I think grub would usually be called after the boot firmware (like UEFI, or coreboot, or a traditional BIOS) has already initialized the hardware. Some of these

I think grub would usually be called after the boot firmware (like UEFI, or coreboot, or a traditional BIOS) has already initialized the hardware. Some of these systems can also directly load your operating system, and I would guess the one of the few thing missing from the boot firmware would be the filesystem support, and not much more