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The Filter Manager provides facilities for you to register filters to be used when a specified task calls Wimp_Poll, or when Wimp_Poll is about to return to that task. These are known - respectively - as pre-filters and post-filters:
Filters need not be applied to a specific task; you can also apply filters to all tasks.
Each filter is a routine that has well-defined entry and exit conditions; it is your responsibility to write the routine.
R1 = &87 (reason code)
All registers preserved
This service call is issued when the Filter Manager starts up. You may then register new filters using Filter_RegisterPreFilter and Filter_RegisterPostFilter.
R1 = &88 (reason code)
All registers preserved
This service call is issued as a broadcast to inform filters that they have been deregistered and that the Filter Manager is about to die.
R0 = pointer to filter name (null terminated)
R1 = pointer to filter routine
R2 = value to be passed in R12 when filter is called
R3 = task handle to which to apply filter (or 0 for all tasks)
All registers preserved
Interrupt status is undefined
Fast interrupts are enabled
Processor is in SVC mode
Not defined
This call registers a pre-filter routine (pointed to by R1), which will be called whenever the specified task calls Wimp_Poll.
The entry and exit conditions of the filter routine are:
R0 = event mask, as passed to Wimp_Poll
R1 = pointer to event block, as passed to Wimp_Poll
R2 = task handle of task that called Wimp_Poll
R12 = value of R2 on entry to this SWI (ie Filter_RegisterPreFilter)
It may clear bits in R0 to provide a new event mask
It must preserve all registers other than R0.
The routine should exit using the instruction:
MOVS pc, r14
None
Adds a new post-filter to the list of post-filters
R0 = pointer to filter name (null terminated)
R1 = pointer to filter routine
R2 = value to be passed in R12 when filter is called
R3 = task handle to which to apply filter (or 0 for all tasks)
R4 = event mask (1 bit masks the event out as for Wimp_Poll)
All registers preserved
Interrupt status is undefined
Fast interrupts are enabled
Processor is in SVC mode
Not defined
This call registers a post-filter routine (pointed to by R1), which will be called whenever the Wimp is about to return from Wimp_Poll to the specified task.
The entry and exit conditions of the filter routine are:
R0 = event reason code, as returned from Wimp Poll
R1 = pointer to event block, as returned from Wimp Poll
R2 = task handle of task that is being returned to
R12 = value of R2 on entry to this SWI (ie Filter_RegisterPostFilter)
Task is paged in, so you can access its memory
It must preserve all registers other than R0.
The routine should exit using the instruction:
Note: Under RISC OS 3, if a filter routine sets R0 to -1 to claim an event and prevent it being passed to its task, then that event is not passed on to any further post filters. From RISC OS 3.5 onwards, claiming an event does not prevent other post filters from being called, but does still prevent the event being passed to the task.
R0 = pointer to filter name (null terminated)
R1 = pointer to filter routine
R2 = value to be passed in R12 when filter is called
R3 = task handle to which to apply filter (or 0 for all tasks)
All registers preserved
Interrupt status is undefined
Fast interrupts are enabled
Processor is in SVC mode
Not defined
This call removes a pre-filter from the list of pre-filters. All values on entry must be the same as those used to originally register the filter (ie those that were passed to Filter_RegisterPreFilter).
None
Removes a post-filter from the list of post-filters
R0 = pointer to filter name (null terminated)
R1 = pointer to filter routine
R2 = value to be passed in R12 when filter is called
R3 = task handle to which to apply filter (or 0 for all tasks)
R4 = event mask (1 bit masks the event out as for Wimp_Poll)
All registers preserved
Interrupt status is undefined
Fast interrupts are enabled
Processor is in SVC mode
Not defined
This call removes a post-filter from the list of post-filters. All values on entry must be the same as those used to originally register the filter (ie those that were passed to Filter_RegisterPostFilter).
*Filters
None
*Filters lists all currently active pre- and post-Wimp_Poll filters.
*Filters Filters called on entry to Wimp_Poll: Filter Task ------ ---- Llama All tasks Filters called on exit from Wimp_Poll: Filter Task Mask ------ ---- ---- Llama All tasks 00000000
None