Jan Brdenbeek has re-released MultiMon – a QL Monitor, Disassembler and Debugger written in 1986/7 – on GitHub.
It was already available from PD websites, but now there’s an English manual and source files too.
MultiMon v2.1 is entirely written in machine code and less than 10K bytes in size.
It may be started with an EXEC command or installed via LRESPR as a SuperBASIC extension called MON.