With the Integrated Language Environment (ILE), a new program model, called the ILE program model, was created. When we first started learning about the new ILE program model, we heard about modules and service programs, activation groups and override scoping. We heard about subprocedures and binding. When it came to binding, we heard about bind-by-copy and bind-by-reference.

Binding is the term used for creating a program under OS/400. Back in the old RPG III days, program source code was compiled, and a program object was created. This single-step compiler produced a *PGM object from a source file member, so there was a one-to-one relationship between a program and its source member.

The ILE program model does not support a direct source-member-to-program compilation. ILE programs are created in a two-phase (or two-step) compile process. You no longer go directly from a source member to a *PGM object. With ILE, the compile process generates a *MODULE object directly from source. That is the first phase of the compiler. The second phase, called binding, uses *MODULE objects (along with service programs and binding directories) to create the *PGM object.

While this seems more complicated, it really is only slightly more complex than the traditional RPG III compiling process because IBM created a series of specialized commands (CRTxxxMOD) that mask the complexities of this two-phased compile process. These command provides us with the same level of simplicity as the traditional PDM option 14 and the CRTRPGPGM command.

To create an ILE program object, you need to use an ILE compiler. Today, on the AS/400 and iSeries, all the major compilers--except the RPG III compiler--have been ported so that they create an ILE module object. None of the ILE-targeted compilers themselves produce *PGM objects; rather, they produce *MODULE objects that are bound into a *PGM object in the second phase of the compile process.

As for RPG III, it is really the only language not ported to the new environment. Instead, as you already know, IBM created a new RPG IV language specification and a new ILE targeted compiler. So to take advantage of ILE stuff, you need to use an ILE targeted compiler, which means CL, COBOL, C, C++, or RPG IV.

Program Structure and the Compilers

As mentioned, ILE program objects are made up of *MODULE objects. When you use an ILE-targeted compiler, it produces a *MODULE object as output. You do the compile by using one of the CRTxxxMOD compiler commands and then binding the module(s) together using the CRTPGM command.

The CRTxxxMOD commands are wired into PDM option 15 for all the languages. The CRTPGM command is not associated with a PDM option, so it must be run from command entry, from a command menu line, or within CL. Of course, CODE/400 (WDSc) and CodeStudio allow you to select which command you want to use to compile your source members.

You can still work in an ILE language just like you did with the traditional version of the compilers, however. You don't need to first create the module and then create the program object. IBM has created a compatibility mode for ILE program objects so that they may be intermixed with traditional/legacy systems. To do this, IBM created an entirely new set of compiler commands that mimic the old CRTxxxPGM commands. These new commands are named CRTBNDxxx where xxx is the language identifier, such as CBL, C, CL, or RPG.

IBM created these CRTBNDxxx commands to support the situation in which you are moving to an ILE language, such as RPG IV, but still have a large number of legacy applications in production--in RPG III, for example. By using the CRTBNDRPG command, you can effectively replace an existing RPG III program with a new RPG IV program. To do this, you need to specify the DFTACTGRP(*YES) parameter. This causes the program object to be supported in the default activation group, which is where all your old RPG III and CLP programs run.

A side effect of the CRTBNDxxx commands is that they get around the two-phase compile process by performing both phases of the compile process. First, these commands create the *MODULE object. If that step is successful, they go on to call the bind phase and create the *PGM object. The *MODULE object is created in the QTEMP library so it isn't left lying around on your system for you to manage. The process that a source member goes through to become a *PGM object is illustrated in Figure 1.

Figure 1: CRTBNDxxx compiler stages

Programs Architecture

A program can be made up simply of one *MODULE and, hence, one source member. But programs may also be created from multiple *MODULE objects and, therefore, multiple source members. With the ILE, you no longer create a program from a source member; you create a *MODULE from a source member. Figure 2 illustrates the one-to-one relationship between a source member of an ILE language, such as RPG IV, and the *MODULE object.

Figure 2: The relationship between a source member and a *MODULE

To create the modules that appear in Figure 2, the following command would be used:

You could also use PDM option 15 to create these modules.

Once modules are created, they may be combined into a program object. Whether they are stored in one program or each in their own program is up to your system design. If the three modules illustrated in Figure 2 were combined into a single program object, it would appear similar to Figure 3.

Figure 3: Program object consisting of multiple modules

The program object in Figure 3 is named ORDENTRY. This program is made up of three *MODULE objects: ORDENTRY, SEARCH, and DATERTN. To create this program, the CRTPGM command was used as follows:

The extra item in the ORDENTRY program is called the "PEP." You'll see the PEP when you are running your program and look at the call stack using the WRKJOB command. The PEP is the piece of code that is called when the program itself is called. Its job is primarily to transfer control to the code in the entry model.

What is the entry module? The entry module is the module that is the main entry point for the program. By default, the entry module is the first module listed on the CRTPGM command. There is also an ENTMOD parameter, which defaults to ENTMOD(*FIRST), meaning it uses the first module in the list of modules as the entry module. In this example, the module named ORDENTRY is the entry module; it receives control when the program is evoked.

When the CRTPGM is run, the modules are physically copied into the program object itself. They also remain out on disk as *MODULE objects. Once your program is working, you may delete the *MODULE objects, as they are of no real use once a program is placed into production.

What happens when you need to do maintenance on one of the modules in a program? The same thing that happens today: You make the changes to the source member, recompile it, test it, and then place the new version into production. You can replace a *MODULE object in an existing program by using the UPDPGM command. Simply specify the program name and the new module name (which must already be part of the program itself) and press Enter. The new module replaces the old module in the program, and you're ready to go. I'll talk more about UPDPGM and its cousin, UPDSRVPGM, next time.

The Last Word

Since OS/400 associates the source file, library, and member name with the object that is created, under ILE, the "create source" information is in the *MODULE, not in the programs. So you cannot view the source that was used to create a program by looking at a program object. Even if that program contains only one *MODULE object, such as those created with the CRTBNDRPG command, you can't view the "create source." To determine what source was used to create an ILE program object, you need to call an OS/400 API and list the module descriptions. A utility to do that very thing will be the subject of the next issue of Midrange Developer.

Bob Cozzi is a programmer/consultant, writer/author, and software developer. His popular RPG xTools add-on subprocedure library for RPG IV is fast becoming a standard with RPG developers. His book The Modern RPG Language has been the most widely used RPG programming book for more than a decade. He, along with others, speaks at and produces the highly popular RPG World conference for RPG programmers.

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