The extremely popular BBC Micro (1981-1986, still in retro use today) had an exceptionally powerful debugger, "Beebmon" by Watford Electronics, capable of breakpoints, IRQ trapping, code stepping, and so on. From a summary:
BEEBMON
A ROM based machine code monitor for BBC Micro. It enables machine code programs to be debugged and altered easily and quickly. Being a ROM, its commands are always readily available and occupy no user memory. Appears to take no base page and only one page of relocatable workspace (256 bytes) and no more anywhere in RAM. Beebmon can do more than any other machine code monitors currently on the market. The special features include facilities like: TABULATE, MODIFY, FILL, COPY, COMPARE, SEARCH (HEX & ASCII) CHECKSUM, DISASSEMBLE, RE-LOCATE and, by emulating the 6502 processor, SINGLE STEP, BREAK POINTS ON READ/WRITE/EXECUTE OF LOCATION. Also BREAKPOINTS ON A, X & Y REGISTERS are provided . HAS WINDOWS INTO MEMORY & TEST WINDOWS. All this and more...
I used this to trace IRQ handling in the OS and other ROMs, and to reimplement disk and keyboard handlers for software which required modified handling. I think it came out very early on, around 1982, and certainly by 1983 (according to the source).
Description source: Watford Electronics flier, 1983, on 4corn.co.uk (my phone won't deobfusticate Google search links for pdfs, so this link tracks via google tracking. Please feel free to edit/fix)
