Event Mask

Has the following bits:

Bits 7-15

Reserved. Set to 0.
Bit 6
Set, if Button 3 is down
Bit 5
Set, if motion with Button 3 down
Bit 4
Set, if Button 2 is down
Bit 3
Set, if motion with Button 2 down
Bit 2
Set, if Button 1 is down
Bit 1
Set, if motion with Button 1 down
Bit 0
Set, if all mouse motion, no buttons down
A set bit has a value of 1.