Jez asked for a emulator that makes internal signals visible together with signals that show the execution of the code. For that you need a trace buffer, thus a real in-circuit emulator and not only a JTAG debugger. It is interesting that a lot of 8051 IP cores come with on-chip debug logic for the debugging that only requires a small and cheap JTAG debugger box, but the task of finding bugs in an embedded system sometimes requires more than some breakpoints. At least if you want to find the bugs quick.

Harald