Does GRUB switch to protected mode?

Question

I would like to ask if it is GRUB that switch the CPU to protected mode during boot up or is it the Linux kernel that does it. And also I would like to ask - is the kernel itself (vmlinuz) an ELF or is it plain binary format? Thanks.

Answer 1

GRUB does drop you in protected mode.

The GRUB Multiboot Specification (version 0.6.96) Section 3.2 tells you this

‘CR0’

Bit 31 (PG) must be cleared. Bit 0 (PE) must be set. Other bits are all undefined.

And CR0 Register mapping tells you that the system should be in protected mode.

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Linux is not a multiboot kernel and does not rely on some bootloader for switching in the PM, it follows the Linux Boot Protocol. But linux does the protected mode switch itself, and does not rely in the bootloader

check : http://lxr.linux.no/#linux+v2.6.39/arch/x86/boot/main.c

Here it calls go_to_protected_mode(); when then calls protected_mode_jump () which then does the CR0 stuff (sets the bit 0)

(The other bit says that paging is disabled)

EDIT

What i can figure out is that GRUB can detect linux boot protocol (GRUB2, and legacy should also) and loads linux on memory, but does not switch to protected mode. Have a look at this link: http://www.gnu.org/software/grub/manual/grub.html#GNU_002fLinux and section 16 of the page in this link.