Compatibility with the Mainframe Environment

This chapter describes facilities provided by Mainframe Express that provide emulation of the following mainframe features:

15.1 Mainframe Pointers

To provide compatibility with mainframe-style pointers in COBOL programs, Mainframe Express:

15.1.1 AMODE Compiler Directive

Mainframe Express also provides and sets the DATA compiler directive that can control the placement of the Data Division of a program compiled using the AMODE compiler directive.

COBOL Dialect	Directive
COBOL 370	AMODE(31) DATA(31)
VS COBOL II (all releases)	AMODE(31) DATA(31)
OS/VS COBOL	AMODE(24)

Note: You cannot see the directives in the Build settings field on the Build Settings dialog box as they are set implicitly.

For more details of the AMODE and DATA directives, see the topics for these directives in the on-line help: Help Topics on the Mainframe Express Help menu, then on the Contents tab click Reference, Development Environment, Compiler Directives.

15.1.2 Accessing Data in a Canceled Program

On the mainframe, a COBOL program can continue to use the data defined in the Data Division of a subprogram even after that subprogram has been canceled. This facility has been emulated in Mainframe Express though we do not recommend it as good practice.

You can enable this facility by checking Enable access to canceled programs' Data Divisions on the COBOL page of the Execution page of the Project Settings dialog box.

Before the subprogram is canceled, you should set pointer variables in the calling program to the addresses of the data in the subprogram that you want to continue to use. Then, after the subprogram is canceled, you can continue to use that data until Mainframe Express releases the Working-Storage of the canceled subprogram. This can occur if:

Any attempt to use a pointer variable that still points to a released Working-Storage area will have unpredictable results.

15.2 Entry Point Mapping for COBOL and Assembler Programs

The Entry Point Mapping facility enables you to specify entry points in COBOL and Assembler programs so that the application can find them at run time.You need to do this for any entry points in programs which will not have been loaded prior to them being called.

To appreciate the functions that this facility performs, you need to understand how calls to entry points are normally resolved using the run-time system.

When a program makes a call to an entry point in which the entry point has not been resolved by statically linking the program, the run-time system looks for the entry point to resolve it dynamically.

When a program is loaded, the name of the program and all the entry points in the program are recorded in a table. When a call to an entry point is made, the run-time system searches the table for the entry point-name. If it does not find the entry point-name, it looks on disk for an executable file of the same name. If it does not find one it returns an error.

Consequently, if your program calls an entry point that is not the name of an executable file or is not an entry point in a previously loaded program, the run-time system cannot find the entry point and your program fails. This is the only case where you need to use the Entry Point Mapper.

This problem often occurs when you take code that has been downloaded from a mainframe to the PC for further development. Mainframe programs often have several entry points in a single program. On the mainframe, entry points are resolved by linking, so calls can be made to a program and the entry point located whether or not the program has been called.

Using Mainframe Express, if the program containing the entry point has not already been called, the call to that entry point fails.

To get round this problem, you need to create an entry-name map file containing an entry point table and make it available to the run-time system. There are two methods of creating an entry-name map file:

If you have an application with a mixture of COBOL and Assembler programs, you can create the table entries for the COBOL programs using the Entry Point Mapping facility and add the table entries for the Assembler programs using the text editor.

The entry point map file is called mfentmap.dat and by default it must be held in the project's loadlib folder. For example, if your project is called myproject and you use the default directory names, the full path is:

The Entry Point Mapping facility automatically creates the file in the correct folder.

To give you more flexibility when working with mfentmap.dat files, you can specify that Mainframe Express searches for them in different locations. The General subtab of theExecution tab of the Project Settings dialog box includes options that enable you to change where Mainframe Express looks for mfentmap.dat files as follows:

The Folder containing MFENTMAP.DAT field enables you to specify an alternative folder containing mfentmap.dat. The folder you specify can be on your local machine or on a network. Specifying a folder on a network is useful when there are a number of Mainframe Express users needing to use a common mfentmap.dat file.
Checking Use concatenated MFENTMAP.DAT search specifies that Mainframe Express looks for an mfentmap.dat file in each of the load libraries defined in a project's different workgrouping levels. If there are a number of mfentmap.dat files, the same entry point could be defined differently in multiple files; in this case, precedence is given to the definition in the mfentmap.dat file that was found first. To change the order in which Mainframe Express finds mfentmap.dat files, change the order in which the load library folders appear in the Workgroup View.

15.2.1 Creating the Entry Name Map using Entry Point Mapping

To create an entry name map using Entry Point Mapping, click Alternate Entry Points on the Build menu. You see the Entry Point Mapping dialog box. Use this dialog box to specify the programs for which you want entry points mapped.

Note: If you check the Use concatenated MFENTMAP.DAT search option on the Project Settings dialog box, you cannot specify a location in the Generated file location field of the Entry Point Mapping dialog box. Instead, this field shows the first folder in the load library search order.

15.2.2 Creating the Entry Name Map Using a Text Editor

If you create your map file with an editor you must manually enter all the text required. For each entry point you need to enter the following three lines:

Consider a run unit with a main program A and subprogram C. Within the run unit, A calls entry point B which is in program C. The table entry in the mfentmap.dat file is:

15.2.3 Making the Map File Available to the Run-time System

If you have created an entry name map for a project, the Entry Name Mapping facility is automatically switched on. If Mainframe Express cannot find an entry point when debugging or executing a program, it searches the map file mfentmap.dat for the name that has been called.

15.2.4 Creating an Alias

For Assembler programs you can use the ALIAS command to create an alias mfentmap.dat entry automatically during the static link process for a load module. For example, to create an alias name of pacct001, the Assembler (.lin) file command line is:

For COBOL programs you must use the manual method since it does not have automatic processing during linking. To do this you can use the Entry Point Mapper to emulate the alias function of an IBM mainframe linkage editor, which redirects the system to execute a program accessed via an alias name. To enter aliases manually, you must edit the file mfentmap.dat using a text editor. You cannot enter aliases into this file using the Entry Point Mapping dialog box.

At run time, Mainframe Express first looks for a program with the original name. If it finds one, it does not invoke the Entry Point Mapper.

Enter the alias on the [ENTRY-POINT] line and the actual program name on the [SUB-PROGRAM] line.

Consider a run unit with a main program named progroot. Within the run unit, progroot issues a call using the name pacct001. The actual name of the subprogram required is tacct001. The table entry in the mfentmap.dat file is:

15.3 Using SMS Data Classes

Mainframe Express includes support for Storage Management Subsystem (SMS) data classes. A data class is a template of data set attributes (such as record length and format) that you can specify is to be used when creating a new data set.

Using a graphical interface (accessable by clicking SMS on the Options menu), you can quickly and easily define these data classes, which you can then refer to using any of the following methods:

DATACLAS parameter of a JCL DD statement
DATACLASS parameter of a TSO ALLOCATE command
DATACLASS parameter of an IDCAMS DEFINE command
data class text unit (TU) of SVC 99 (DYNALLOC)

Data class definitions apply on a project-by-project basis, with definitions for a project stored in a file. You can make this definitions file available to multiple Mainframe Express users, ensuring consistency of data class definitions. Whenever the data class specification is missing, Mainframe Express uses a default data class called MFEDFTDC, if it exists. This data class must exist in a project's data class definitions file for it to be available for that project.

For full information on SMS, see the DFSMSdfp Storage Administration Reference available from the IBM BookManager Server Library.