Extract stacked state (ESTA) instruction

A linkage stack entry includes the contents of the ARs and the GPRs, as well as other information. The EREG instruction copies the ARs and GPRs from the current entry; the ESTA instruction copies the rest of the information (that is, the non-register information) from the current entry. The non-register information is divided into four eight-byte information fields and is identified to the ESTA instruction by a code.

Figure 1 shows the code for each of the four information fields and the format of the fields. The format of the third field varies depending on whether a BAKR or a stacking PC instruction caused the entry.

Figure 1. Format of the information fields

The ESTA instruction copies one of the fields in the current entry into an even/odd pair of GPRs. It returns a condition code that tells whether the entry on the linkage stack was formed by the BAKR (CC=0) or stacking PC (CC=1) instruction.

In the following example, the load address instruction (LA) loads a code of "1" into register 9, where "1" identifies the information field that contains the PSW. The ESTA instruction then copies this field into general registers 4 and 5. The BZ instruction causes a branch if the stack entry was formed by a BAKR.

Figure 2. Example of an ESTA instruction

* Program entered through a stacking PC or BAKR
* Code of 1 identifies the PSW in the linkage stack entry
    .
    .
    LA   9,1         Load the code of 1 into general register 9
    ESTA 4,9         Load the PSW into general registers 4 and 5
    BZ   BAKRTYPE    If CC=0, then BAKR formed the stack entry
                     If CC=1, then stacking PC formed the entry

Another example of the ESTA instruction appears in Figure 1.