Shift States - IBM 5150 Technical Reference

Shift States
Most shift states are handled within the keyboard routine,
transparent to the system or application program. In any case,
the current set of active shift states is available by calling an entry
~
point in the ROM keyboard routine. The key numbers are shown
on the keyboard diagram in Section 4. The following keys result
in altered shift states:
Shift
This key temporarily shifts keys 2-13,15-27,30-41,43-53,55,
59-68 to uppercase (base case if in Caps Lock state). Also, the
Shift -key temporarily reverses the Num Lock or non-Num-Lock
state of keys 71-73, 75, 77, and 79-83.
Ctrl
This key temporarily shifts keys 3, 7, 12, 14, 16-28,30-38,
~
43-50,55,59-71,73,75,77,79, and 81 to the Ctrl state. Also,
the Ctrl key is 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 in
"Special Handling" on the following pages.
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 0 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
System BIOS 5-19