I thought Mcode was just a generic term for any machine language. The HP-41 hand-held calculator/computer of the 1980's offered Mcode (and called it exactly that) for users who wanted to go that deep. I almost got there, but not quite. One of the modules (the ZENROM) plugged into my HP-41cx has tools for directly keying in synthetic instructions, and also an assembler. I got Ken Emery's book, "HP-41 Mcode for Beginners" but didn't get very far into it and never did any real Mcode programming. There's a ton of info on it on the 12GB DVD set (1400 documents, no video) offered by the
HP Museum website. The entire listing of the "mainframe's" and other ROMs is available not just for examples but so you can use the various routines' entry points in your Mcode programming. There are quite a few experts at Mcode on the very active
HP Museum forum, and a couple are making and selling new modules for the HP-41 with large memory spaces and microSD cards and USB.
Part of my justification in mentioning it is what I put in
one of my project pages here: the inspiration the HP-41 and HP-71 handhelds have given me for my 6502 projects. Both have expandable operating systems, file chains in non-volatile RAM, expansion ports, the interface loop (HP-IL), and tons of other features that serve as outstanding models of engineering.