Texas Instruments TI-83 Plus Manual page 78

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The É key has a key code of 0 and the flag indicating that it was pressed is
–
also set.
The key code returned can be either one or two bytes. The ACC is checked to
–
see if a one or two byte key code is returned.
There are two values returned that signal a two byte key code:
There is a table for each of these keys that list the second byte key values
associated with them which can be found in the include file, TI83plus.inc.
If either of the above values are returned, the second byte of the key code is
located in the RAM location (keyExtend).
For example, the key code for DrawF are the two bytes kExtendEcho and
kDrawF. GetKey would return the ACC = kExtendEcho and (keyExtend) =
kDrawF.
– Lowercase Alpha keys
When the following flag is set, consecutive presses of the ƒ key will become
the mechanism for lowercase alpha key entry.
This flag should be reset when lowercase is not needed. It should also be reset
before exiting the application.
The lowercase alpha keys are two byte key codes with the first byte being
kExtendEcho2.
TI-83 Plus Developer Guide
onInterrupt, (IY + onFlags)
kExtendEcho and kExtendEcho2
lwrCaseActive, (IY + appLwrCaseFlag)
Chapter 2: TI-83 Plus Specific Information
Third Release May 28, 2002