Message-ID: <34861C8E.FBE@mailexcite.com>
Date: Wed, 03 Dec 1997 21:59:26 -0500
From: Doug Gale <dgale AT mailexcite DOT com>
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Subject: Re: (none)
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Tan Pinghui wrote:
> I'm very confused by the 386's 16-bit & 32-bit mixing coding.
> Here are some questions, please help.
> 
> (1) cs selector refers to 16-bit segment, and ds/es/fs/gs selectors to
> 32-bit segments (D=1), ss to 16-bit segment, how do the instructions
> operate? the stack? the data?
> (2) cs = 16-bit, ds/es/fs/gs = 32-bit, ss = 32-bit?
> (3) cs = 16-bit, ds/es/fs/gs = 16-bit, ss = 32-bit?
> (4) cs = 32-bit, ds/es/fs/gs = 16-bit, ss = 16-bit?
> (5) cs = 32-bit, ds/es/fs/gs = 16-bit, ss = 32-bit?
> (6) cs = 32-bit, ds/es/fs/gs = 32-bit, ss = 16-bit?
> 

Are you are referring to the fact that the segment registers are 16-bit?

Some terminology:

	Segment - an area of memory

	Selector - a value that you stick into a cs, ds, es, fs, gs, or ss to
access a segment

	Descriptor - a sort of struct that defines a segment, stored in arrays
managed by the DPMI server (in the DJGPP case) called descriptor tables.

The segment registers tell the CPU which descriptor to use when
accessing memory.

The CPU uses the segment register as an offset into a table of
descriptors, reads the descriptor to determine the base address of the
segment, whether it is a 16-bit or 32-bit segment, etc.

So, to answer your question (I hope), if cs (or any other segment
register) holds a selector that refers to a descriptor that specifies it
is 32-bit segment, then references using cs will DEFAULT to a 32-bit
addressing mode.

By the way, making a segment 32-bit just means that the DEFAULT
_addressing_mode_ is 32-bit. You can still use a 16-bit addressing mode,
but it requires an override prefix and an extra cycle. The same applies
to a 16-bit segment (32-bit addressing mode requires an override
prefix).

Now, why do you care? :-)