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Why (or why not) is important enable the A20 line?
Is necesary to work in protected mode?
Which CPU's support teh A20 line?

Sorry, I am very new with A20...

Thank you.

Every CPU over like the 286 has an A20 line, it IS protected mode, and is required to access more the 16MB of RAM IIRC.

A20 has nothing to do with protected mode. A20 for emulation of an 86 processor. It had a 20 bit bus, and many programers used it's feature: address
0xffff:0xffff = 0xffff0+0xffff = 0x10FFEF = ( beacause of the 20-bit bus)= 0xFFEF.
Starting with a 286 A20 apeared, because the effect above would not apear overwise.
Anton

A20 was disabled by default on AT architecture when introduced because of compatibility issues with the XT architecture.  XT when you try to reference memory passed 1 meg would wrap back to 0.  So you must enable it to access anything over 1 meg to prevent wrapping back.  IIRC it's actually 21 address lines(start from 0, 20 is the 21st address line).

If A20 has nothing to do with protected mode, then may I switch to protected mode and access the 4GB of memory?

A20 lets you access every other megabyte of memory. You could enter protected mode without enabling A20, but then you wouldn't be able to access memory at 1MB-2MB, 3MB-4MB, 5MB-6MB, etc.

That's exactly what i wrote.

Thank you for everybody, I know now what A20 is.

I get a very interseting code at http://lists.insecure.org/linux-kernel/ ... /1123.html it works very fine and maybe someone else like to study it.

Thank you again!