Comment by flohofwoe

Here's a similar (and much more indepth) opcode decoding recipe for Z80, very useful for emulator development:

http://www.z80.info/decoding.htm

For actually programming in machine code this understanding of the internal opcode structure isn't all that useful though, usually - without an assembler at hand - you had a lookup table with all possible assembly instructions on the left side, and the corresponding machine code bytes on the right side.

Programming by typing machine code into a hex editor is possible, but really only recommended as absolute fallback if there's no assembler at hand - mainly because you had to keep track of all global constant and subroutine entry addresses - e.g. the main thing that an assembler does for you, and you had to leave gaps at strategic locations so that it is possible to patch the code without having to move things around.