IBM XT 5160 Technical Reference page 105

Alt
This key temporarily shifts keys 2-13, 16-25,30-38,44-50, and
59-68 to the Alt state. Also, the Alt key is used with the Ctrl and
Del keys to cause the system reset function described in "Special
Handling" on the following pages.
The Alt key has another use. This key allows the user to enter
any ASCII character code from 1 to 255 into the system from the
keyboard. The user holds down the Alt key and types the decimal
value of the characters desired using the numeric keypad (keys
71-73,75-77, and 79-82). 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
transmitted through the keyboard routine to the system or
application program. Alt is handled within the keyboard routine.
Caps Lock
This key shifts keys 16-25, 30-38, and 44-50 to uppercase.
Pressing the Caps Lock key a second time reverses the action.
Caps Lock is handled within the keyboard routine.
Scroll Lock
This key is interpreted by appropriate application programs as
indicating that use of the cursor-control keys should cause
windowing over the text rather than cursor movement. Pressing
the Scroll Lock key a second time reverses the action. The
keyboard routine simply records the current shift state of the
Scroll Lock key. It is the responsibility of the system or
application program to perform the function.
Shift Key Priorities and Combinations
If combinations of the Alt, Ctrl, and Shift keys are pressed and
only one is valid, the precedence 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.
4-10 83-Key Keyboard