Using save-area module linkage conventions

VTAM® traces the flow of the execution of three VTAM components, SSCP, PUS, and LUS, by saving the work areas of modules in these components. The addresses of the module work areas are stored in either of these control blocks:

Network configuration services parameter list (NCSPL)
Request/response unit processing element (RUPE)

In the RUPE, the work area address can be found at RUPEDAP. In the NCSPL the work area address can be found at NCSPLWKA. For the hex offsets of these fields, see z/OS Communications Server: SNA Data Areas Volume 1.

The NCSPL or RUPE work area contains the work and save-areas for each module invoked for the command that the NCSPL or RUPE represents. The module work and save-areas provide status information that pertains to both the processing of that command and any interruptions in the processing.

This status information includes a record of which modules were entered, which modules returned to their callers, and which modules returned with a return code. Each module save-area contains the 4th, 5th, 7th, and 8th characters of the module name and the register 15 value that includes a pointer to the last module called by this module. If this address is not in the dump, the module can be obtained by comparing the address to the addresses in the VTAM module list pointed to by ATCMDLST out of the ATCVT. See VTAM load module list.

The high-order byte of the register 15 save-area also indicates the status of the last module called. (In 31-bit mode the address fills register 15, causing the status to overlay the high-order byte of the address in the register 15 save-area.)

Byte value	Status indicated
FE	The called module has returned to this module without a return code.
FF	The called module has not returned to this module.
nn	The called module has returned to this module with a return code of nn.

Figure 1 is an example of what the NCSPL or RUPE work area might contain for modules invoked for a VTAM process using save-area module linkage conventions. Using this convention, the save-area contains a 4-byte module identifier, such as ACRT, at the location pointed to by register 13 for each entry in the chain.

Figure 1. Save-area module linkage conventions—subarea

Diagram of save-area module likage conventions--subarea.