store itdr on x64

Question

I tried to get idt address in my driver, I made function in asm which returns what idtr contains:

.data
  myData dq 0

.code
Function PROC
  sidt myData
  mov rax, myData
  ret
Function ENDP
END

But the address which I get is weird, for example in windbg:

r idtr
idtr=fffff80000b95080

However my driver shows:

idtr = f80000b950800fff

I read that on x64 IDTR contains 64-bit base address of IDT table. I would appreciate if anyone explain why my output is not the same as from WinDbg.

Answer 1

This is what the Intel docs say about the SIDT instruction:

In 64-bit mode, the operand size is fixed at 8+2 bytes. The instruction stores 8-byte base and 2-byte limit values.

and:

DEST[0:15] <- IDTR(Limit);
DEST[16:79] <- IDTR(Base);

This means your myData variable needs to be 10 bytes long, and the instructions stores the limit in the first 2 bytes and base address in the next 8 bytes. This also explains why your value matches with WinDbg's value after the first ffff bytes.