The address space referenced by an ALET of zero changes as the primary address space changes. When a space-switching PC instruction, defined with SASN=OLD, makes a different address space the primary address space, an ALET of zero references the new primary address space. If the PC routine was defined with SASN=NEW, then an ALET of zero and an ALET of one both reference the same address space after a space switch.

The change in the meaning of an ALET of zero and one is important when your program issues a space-switching PC instruction. If you pass an ALET of zero to the routine in the target address space, the zero now refers to that address space. If the parameter is in the address space that the program switched from and the PC was defined SASN=OLD (or used the default), your program must change the value of the ALET from zero to one. An example of this change is in Example of using TESTART.

When you use nested PC routines (one PC routine invokes another PC routine, and that PC routine invokes yet another PC routine, and so on), using the special ALET values is not sufficient to maintain addressability to any address space other than the current primary and secondary address spaces. If you use nested PC routines, use the ALESERV macro to add entries to the DU-AL and pass ALET-qualified addresses. You can use the ALESERV macro with the ADDPASN parameter to add the current primary address space to the DU-AL.

After a program issues a SSAR instruction, an ALET of one references the new secondary address space.

The meaning of the ALET with the value of two (for the home address space) does not change at a space switch.