IBM XT 5160 Technical Reference page 136

Alt:
This key temporarily shifts keys 1 through 29, 31 through
43,46 through 55,75 through 89,95, 100, and 105 through 124
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
o
to 255. 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.
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.
Num Lock:
This key shifts keys 91 through 93,96 through 99,
and 101 through 104 to uppercase. When Num Lock is pressed
again, it reverses the action. Num Lock is handled internal to the
keyboard routine.
Shift Key Priorities and Combinations:
If combinations of the
Alt, Ctrl, and Shift keys are pressed and only one is valid, the
priority is as follows: the Alt key is first, the Ctrl key is second,
and the Shift key is third. The only valid ·combination is Alt and
Ctrl, which is used in the system-reset function.
lOl/102-Key Keyboard 4-41