Actually, I think it's as86 syntax - kind of a weird hybrid of Intel and AT&T. Not for Nasm, in any case!
"rep movw", -> "rep movsw"
"seg fs", -> just "fs" probably?
"lgs si,(bx)", -> "lgs si, [bx]"
"j load_setup" -> just "jmp", I would guess (maybe "jmp short"?)
Some "context" might help - we know what a bootsector "has to do", even if the syntax is unfamiliar. Got a link to the exact "bootsect.s" you're looking at? (save us looking it up...)
Best,
Frank