IBM AT 5170 Technical Reference page 192

Ctrl: This key temporarily shifts keys 3, 7, 12, 15, 17 through 29,
31 through 39, 43, 46 through 52, 91 through 93, and 101
through 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 are described under "Special Handling" later
in this section.
AIt: This key temporarily shifts keys 1 through 13, 17 through
26,31 through 39, and 46 through 52 to the Alt state. The Alt
key is also used with the Ctrl and Del keys to cause a system
reset.
The Alt key also allows the user to enter any character code from
1 to 255.
Note:
Character codes 97-122 will display uppercase
with Caps Lock activated.
The user holds down the Alt key and types the decimal value of
the characters desired on the numeric keypad (keys 91 through
93,96 through 99, and 101 through 103). The Alt key
is
then
~
released.
If
the number is greater than 255, a modulo-256 value
is used. This value is interpreted as a character code and is sent
through the keyboard routine to the system or application
program. Alt is handled internal to the keyboard routine.
Caps
Lock:
This key shifts keys 17 through 26, 31 through 39,
and 46 through 52 to uppercase. When Caps Lock is pressed
again, it reverses the action. Caps Lock is handled internal to the
keyboard routine. When Caps Lock is pressed, it changes the
Caps Lock Mode indicator. If the indicator was on, it will go off;
and if it was off, it will go on.
Scroll Lock:
When interpreted by appropriate application
programs, this key indicates that the cursor-control keys will
cause windowing over the text rather than moving the cursor.
When the Scroll Lock key is pressed again, it reverses the action.
~
The keyboard routine simply records the current shift state of the
Scroll Lock key.
It
is the responsibility of the application
program to perform the function. When Scroll Lock is pressed, it
5-20 System BIOS