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From: ballou@brahms.Berkeley.EDU.UUCP
Newsgroups: comp.sys.ibm.pc
Subject: Re: 286 protected virtual address mode
Message-ID: <829@cartan.Berkeley.EDU>
Date: Mon, 26-Jan-87 17:25:24 EST
Article-I.D.: cartan.829
Posted: Mon Jan 26 17:25:24 1987
Date-Received: Wed, 28-Jan-87 07:09:34 EST
References: <4207@utah-cs.UUCP> <1606@cit-vax.Caltech.Edu>
Sender: daemon@cartan.Berkeley.EDU
Reply-To: ballou@brahms.Berkeley.EDU (Kenneth R. Ballou)
Organization: Math Dept. UC Berkeley
Lines: 44
Keywords: 80286 assembler


>The assembler will, of course, only support the mode that it can access.  DOS
>can not (as of ver 3.x) access the proteced mode, so having the assembler
>support it would be foolish.

 	1)  Why would it be foolish?  Allow me to clarify your usage of the
expression 'access the protected mode.'  It is true that DOS does not run
in protected mode.  However, it is still quite possible to put the 80286 into
protected mode from DOS; you (simply?) have had to set up your own operating
system while in real mode to deal with the fact that you make a clean break
once you set the protected mode bit in the MSW.  Why would it be foolish to
have an assembler assemble protected mode instructions?  Let me also point
out that there are programs which start out running under DOS but which
use protected mode of the 80286, e.g., Gold Hill Common Lisp Development
System.

	2)  Please note also that the latest version of the Microsoft Macro
Assembler accepts 80286 protected mode instructions.

>			 Xenix, which works in protected mode, supports
>the protected mode commands, but not the real mode.  You don't get both in 
>either, because of the way the chip is designed.

	Excuse me, but this is a crock.  The protected mode instruction set
is a superset of the real mode instruction set.  The main difference between
protected mode operation and real mode operation is that segment
registers have a different meaning.  Instead of containing a paragraph
index which refers to an absolute location of memory, they contain
segment selectors, which are indices into either the GDT (Global
Descriptor Table) or the LDT (Local Descriptor Table).  These table
entries contain information the processor uses in handling segments,
notably the type of the segment, read/write permission, access
permission, and whether the segment is present in memory.  Note that
this is one of the big obstacles to simply setting up table entries to
DOS and using DOS routines in protected mode.  In order to preserve
user registers, DOS accesses data using a CS segment override prefix.
This works fine in real mode, but since code segments are not writable
in protected mode, the 80286 responds to this with a protection exception.

--------
Kenneth R. Ballou			ARPA:  ballou@brahms.berkeley.edu
Department of Mathematics		UUCP:  ...!ucbvax!brahms!ballou
University of California
Berkeley, California  94720