Your z/OS® system might appear to be one large block of code that drives your CPU. Actually, z/OS is a complex system comprising many different smaller blocks of code. Each of those smaller blocks of code perform a specific function within the system.

Each system function is composed of one or more load modules. In a z/OS environment, a load module represents the basic unit of machine-readable, executable code. Load modules are created by combining one or more object modules and processing them with a link-edit utility. The link-editing of modules is a process that resolves external references and addresses. The functions on your system, therefore, are one or more object modules that were combined and link-edited.

To see where the object modules come from, llook at the example in Figure 1.

Figure 1. Creating load modules

Most of the time, object modules are sent to you as part of a product. In this example, the object module MOD1 was sent as part of the product. Other times, you might need to assemble source code sent to you by product packagers to create the object module. You can modify the source code and then assemble it to produce an object module. In the example, SRCMOD2 is source code that you assemble to create object module MOD2. When assembled, you link-edit object module MOD2 with object module MOD1 to form the load module LMOD1.

In addition to object modules and source code, most products distribute many additional parts, such as macros, help panels, dialog elements, and other z/OS library members. These modules, macros, and other types of data and code are the basic building blocks of your system. All of these building blocks are called elements.