I'm not sure specifically on the z-160 but my z-150 had it (iirc and it's been a while ctrl+alt+ins or ctrl+alt+enter) I think put you in the diagnostic rom which had a debugger. Curiously though i dont know if that is a hardware function or a feature of zdos? -------- Original message --------From: Fred Cisin <cisin at xenosoft.com> Date: 12/11/16 6:59 PM (GMT-06:00) Don't those machines have a debugging monitor in ROM?? (something that IBM does NOT)? Otherwise DEBUG.COM should work.