GDTR and GDT

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system error

Joined: 01 Sep 2013
Posts: 670

system error 26 Jul 2017, 18:40

I don't know where to ask this question. This has nothing to do with OS construction btw. Just out of curiosity.

I was told that GDTR is 10 bytes for 64-bit CPU and 6 bytes for 32-bit. I got some informative returns when I used it from 64-bit, but zeros from 32-bit. Any particular reason why? I tested this on Win64.

Code:

mov rax,gdt_val
sgdt [rax]

gdt_val rb 16

Returns: BASE=0xFFFFxxxxxxxxxxxx, LIMIT=87

But the same 32-bit setting returns nothing.

Code:

mov eax,gdt_val
sgdt [eax]

gdt_val rb 8

Returns: BASE=0, LIMIT=0

Did I miss something?

26 Jul 2017, 18:40

Azagaros

Joined: 18 Jan 2013
Posts: 26

Azagaros 02 Aug 2017, 22:37

The 64 bit version does not hardware task switching so the four bytes of the TSS of the GDT entry is consumed to accommodate the 64 bit address. OSdev wiki pages have a pretty good difference in the 32 bit models and the 64 bit models.

02 Aug 2017, 22:37

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