IBM 5150 Manual To Operations page 56

Below are a few examples of "CONTROL" functions:
"CTRL
+
NUM LOCK"
=
SUSPEND
Suspends system operation so you can read the display.
Press any key to continue.
"CTRL
+
PRTSC
=
ECHO
Echos and prints what you type and what the system
displays until you press these keys again.
"CTRL
+
SCR LOCK/BREAK"
=
BREAK
This stops your command program while it's running.
Keyboard 3-10