Author: Dave Lyons
Year: 1989
... discusses an anomaly with the keyboard modifiers register at location $C025 which prevents it from always properly reflecting the state of the Control and Shift keys.
Apple II
Technical Notes
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Developer Technical Support
Apple IIGS
#58: Keyboard Modifiers Register Anomaly
Written by: Dave Lyons July 1989
This Technical Note discusses an anomaly with the keyboard modifiers register
at location $C025 which prevents it from always properly reflecting the state
of the Control and Shift keys.
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There are two cases where pressing the Control key turns on the Shift bit
instead of the Control bit in the keyboard modifiers register:
o An arrow key (or a Control key equivalent to an arrow key) is
being held down and is repeating
o The Space bar or Delete key is being held down and repeating with
the Fast Space/Delete option selected in the Control Panel
Since the Event Manager reads the modifiers byte, desktop applications may be
affected by this anomaly.
Further Reference
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o Apple IIGS Hardware Reference