IBM 5150 PC Technical Reference (6025005, August, 1981) (PDF)

B. Break
The combination CTRL BREAK will result in the keyboard
routine signaling interrupt -lAo Also, the extended characters
(AL =DOH, AH = DOH) will be returned.
Power up initialization, this interrupt is set up to cause the
break sequence to be ignored. It is up to the system or
application initialization code to change the interrupt
vector in order to support an actual "break" function.
C. Pause
The combination CTRL NUM-LOCK will cause the keyboard
interrupt routine to loop, waiting for any key except
NUM-LOCK to be pressed. This provides a system/
application transparent method of suspending list/print/
etc. temporarily, and then resuming. The "Unpause" key
is thrown away. Handled internal to keyboard routine.
D. The following keys will have their typematic action
suppressed by the keyboard routine: CTRL, SHIFT, ALT,
NUM-LOCK, SCROLL-LOCK, CAPS LOCK, INS.
E. Print Screen
The combination SHIFT-PRINT SCREEN (Key 55) will
result in an interrupt invoking the print screen routine.
This routine works in alpha/graphics mode, with unrecognizable
characters printing as blanks.
The keyboard routine does its own buffering. The buffer is big
enough to support a fast typist. If a key is entered when the
buffer is full, the key will be ignored and the "bell" will
be sounded.
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