OSDev.org

The A20 line:
When the 21'st address line is enabled on entry to protected mode, alowing access to all memory above the one megabyte mark, are you just enabling the 21'st address line (A20), or the address lines form 21 to 32? Or, was the 21'st line the only line that was dissabled for compatability in the first place. Also, is it possible to stay in real mode, and just enable the 21'st address line, allowing you to access all lines, and therefore the 4 megabytes? Or am I comfused and mistaken. Thanks, and sorry for so many questions.

Tyler

Ty wrote: are you just enabling the 21'st address line (A20), or the address lines form 21 to 32?

"the 21'st line the only line that was dissabled for compatability in the first place."

Ty wrote: Also, is it possible to stay in real mode, and just enable the 21'st address line, allowing you to access all lines, and therefore the 4 megabytes?

Yes, this is possible, and this is called the UNREAL mode. (But you have to first enter the pm, set the limits of the segment descriptors to the maximum, and then return to real mode)

Ty wrote: Thanks, and sorry for so many questions.

No problem.
Anton.

yes but in unreal mode you lose all the benefits of pm and
you cannot create long code segs(?) only data and it can
easily be damaged forcing you to reenable unreal mode
and rm is a LOT slower than pm and has no memory management
and no kernel protections or hardware protections

Thanks for your thorough replies!
That really helped. (I was really confused)

Ty