I'm not a mac user anymore. But I used to be one when I was a kid.
My father had a Macintosh Performa 6200 running MacOS 7.5.3.
On said MacOS you could press command + the turn on key on the keyboard, and a white rectangle with a textcursor would appear.
Does anyone know what type of commands (and in what language) this box expected?
I googled around and found no mention of this textbox. Until this day I have not the slightest idea what that thing was.
Best Answer
Yes, that was MacsBug! (IIRC it was a System Extension that had to be installed/enabled -- although it's been quite a number of years...)
I found a command reference for it here: MacsBug Reference.
There's some more info over on Wikipedia.