* Task handle of new task, or NUL
* All registers are preserved.
Notes: If a task handle is supplied in R0, this code will affect a task switch. It is normally called in this manner on return from SWI Wimp_Poll. If, however R) is NUL, then this will re-enable multithreaded operation if it has been turn off by SWI Thread_Off.
Typical action for a task whilst doing a WIMP Poll would be:
thread_off();
wimp_poll(blah, blah, blah);
if (event_got == Redraw) handle_the_redraw_event();
thread_on(my_task_handle); /* Do a context switch back */
thread_on(NULL); /* Turn threads back on */
The action of turning threads back on has been split into two parts because, on a multiprocessor system, the overheads in doing a task switch are great.
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