Re: How to enter Protected Mode from Assembly. 1 Q.

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#1393

From: Boyer Francois-Raymon
Date: Thu, 05 May 1994 18:03

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SIR P/L (sir@usage.csd.unsw.OZ.AU) wrote:

: Hello everybody,
: 		What follows is my idea of all that is necessary to switch
: a 486 (or 386 for that matter) into "Flat" protected-mode.
: 		
: 		IT does NOT work.

: 		I would be *most* grateful if anyone could take a look at it
: and give me some idea what I'm doing wrong.  It simply hangs the machine, and
: the top left-hand corner of the screen remains un-changed.

: 		I'm using MASM 6, but this should work on most versions of
: most assemblers that accept protected mode instructions.

: 	Alternativly, if anyone has a tiny .EXE or .COM program to (1) switch
: the CPU into protected mode, and (2) execute some trivial instructions in an
: endless loop, I'd love to get a copy.  I'm much better with .ASM when I've got
: some working code to disect.

: 		All help grately appreciated.

: Chris.
: sir@usage.csd.unsw.oz.au

 With a quick look I've found things that are wierd.

You're loading the GDT instead of the GDT descriptor.
The data_selc should be 8h if you look at your gdt_tab.
You should use a 'smsw ax' instead of a 'mov ax,bx'.
You can't use a far jump because it will generate the wrong segment.

There are surely other errors. Have you read a book on 386 protected mode? It
could help...

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From: boyerf@JSP.UMontreal.CA (Boyer Francois-Raymon)
Subject: Re: How to enter Protected Mode from Assembly.  1 Q.
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Date: Thu, 5 May 1994 18:03:51 GMT
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