[immibis]

What's wrong with depending on the Windows PE format, FAT filesystem and UEFI? You're always going to have some dependencies. FAT32 is better than having the first sector load some magic reserved sectors. Windows PE is better than a fixed memory address.

[rep_lodsb]

It's adding pointless complexity, and baking assumptions about how an OS should work into the firmware. Loading a sector at a fixed memory address and jumping to it (with some function provided so that your code can go on to load other sectors) is both easier to understand, and doesn't require you to use some multi-megabyte toolchain.

[p_l]

Unless you need to load anything else other than what's in that sector, because you can't fit enough I/O drivers into one sector to load the rest of the OS.

Unless you're targeting S/360, but even there XA and newer made it a bit more complex.

[immibis]

Wouldn't it be better to use a header to specify to load multiple sectors at multiple memory addresses?

[ForOldHack]

Yes, and considering that your boot sector is actually a boot cluster... You need not use compression. You could even steal the entire first cylinder. (Now that is old school)...