OSDev.org

Hi,

I am not there with the implementation yet, but am thinking about the design of the second-stage boot loader for my 64 bit OS. This is probably a simple yes/no answer, but can I do the following from 64-bit compatiblity mode:
or will I have to initially jump to a 32 bit offset to enable long mode and do a second jump to a 64 bit offset once long mode is enabled?

Thanks for any help,
Adam

Hi,

AJ wrote:I am not there with the implementation yet, but am thinking about the design of the second-stage boot loader for my 64 bit OS. This is probably a simple yes/no answer, but can I do the following from 64-bit compatiblity mode:

Code: Select all

jmp [long mode segment]:[64 bit offset]

Simple answer is "no" - a (32-bit or 16-bit) compatability code segment in long mode can't use REX prefixes, and therefore can't use 64-bit addressing.

The more complex answer would be that you might be able to fake it by doing a SYSCALL (without any SYSRET) or a software interrupt (without any IRET), where the target code removes anything the CPU pushed on the stack. This is probably more hassle than it's worth though...


Cheers,

Brendan

Hi,

As I hadn't used syscall/sysret before, I was a little unsure about what to do here. For anyone else trying the same thing, I have just implemented the syscall mechanism to call my kernel which is at 0xF000000000 from 32 bit code and it works very nicely.

Thanks Brendan,
Adam