External procedures are called using the CALL instruction and are returned using the RET and RESTORE sequence.
- The first six parameters are passed in the output registers; that is, the first parameter is passed in register o0, the second in register o1 and the sixth one in register o5. Parameters beyond the sixth are passed on the stack.
- If OPTIONS(VARIABLE) is used, any missing arguments in the call to the procedure are replaced by null pointers.
- The external procedure is called by executing a CALL instruction.
