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From: [Jongware] on 19 Feb 2010 09:08
David Given wrote:
> I have some deeply strange code that I have just discovered is accessing
> some values via GDT segment descriptors.
>
> The library that I am implementing has an API for allocating a new
> segment descriptor from the operating system, and setting it up to point
> at a block of memory in user space; so, once %es has been loaded with
> the new segment descriptor, accessing %es:0 is equivalent to accessing
> the first byte of this block of memory, etc.
>
> Does Windows (or, preferably, the Windows NT kernel) have an API for
> this? I'm actually from a Unix background, so I don't know the Windows
> terminology for these things, so I don't know what keywords to search
> for to get information...

GDT, LDT :-)

Not an answer; but I have seen MS own VC compiler perform a /similar/
trick using the fs: register -- it typically accesses stuff like fs:[0].
I have a vague notion it was somehow related to TSL (= thread local
storage, IIRC).
I have no idea how the underlying system works -- for example, where,
how and for what the fs: segment gets its "active" value before stepping
into the thread.

However, just idly thinking about it: fs: sounds more like a sane choice
for a system like this (and yours) than es: -- that one is highly likely
to be in use for stuff such as string comparing and moving (the
lods/stos/scas/cmps family of operands).

> This is all 32-bit code and doesn't have to be portable.

You mean, between Linux & Windows? What a relief -- it sounds like
extremely low level close-to-metal stuff such as system kernels and drivers.
Speaking of which: you could wander over to the Microsoft site and
browse a bit through their Device Driver Kit -- DDK -- as it is quite
more probably a segment register specification pops up in there than it
is in the 'regular' APIs (personally, I've never encountered a register
spec in the regular APIs ;o)
You could also try to post in a DDK dedicated group -- no harm in trying.

[Jw]
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