IBM 5170 Technical Reference page 161

Ctrl:
This key temporarily shifts keys 3, 7,13,15,17-28,31-39,
46-52, 106,65-74,42, 101,92,102,91,93,95, 100, and 103 to
the Ctrl state. The Ctrl key is also used with the Alt and Del keys
to cause the system-reset function; with the Scroll Lock key to
cause the break function; and with the Num Lock key to cause
the pause function. The system-reset, break, and pause functions
r--...
are described under "Special Handling" later in this section.
Alt: This key temporarily shifts keys 1-13, 17-26,31-39,46-52,
and 65 -74 to the Alt state. The Alt key is also used with the Ctrl
and Del keys to cause the system reset function.
The Alt key also allows the user to enter any character code from
0-255 into the system from the keyboard. The user holds down
the Alt key and types the decimal value of the characters desired
on the numeric keypad (keys 91-93, 96-98, and 101-103). The
Alt key is then released. If more than three digits are typed, a
modulo-256 result is created. These three digits are interpreted as
a character code and are sent through the keyboard routine to the
system or application program. AIt is handled internal to the
keyboard routine.
~
Break:
The combination of the Ctrl and Break keys results in the
keyboard routine signaling interrupt hex 1A. The extended
characters AL=hex 00, AH=hex 00 are also returned.
Pause:
The combination of the Ctrl and Num Lock keys causes
the keyboard interrupt routine to loop, waiting for any key except
Num Lock to be pressed. This provides a system- or
application-transparent method of temporarily suspending list,
print, etc. and then resuming the operation. The key used to
resume operation is thrown away. Pause is handled internal to the
keyboard routine.
Print Screen:
The combination of the Shift and PrtSc keys
results in an interrupt invoking the print screen routine. This
routine works in the alphanumeric or graphics mode, with
unrecognizable characters printing as blanks.
Caps Lock:
This key shifts keys 17-26, 31-39, and 46-52 to
uppercase. When Caps Lock is pressed again, it reverses the
action. Caps Lock is handled internal to the keyboard routine.
System BIOS 5-19