This article is from the Macintosh system software FAQ, by Elliotte Harold elharo@shock.njit.edu with numerous contributions by others.
On Macs that don't have a physical programmer's switch you
can restart the computer with Command-Control-Power and drop
into the debugger with Command-Power. Also note that in System 7
Command-Option-Escape will force most applications (including
the Finder) to quit so you no longer need to activate the debugger
just to kill a frozen application.
 
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