64-Bit bootloader

[Fudlite]

Hi! I'm relatively new to bootloaders, and I'm looking for some help with an example bootloader that just attempts to go into 64-bit mode, and then runs the kernel. If it fails, then it just runs the 32-bit kernel in its place.

I know I'm probably asking a lot, but all the 64-bit bootloader tutorials I've seen around show random snippets, but nothing I've been able to make work.

My goal is to write a hybrid microkernel as a personal project, but I need to get the thing to load =P

Thanks very much!

Fud

[Fudlite]

That's probably what I'll end up doing lol

[StephanvanSchaik]

Hi,

Generally, the boot process (I use) is as following:

• Set up the segment registers and the stack pointer.
• Store BIOS information.
• Set up VGA.
• Test if the CPU is at least a 80386.
• Validate that the boot loader is entirely loaded, otherwise try it manually when possible.
• Test if the CPU supports certain instructions I need.
• Enable the A20 gate.
• Enter unreal mode (real mode with 32-bit data descriptors).
• Attempt to retrieve the memory regions.
• Set up a minimal memory manager.
• Parse the file system looking for KERNEL32 and KERNEL64 and load them both.
• Test if the CPU is a 64-bit processor.
• Prepare to jump to KERNEL64, when it was loaded i.e. exists.
• Otherwise prepare to jump to KERNEL32.

Information dedicated to testing if a x86-processor is also a x86-64-processor, entering long mode and preparing the execution of the kernel can be found here.


Regards,
Stephan J.R. van Schaik.

[StephanvanSchaik]

• Parse the file system looking for KERNEL32 and KERNEL64 and load them both.
• Test if the CPU is a 64-bit processor.

Why not reverse these two and skip loading the not-needed kernel altogether?

Actually, this was what I was doing several months ago, but I am not using 64-bit at the moment and it took me some time to rethink what I exactly did in my older boot loaders. My new boot loader currently only supports 32-bit, but will use the exact method I described here since I am trying to merge both architectures into one file.


Regards,
Stephan J.R. van Schaik.