I learned basic 16 bit x86 assembly by making simple COM executables and boot sectors with MASM. I don't know if its still possible run COM on modern Windows (t

I learned basic 16 bit x86 assembly by making simple COM executables and boot sectors with MASM. I don't know if its still possible run COM on modern Windows (this was in the 32 bit XP days), but getting started with boot sectors using BIOS interrupt service routines for IO should be an interesting project. You should be able to use the assembler output directly in a VM as a disk image this way, or dd it to something and watch it run on bare metal.