what	how
build boot sector	nasm main.s -fbin -o boot_sect.bin
'run' bare boot sector	qemu-system-x86_64 -fda boot_sect.bin
build kernel entry	nasm enter_kernel.s -felf -o enter_kernel.o
link entry with source	ld -o kernel.bin -m elf_i386 -Ttext 0x1000 enter_kernel.o kernel.o --oformat binary
compile kernel source	gcc -ffreestanding -m32 -fno-pie -c kernel.c -o kernel.o
construct kernel image	cat boot_sect.bin kernel.bin > os-image
everything automatically	make / make run / make [target]

https://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf