Indirect calls to routines must pass through the routine's transition vector. A transition vector is a data structure in the called fragment's data section whose first element is the address of the routine to be called. The basic structure of a transition vector is shown in Figure 2-3 . A routine's transition vector is accessed through the base register, just like any other piece of data. As with other data, it is generally more efficient to place the transition vector in the direct data area.
Control can then pass from the transition vector to the called routine.
The transition vector can contain any number of elements in addition to the routine address. These other elements may be used by the called routine in any way useful. Inc. All rights reserved.