Setting the EAX value

The EAX is an index that is similar to the cross memory authorization index (AX), and is obtained in the same manner, through the AXRES macro (see z/OS MVS Programming: Authorized Assembler Services Reference ALE-DYN and Establishing cross memory communication for more information about AXRES). Unlike an AX, which is associated with an address space, an EAX is associated with a PC routine and is available to the programs that call the PC routine.

Figure 1 shows the AX value for an address space and the EAX value of a program in supervisor state or PSW key 0 - 7. Assume that the ALESERV ADD macro included CHKEAX=YES, or the default, and that the entry for address space AS2 is a private entry, ACCESS=PRIVATE.

The AX value of AS1 indexes into the AT of AS2. The system checks this value on PT or SSAR instructions to find out if a caller in AS1 has the authority to (1) PT to AS2 or (2) set AS2 as its secondary address space. If the entry in AS2's authority table has the PT authority, PGM1 can PT to AS2; if SSAR authority is on, PGM1 can set AS2 as its secondary address space.
The EAX value of PCRTN also indexes into the AT of AS2. The system checks this value when PCRTN uses the ALESERV ADD macro with the CHKEAX=YES option to add an access list entry for the address space to either its DU-AL or its PASN-AL. The system also checks this value when the program tries to reference AS2 through the entry (the entry is a private entry). Because the entry in AS2's authority table indicates SSAR authority, the caller of PCRTN is considered to be EAX-authorized to AS2.

In Figure 1, the value of the AX and EAX is 4. The value 4 is an arbitrary value chosen for illustrative purposes. You obtain an AX or EAX value from the AXRES macro. The entry that the AX and EAX indexes into indicates SSAR authority is on, which means that PGM1 is EAX-authorized to the address space.

The example also shows the difference between cross memory data movement with a move to primary (MVCP) and a data movement performed through ARs and the MVC instruction. PGM1 uses the SSAR instruction to establish AS2 as the secondary address space, then it uses MVCP to move data from AS2 to AS1. PCRTN issues the SAC instruction to change the ASC mode to AR mode. Having loaded the addresses and ALETs into the AR/GPR correctly, PCRTN uses MVC to move data from AS2 to AS1.

Note:

If PCRTN had used CHKEAX=NO on ALESERV, the system would not have checked the EAX.
If PCRTN had used ACCESS=PUBLIC on ALESERV, the system would not have checked the EAX value when programs referenced that address space through that access list entry.
Consider the storage key and data access and integrity issues when you add entries for address spaces. Most problem state programs execute with a PSW key of 8, which allows them to use public access list entries to modify data in storage that has storage key 8.
The EAX value can be the same as the AX value.

In some cases, supervisor state or PSW key 0 - 7 programs in the target address space can change the EAX checking that the system does for programs accessing data in their address space. For example, in Figure 1, a program in AS2 could use the ATSET macro to change SSAR authority in the fifth entry in the authority table. Because the entry was added as CHKEAX=YES and ACCESS=PRIVATE, if the program turned SSAR authority off, PCRTN could no longer access the address space through that access list entry. If the entry had been added CHKEAX=NO and ACCESS=PUBLIC, programs in the target address space would be unable to prevent access through those access list entries.

Figure 1. Comparison of an AX and an EAX