The hardest part of writing an Operating System is not writing an Operating System, it's getting other people to use it.
From what I recall (it was a *very* long time ago) the hardest parts about booting an OS were dealing the bullshit involved in switching from 16-bit mode into protected mode. (Yes, all x86 based PCs *still* start in 16-bit mode.) Not an impossible task, obviously, but it could have been easier.
That's not that hard ... certainly not as hard as switching back again (without rebooting) used to be.
I did have fun writing the floppy (that's how long ago this was) boot loader though.
The bootloader works in the same way on a hard disk (in pre-UEFI machines), fortunately the BIOS does the actual reading and writing of the disk. The standard hard drive bootloader is only 512 bytes (including the partition table) but the rest of track 0 is unused, so you can use your bootloader to load a more sophisticated bootstrap from the rest of that track. I've worked on bootloaders that enabled booting from an encrypted drive, and they work like that.
Biting the hand that feeds IT
