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
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