=> R0 = handler number
R1 = new address, or 0 to read
R2 = value of R12 when code is called, or 0 to read
R3 = pointer to buffer if appropriate, or 0 to read
<= R0 preserved
R1 = previous address
R2 = previous R12
R3 = previous buffer pointer
This is a single routine which performs the actions of OS_Control, OS_SetEnv, OS_CallBack, OS_BreakCtrl and OS_UnusedSWI. In fact all of those routines use this call and are only kept for backwards compatibility, so you should use this call in preference.
Note, OS_ChangeEnvironment vectors through ChangeEnvironmentV. A routine linked into this vector can stop the change from happening by setting R1 to zero and passing the call on.
If you set up any handlers, you must provide Error, Exit and UpCall handlers so that you can restore all handlers on any kind of program exit (even X SWIs can generate serious errors).
See PRM 1, page 315.
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