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Kempston Mouse for the ZX Spectrum
Kempston Electronics
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The X and Y counters are incremented or decremented with each mouse movement and
simply wrap around if they overflow.


X-AXIS

Input port 64479 = $FBDF = %xxxxx011xx0xxxxx
Return value = Movement counter (0 - 255)


Y-AXIS

Input port 65503 = $FFDF = %xxxxx111xx0xxxxx
Return value = Movement counter (0 - 255)


BUTTONS

Input port 64223 = $FADF = %xxxxxx10xx0xxxxx
Return value:
+---+---+---+---+---+---+---+---+
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
+---+---+---+---+---+---+---+---+
  |   |   |   |   |   |   |   |
  |   |   |   |   |   |   |   +---- 0=Right pressed
  |   |   |   |   |   |   +-------- 0=Left pressed
  +---+---+---+---+---+------------ X=Undefined