Opening the A20 gate

[f2]

Because my OS is going to be in Protected Mode, I'm going to have to open the A20 gate. Isn't it to use more than 1 meg of memory?

Anyway, can somebody tell me how to do it? Not just giving me code, but explaining what is needed and what is done and why and how. Thanks!

[df]

There is code in the OS faq for doing it
http://www.mega-tokyo.com/os

[virtus]

In real mode, you can only access memory below 1 Meg, in protected mode, you can access up to 4Gigs, when you have enabled the a20 gate..
a20 is controlled thru the keyboard controller..
A very good tutorial can be found at:

http://www.win.tue.nl/~aeb/linux/kbd/A20.html

[f2]

Does that mean you HAVE to be in Protected Mode when opening the A20 gate, or can you be in Real Mode to open it?

[The_Legend]

You can open it in real mode, then you can access even 64kb - 16 bytes more ram than 1MB, but some programs crash if you don't close it again (no problem if you are not running dos programs at all).

[f2]

Well, what I'm going to do is just open the A20 gate (in my bootstrap) and then choose the file system and then load the kernel, so I don't have to worry about opening other programs and causing the system to crash. Thanks!