Figure 1 shows PGM1 that runs in AS1. The figure shows AS1's PASN-AL and PGM1's DU-AL. PGM1 shares the PASN-AL with other programs that execute in AS1. It does not share its DU-AL with any other programs. The PASN-AL contains entries to address/data spaces that program(s) placed there. PGM1 (either problem or supervisor state) has an entry for Space X to its DU-AL and an ALET for Space X. PGM1 received an ALET for Space Y from a program in supervisor state. Assuming PGM1 has authority to Space X and Space Y, it has addressability to Space X through its DU-AL and Space Y through its PASN-AL; it can access data in both Space X and Space Y. Therefore, with one MVC instruction, PGM1 can move data from a location in Space X to a location in Space Y.

Figure 1. Comparison of addressability through a PASN-AL and a DU-AL

What happens to a program's PASN-AL and DU-AL at an address space switching operation? When a program issues a PC instruction that causes the primary address space to change, the system does not dispatch a new TCB or SRB. The PC routine that gets control runs under the same TCB or SRB as the program that issued the PC. Therefore, the PC routine keeps the DU-AL that is associated with the original program. However, the PC routine has the PASN-AL of the new primary address space. This is a different PASN-AL than the one used by the original program (except for the entries for SCOPE=COMMON data spaces).

For example, consider PGM1 in Figure 1. It has addressability to Space X through TCB A's DU-AL and Space Y through its PASN-AL. Figure 2 shows PGM1 issuing a PC to PGM2 in another address space. The figure shows how addressability through PASN-ALs and DU-ALs changes over a space-switching PC instruction. After the PC instruction, the PC routine PGM2 still has addressability to Space X through TCB A's DU-AL. Because the primary address space has changed, PGM2, however, does not have addressability to Space Y or Space Z. AS2 has its own PASN-AL, which is available for programs that have AS2 as their primary address space. PGM1 and PGM2 can access Space W, a SCOPE=COMMON data space, using the same access list entry token (ALET).

Figure 2. PASN-ALs and DU-ALs at a space switch