From: "A D" <a_d_249@hotmail.com>
To: fbkotler@verizon.net
Cc: linux-assembly@vger.kernel.org
Subject: Re: Segment override and lldt instruction
Date: Wed, 30 May 2007 21:21:00 -0400 [thread overview]
Message-ID: <BAY133-F856FD426A3078A80C5250F52D0@phx.gbl> (raw)
In-Reply-To: <465E1AC6.2020906@verizon.net>
>Frank Kotler wrote:
>>A D wrote:
>>Hi! I have couple of questions regarding gnu assembly. I've heard the word
>>segment override. Where segment register can be manually overriden. so how
>>can i
>>override say ds register? can segment register be overriden to my
>>preferred
>>memory address location?
>>
>>mov $0xf800, %ax
>>mov %ax, %es
>>mov %ds, %bx
>>
>>But I get segmentation fault error. How can i do it without error?
>
>Use a valid selector. You appear to have "heard about" some 16=bit real
>mode stuff that is not true in protected mode. Take a look here:
>
>http://my.execpc.com/~geezer/johnfine/segments.htm
>
>In "Linux assembly", there is no reason you'd *want* to use a segment
>override, or alter a segment register.
>
>>
>>Also I was looking at the function of lldt instruction. The manual says
>>that:
>>"The source operand (a general-purpose register or a memory location)
>>contains a segment selector that points to a local descriptor table
>>(LDT)."
>>
>>How can one make segment selector point to ldt?
>
>"lldt ax" or "lldt [mem]" (16-bits)... I guess. But why? The limit is
>0xffffffff and the base is 0. (Windows uses fs in a way that might be
>"interesting"..._
>
>If you're developing your own OS (using Linux), starting from a bootsector
>- or GRUB - these instructions might be useful, but for "Linux assembly",
>forget that segment registers exist! (and be glad! :)
>
>Best,
>Frank
>
Thanks Frank. I was just curious how these things work. Thanks for your
pointer.
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next prev parent reply other threads:[~2007-05-31 1:21 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-05-30 22:31 Segment override and lldt instruction A D
2007-05-31 0:45 ` Frank Kotler
2007-05-31 1:21 ` A D [this message]
2007-06-04 23:52 ` Hendrik Visage
2007-06-11 0:33 ` A D
2007-06-11 1:14 ` Frank Kotler
2007-06-11 5:30 ` Brian Raiter
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