Switching to x86 mode by iretq doesnt work

NTDEV
1

Hello Experts!

I have some problem with switching to x86 mode in my driver.

I use command iretq to jump to another segment. Right after jump I get needed value fo CS and SS registers, but next instruction reverts back my segments to x64 mode (0x10 and 0x18).

What could be a reason?

Here is my code:

push 38h
push rdx
pushfq
push 8h
push rcx
iretq

Thanks in advance!

With respect, Eugene.

2

What on earth is the purpose of this? You can’t run x86 code in kernel mode on a 64-bit system.

3

Hello Tim, thank you for the response. Why do you think so? CPU can be switched into Compatibility mode and so 32 bit code can be executed in this mode.

4

Last time I looked, segments 0x08 and 0x38 were unused, and there was no GDT entry for a code segment in compat mode in ring 0. Have you written your own entries to the GDT?

5

Yes, I configured these descriptors by my custom.

6

The question is solved, thank you, Tim!

7

malware? I can’t think of another reason to attempt this

8

Maybe, but not necessarily. I remember 100 years ago in Windows 3.1, our display drivers often shifted the chip into 32-bit mode for better performance.