A PC routine might want to check the validity of the ALETs that a calling program passed and also check the EAX-authority of the calling program. Making such checks is a good programming practice for PC routines that change the EAX value and space-switching PC routines.

Input to TESTART is the ALET that it received and the EAX of the calling program. To get the EAX, a program can issue the extract stacked state (ESTA) instruction to retrieve the EAX from the current linkage stack entry. The first information field in the linkage stack entry contains the EAX of the caller. (If the EAX is 0, the ALET is for a public entry.) See Extract stacked state (ESTA) instruction for a description of ESTA instruction, an example of its use, and the format of the information field. Figure 1 shows an example of PGM1 (in problem state) requesting service from PCRTN (in supervisor state).

Figure 1. Checking the Validity of an ALET

Having received the ALET from another program, PGM1 passes the ALET to PCRTN for PCRTN's use. Before it uses the ALET, PCRTN issues the TESTART macro to test its validity.

To get the EAX value of the caller, PCRTN issues the ESTA instruction. PCRTN determines which of the return codes is acceptable for its purposes. For example, it might accept only ALETs that index public entries on a DU-AL.