OS_Pointer 2 (changes)
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OS SWI Calls
» OS_Pointer
» OS_Pointer 2
OS_Pointer 2
(SWI &64)
| Entry | |
|---|---|
| R0 | 2 |
| Exit | |
|---|---|
| R0 | Signed X position of alternate device (positive = right) |
| R1 | Signed Y position of alternate device (positive = “up”/away from user) |
Use
This call returns the position of the “alternate positioning device”, i.e. scroll wheel.
Notes
The returned values are an accumulation of all the delta position updates received from the pointer device driver. However if the values overflow their signed 32bit range then they will be reset to zero. Software which attempts to compute wheel movement deltas by subtracting a new accumulator value from an old accumulator value must be careful to take this behaviour into account, otherwise it will mistakenly see a very large jump in wheel position.
Many versions of RISC OS contain a bug which will cause OS_Pointer to malfunction or crash if called with an unsupported reason code. Therefore, care must be taken when attempting to make use of reason codes 2 and above.
In addition to polling this SWI, software can use Event 21,4 to detect when the alternate positioning device moves.
See also
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