x86 How to change gdt after it has been already loaded?

Question

I have my idtr addressing defined as

gdt_pointer:
    DW 0;size_of_gdt
    DD 0;start_of_gdt

and I can load it with LGDT [gdt_pointer]. But how can I change the contents of gdt later if I have to? Can I use the LGDT instruction again after setting the proper values?

EDIT: I'm planning to keep my gdt_pointer start address same all the time, but I'll be changing the size parameter. Also I'm loading and changing the gdt from 32 bit protected mode.

score 2 · Accepted Answer · answered Aug 25 '14 at 07:31

2

If you change the contents of the GDT (but leave it at the same address), then you only have to load segment registers that would be effected by the changes (if any).

If you change the (virtual) address of the GDT (but leave its contents the same), then you only have to do lgdt again.

If you change both the (virtual) address of the GDT and its contents, then you'd need to do lgdt and also load any effected segment registers.

answered Aug 25 '14 at 07:31

Brendan

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How do I load the segment registers? I don't have any clue. And yes I'm planning to keep the gdt at same address all the time and just change its contents. – The amateur programmer Aug 25 '14 at 09:49
Also I'm not using virtual addresses (at least not yet). – The amateur programmer Aug 25 '14 at 10:09

x86 How to change gdt after it has been already loaded?

1 Answers1