IBM AT 5170 Technical Reference page 194

Pause
The Pause key (Ctrl and Num Lock) causes the keyboard
interrupt routine to loop, waiting for any key except Num Lock to
be pressed. This provides a method of temporarily suspending an
operation, such as listing or printing, and then resuming the
~
operation. The method is not apparent to either the system or the
application program. The key stroke used to resume operation is
discarded. Pause is handled internal to the keyboard routine.
Print Screen
The PrtSc key results in an interrupt invoking the print-screen
routine. This routine works in the alphanumeric or graphics
mode, with unrecognizable characters printing as blanks.
System Request
When the System Request (Sys) key is pressed, a hex 8500 is
placed in AX, and an interrupt hex 15 is executed. When the Sys
1"'"""'-\
key is released, a hex 8501 is placed in AX, and another interrupt
hex 15 is executed.
If
an application is to use System Request,
the following rules must be observed:
Save the previous address.
Overlay interrupt vector hex 15.
Check AH for a value of hex 85:
If
yes, process may begin.
If no, go to previous address.
The application program must preserve the value in all registers,
except AX, upon return. System Request is handled internal to
the keyboard routine.
5-22 System BIOS