Run, pause, and the step family

Action	Key	Toolbar
Run / Continue	F5	F5: Continue
Pause / Break	F9	F9: Break
Single Step (into)	F6	F6: Single Step
Step Over	F7	F7: Step Over
Step Out	F8	F8: Step Out

The same actions are in the Debug menu.

Run / Continue resumes free execution. When the machine is stopped on a breakpoint, that breakpoint does not stop it again for the instruction you are standing on — otherwise F5 would put you straight back where you were, having executed nothing. The suppression is exactly one instruction wide: a breakpoint on the next instruction still stops you, and coming back round a loop to the same breakpoint stops you again. Step Over, Step Out, Run to Here and Run to EOF/EOSL all resume through the same mechanism, so they step off the breakpoint under the PC too. One consequence worth knowing: Run to Here on the line you are already stopped at runs a full lap and stops when control comes back.

None of that applies when the machine is not stopped. Pressing F5 while a program is running — the emulator window takes F5 too, whenever the debugger is enabled — does nothing at all. Your breakpoints stay live and stop the machine as normal, and a pending Run to Here or step target is left alone, so a stray F5 while you wait for one cannot cancel it. The key is still swallowed by the debugger either way, so it never reaches the emulated machine.

Single Step executes exactly one instruction, entering calls, traps and interrupt handlers.

Step Over sets a one-shot breakpoint after the current instruction and resumes, so a subroutine runs at full speed and you land on the following instruction. It only does this for instructions that can come back — CALL nn, CALL cc,nn, RST n and DJNZ. For anything else it is identical to Single Step. Note the consequence for DJNZ: stepping over it runs the whole loop.

Step Out resumes until a return instruction — RET, a taken RET cc, RETI or RETN — pops the stack back past its level when you pressed it; that is, until the current subroutine returns rather than a nested one. A subroutine called after you press F8 returns onto that level, not past it, so its own RET does not end the step; neither does an interrupt handler's RETI, nor a bare POP that happens to move SP the same way.

If a subroutine never returns (it jumps away, or is waiting on input that never arrives) Step Over and Step Out simply keep running; press F9 to take back control.