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From: des@dtg.nsc.com (Desmond Young)
Newsgroups: comp.os.minix
Subject: Protected Mode Entry/Exit
Keywords: Protected Mode
Message-ID: <726@blenheim.nsc.com>
Date: 5 Mar 90 07:53:19 GMT
Organization: National Semiconductor, Santa Clara
Lines: 32

Hi,
  just a few simple (silly) questions.

  How does Minix exit protected mode ?  Does it ever need to ?
	To enter protected mode I believe you execute the appropriate
	instruction. But, how do you exit it ?
	I thought AT's sent an instruction to the keyboard to issue a
	reset, with the program having already stored a flag in the
	CMOS RAM. When re-booting, the program checks the RAM flag, and
	if set appropriately, does not perform a full reset/re-boot.

	Since this can be slow..... (reset time non-trivial), there must
	be a better way. Is this a closely guarded secret at Microsoft?

	The only way I can think of is to cause a multiple MMU fault.
	For example, access a location that is invalid, and has an
	invalid page entry in local and global descriptor tables. The
	286 datasheet does not define what happens in this case. Does
	the chip reset?

	Does Minix ever need to get out of protected mode?

	In case you are wondering why these questions. If you have an
	"XT" machine with a 286 processor, you do not have a keyboard
	to give you a reset, and you do not have a CMOS RAM to store a
	flag. So, the questions are to determine whether you may run
	Minix in protected mode on a 286-based non-AT machine.

Thanks in advance for any advice,
Des.
     des@dtg.nsc.com   or des@gpo.nsc.com
     No opinions - no disclaimer.