There are also very useful instructions that don't have semantics for most programming languages and have little or no use outside of booting a system. Like LG

There are also very useful instructions that don't have semantics for most programming languages and have little or no use outside of booting a system. Like LGDT and some of the tricks to switch from real mode to protected mode. What's maybe more curious, some instructions in certain modes aren't even implemented by some assemblers. I don't think it's entirely unusual to see hand crafted opcodes in an OS boot sequence. When the assembler doesn't do it, the compiler certainly isn't going to.