 lines of cut-and-paste assembly to stuff into your bootloader.

Hello,

Can you write example of code to activate A20 adress line ?

It's code to put before LGDT ? (in real mode) ?

Thank you !

Please seearch the wiki and google before posting - there are countless examples available.

A20 Line