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

ALT - Temporarily shifts keys 2-13, 16-25, 30-38,44-50, and
59-68 to ALT state. Used with CTRL and DEL to cause
system reset function described in Section 1.3.
ALT has a special use to allow the user to enter any character
code (0-255) into the system from the keyboard. The user
holds down the ALT key and types the decimal value of
characters using the numeric keyboard (keys 71-73, 75-77,
79-82). The ALT key is then released. If more than three
digits are typed, a modulo 256 result is created. These three
keys are interpreted as a character code (000-255) and are
transmitted through the keyboard routine to the system or
application program. ALT is handled internal to keyboard
routine.
CAPS LOCK - Shifts keys 16-25, 30-38, 44-50 to upper case
A second depression of CAPS LOCK reverses the action.
Handled internal to keyboard routine.
NUM LOCK - Shifts keys 71-73, 75-77, 79-83 to numeric
state. A second depression of NUM LOCK reverses the
action. Handled internal to keyboard routine.
SCROLL LOCK - Interpreted by appropriate application
programs as indicating that the use of the cursor control
keys should cause windowing over the text rather than
cursor movement. A second depression of SCROLL LOCK
reverses the action. The keyboard routine simply records
the current shift state of SCROLL LOCK. It is up to the
system or application program to perform the function.
C. Shift Key Priorities and Combinations
If combinations of ALT, CTRL and SHIFT are pressed and
only one is valid, the precedence is as follows: Highest is
ALT, then CTRL, then SHIFT. The only valid combination
is ALT CTRL, which is used in system reset.
Special Handling
A. System Reset
The combination of ALT CTRL DEL (Key 83) will result in
the keyboard routine initiating the equivalent of a system
reset/reboot. Handled internal to keyboard routine.
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