Re: switching from long mode to compatibility mode

["tejas khatiwala" <socretez@xxxxxxxxx>]

I got it fixed. As I said earlier, the
difference between compatibility and 64bit mode is L/D bits in CS
descriptor. However, while making far jump in 64bit mode, the operand
size _must_ be set to 64bit (which is, by default, 32bit unless
explicitly specified). So I had to put a "operand size prefix" in front
of "retf" ("o64 retf" in NASM) and it worked fine.

Thanks guys,
/tejas

On Wed, Jun 4, 2008 at 6:34 PM, tejas khatiwala <socretez@xxxxxxxxx> wrote:

Hello,

I am writing small code that boots, jumps to 32bit protected mode (no-paging enabled) --> 64bit mode (pml4 paging) --> perform some 64bit test --> jump back to 32bit protected mode (no-paging enabled).

I got successful getting into 64bit mode and I'm trying to switch out of it to 32bit protected mode.

According to Intel document the first step is to "Switch to compatibility mode" which after several efforts, I'm not able to do successfully. Although I was successful in jumping in and out of compatibility mode from 32bit protected mode (no-paging enabled).

Only diff. betwn 64bit page-protected mode and compatibility mode is L and D bit in code segment. In other words, (according to my understanding) if I'm in 64bit mode and if I want to jump to compatibility mode, all I'm suppose to do is to make far jump to compatibility mode Code-Segment (please correct me if I'm wrong). The CPU freezes when I make the jump.

While making jump,
1) PML4 paging is still in place (hence, IA32_EFER.LMA = 1)
2) PAE and PE is still enabled
3) IA32_EFER.LME = 1

Can somebody please help me with this? Am I suppose to re-arrange paging or do something about gdt?

Thanks,
/tejas

NOTE: When I jump back to 32bit protected mode, I want to keep paging disabled.