Segment override and lldt instruction

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?

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?

Thanks for your time.

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