Re: Segment override and lldt instruction

["A D" <a_d_249@xxxxxxxxxxx>]

> 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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