BBC BASIC Programmers' Reference

by Richard Russell, August 2014

Normally, event interrupts (e.g. ON MOUSE, ON MOVE, ON SYS and ON TIME) take place as a result of user actions such as mouse clicks, menu selections, moving/re-sizing a window or (in the case of ON TIME) periodically. The events are initially added to a 32-event FIFO queue, and then automatically dispatched to the specified BASIC handler routine (if any).

For test purposes, or other special uses, it is possible to insert a 'pseudo event' into the queue (if there is enough space). You can do that using the following code:

      SYS @fn%(18), handler%, msg%, wparam%, lparam%
      @flags% OR= &20000000

Here handler% is the address of the event handler (see below), msg% is the value which will ultimately be received by the handler in the @msg% system variable, wparam% is the value which will be received in @wparam% and lparam% is the value which will be received in @lparam%.

The handler% parameter can be the address of a custom handler or can specify one of the standard ON… event handlers as follows:

      handler%=!388 : REM ON TIME
      handler%=!396 : REM ON MOVE
      handler%=!400 : REM ON SYS
      handler%=!404 : REM ON MOVE

(if no relevant ON event statement has yet been executed, or an ON event OFF has been executed, handler% will be set to zero).

If you want to determine whether the event was successfully added to the queue you can test the return value:

      SYS @fn%(18), handler%, msg%, wparam%, lparam% TO ok%
      IF ok% @flags% OR= &20000000

If ok% is non-zero the event was added, if it is zero the queue was already full.

Here is a simple example of the use of this technique:

      ON SYS PRINT @msg%,@wparam%,@lparam% : RETURN

      SYS @fn%(18), !400, 123, 456, 789 TO ok%
      IF ok% @flags% OR= &20000000

      REPEAT
        WAIT 0
      UNTIL FALSE