[simon1ltd]

It was AtEase. In the earlier macs, those with a button to trigger a debugger in firmware, you could completely exit AtEase by entering the debugger and typing “G H”

I know it worked on System 7, and I think there was a variation that worked in System 8 as well.

[exikyut]

What does 'H' do? The other system monitor references in the comments here don't quite do it this concisely.