Nah, my idea is to to have something more "intelligent". Involving logic (possibly programmable) and maybe even an another microcontroller.

For example, the simplest idea, to have a latch (loadable somehow from PC) and a digital comparator - the processor attempts to fetch from this address and voila, a flipflop is flipped and a LED is lit.

Just one step beyond this, upon flipping the flipflop, the "EPROM" is not outputting the data it is supposed to output as a well-behaving EPROM emulator anymore, but throws out a custom written code, causing for example an infinite loop - voila, we have breakpoints without having to mess with the microcontroller itself. We can even recover the register's content somehow, e.g. making the processor fetching from addresses depending on content of registers; and vice versa, we can relatively easily modify registers needed and then nicely jump back where we left out. Voila, what a nice monitor. Plus, we can run a full execution trace, if we want - after all it's trivial to log the addresses the processor was visiting recently... :-)

Unfortunately, stuff like this is not interesting anymore today... :-(

JW