Chapter 16: MVS Emulation

This chapter describes how MSS emulates various facilities provided under MVS, OS/390 and z/OS on the mainframe.

MVS Control Blocks

MSS emulates those MVS control blocks that are accessible to applications running in problem program state. The information stored in these control blocks is limited to that which has value and meaning to applications running in problem program state.

Control blocks are accessed in MSS in the same way as they are on a mainframe, by referencing the appropriate field in the Communication Vector Table (CVT). The address of the CVT is stored at absolute address 16 in memory, as it is on the mainframe.

Low-memory Address Emulation in Assembler and COBOL

On the mainframe the prefixed save area (PSA) is mapped at absolute location 0. Many application programs access its contents. We provide software that allows your COBOL programs to access it without as if they were running on the mainframe.

Control Blocks Supported

Table 16-1 lists the MVS control blocks that are emulated by MSS and specifies any differences between the non-mainframe versions and the mainframe versions.

Table 16-1: MVS Control Blocks Emulated by MSS
Name	Description	Differences from mainframe
IHAACEE	Accessor Control Element	ACEEVRSN is set to 2
IHAASCB	Address Space Control Block
IHAASVT	Address Space Vector Table
IHAASXB	Address Space Extension Block
CVT	Communication Vector Table	CVTTOD is set to X'20', indicating that the Time Of Day clock is operational. CVTMVSE - the X'80' bit in CVTDCB is always on, indicating that you are running under MVS/XA or later. CVT4MPS - the X'04' bit in CVTDCB is on (indicating that you are running on System/360 Model 65 MP) only if the MVS control blocks are above the 16MB line (that is, all three-byte addresses are stored as four-byte addresses in the same location). CVTDATE is set with the century number (that is, it uses "0cyyddds" format, where "c" is century number - 0 for 20th, 1 for 21st, etc.)
CVT	CVT Prefix	The CVT prefix is set to all binary zeros.
IHACDE	Contents Directory Entry	One "null" CDE is allocated. So it appears to be using a program for which the load module has not been loaded.
IDADDSA	Dynamic Storage Area (FSA - First Save Area)	This is an 88-byte save area pointed to by TCBFSA. It is used as the TCB First Save Area.
IEFJFCBN	Job File Control Block
IEZJSCB	Job Step Control Block
IHAPSA	Prefixed Save Area	The X'01' bit in byte 7 (the first byte being byte 0) is on only if the MVS control blocks are above the 16MB line (that is, all three-byte addresses are stored as four-byte addresses in the same location).
IEESMCA	SMF Control Area	SMCASID is set to 'MSTR'.
IDADTCA	Task Communication Area	The PRV address is always zero. The storage overflow routine does not provide overflow segments; it merely abends with the abend code S80A. You must ensure that the ISA setting is adequate for your application.
IDADTCA	TCA Prefix
IKJTCB	Task Control Block	The TCB protect key is set to key 8.
IEFTIOT1	Task I/O Table	A maximum of 256 TIOT entries (representing the first 256 DDs in the job step) are supported.

Accessing MVS Control Blocks from COBOL Programs

MSS enables you to access MVS control blocks from a COBOL program. The following sample program shows how you can access the job, step, and program names for the currently running program.

 IDENTIFICATION DIVISION.
 PROGRAM-ID. JOBINFO.

 DATA DIVISION.
 WORKING-STORAGE SECTION.
 01 JOB-NAME             PIC X(8).
 01 PROGRAM-NAME         PIC X(8).
 01 STEP-NAME            PIC X(8).

 LINKAGE SECTION.
 01 PSA.
    05 FILLER     PIC X(540).
    05 PSATOLD    POINTER.

 01 TCB.
    05 FILLER     PIC X(12).
    05 TCBTIO     POINTER.
    05 FILLER     PIC X(164).
    05 TCBJSCBB   POINTER.

 01 TIOT.
    05 TIOCNJOB   PIC X(8).
    05 TIOCSTPN   PIC X(8).

 01 JSCB.
    05 FILLER     PIC X(360).
    05 JSCBPGMN   PIC X(8).

 PROCEDURE DIVISION.
*    Address PSA
     SET ADDRESS OF PSA TO NULL

*    Address TCB
     SET ADDRESS OF TCB TO PSATOLD

*    Address TIOT
     SET ADDRESS OF TIOT TO TCBTIO

     MOVE TIOCNJOB TO JOB-NAME
     MOVE TIOCSTPN TO STEP-NAME

*    Address JSCB
     SET ADDRESS OF JSCB TO TCBJSCBB

     MOVE JSCBPGMN TO PROGRAM-NAME
     DISPLAY 'JOB NAME     = ' JOB-NAME
     DISPLAY 'STEP NAME    = ' STEP-NAME
     DISPLAY 'PROGRAM NAME = ' PROGRAM-NAME
     GOBACK
     .

MVS External Writer

Under MVS you can run a program that dynamically outputs a JCL job stream in the form of a SYSOUT data set, and you can specify that an external writer is to process the job stream. You can use the IBM-supplied external writer, INTRDR, or you can supply your own.

You can request an external writer in any of the following ways:

In JCL. The name of the external writer is a parameter in the DD statement for the SYSOUT data set
In a TSO ALLOCATE command
By issuing the DYNALLOC (SVC 99) service

MSS emulates the IBM-supplied external writer, INTRDR. It does not support user-supplied external writers.

The MSS emulation of INTRDR differs in the following ways from IBM's INTRDR:

IBM's INTRDR allows a program to submit a job without closing the data set, by writing records that contain control statements such as /*EOF, /*DEL, /*PURGE, and /*SCAN. Each of these statements effectively indicates that all data up to the statement is one or more complete jobs and that the action indicated by the statement should be carried out, for example, /*EOF causes the job to be submitted and is therefore equivalent to closing the data set. However, with the MSS emulation of INTRDR, the program must write an entire jobstream to the SYSOUT data set and then close the data set; the job is then submitted for execution. The program may then reopen the SYSOUT data set, write another complete jobstream and then close the data set for job submission.
IBM's INTRDR supports multiple jobs in one jobstream, regardless of whether the submission is triggered by a control statement or by the closing of the data set. The MSS emulation of INTRDR does not support multiple jobs in one submission, since MSS itself cannot handle such jobs. Each submission must contain only one job.
With IBM's INTRDR, the class of the submitted job defaults to the class of the SYSOUT data set. With the MSS emulation of INTRDR, the class of the submitted job is the same as it would be if the job were submitted without a class being specified.
You cannot request an external writer by issung the DYNALLOC (SVC 99) service

DB2 Call Attach Facility

You use the DB2 Call Attach Facility (CAF) to access DB2 subsystems and the data they manage. You do this by calling DSNALI with the appropriate series of functions. The MSS emulation of this facility behaves exactly as on the mainframe, with the following exceptions:

CONNECT function.
- The subsystem parameter (parameter number 2) must be present but is otherwise ignored.
- The termination Event Control Block (ECB) parameter (parameter number 3) must be present but is otherwise ignored. This ECB is never posted.
- The starting ECB parameter (parameter number 4) must be present but is otherwise ignored. This ECB is never posted.
- The Release Information Block (RIB) pointer (parameter 5) must be present and is set to point to a constant value which contains items such as an eye-catcher "RIB" and the DB2 subsystem release number of 710 (for DB2 V7.1.0).
OPEN function.
- The subsystem parameter (parameter number 2) must be present but is otherwise ignored.
TRANSLATE function.
- No translation is performed. In the SQLCA, SQLCODE is always set to -924, and SQLSTATE is always set to 58006.