[sigcode]

BIOS just runs whatever is in the first 512 byte sector.

Unless it fits in 512 bytes this is not a "kernel" but is just instructions to jump to another address, where there is more small bootstrap code that loads another program, maybe a bootloader that loads another program, maybe a kernel.

Those who multiboot Windows with some other OS that uses disklabels may notice that Windows expects to always be the first OS. Any other bootstrap code put into that first sector will be overwritten by a Windows install.