A functional subsystem (FSS) is an extension of JES2 that runs in an address space separate from the JES2 address space. An FSS provides support for a function peripheral to JES2 processing, such as a peripheral device or other component. A functional subsystem application (FSA) executes within the functional subsystem address space and is defined to provide application-specific support to peripheral functions. An FSA allows devices to operate outside of direct JES2 control. The support contained in the FSS/FSA, then, is specific to the task. There is no limit to the number of FSSs that you can define, nor is there a limit to the number of FSAs that you can define to run under an individual FSS.

A functional subsystem can be defined to relieve JES2 of device-specific support associated with page-mode data. For example, because page-mode printers have processing requirements not required by line-mode printers (such as translating complex data attributes), the Print Services Facility™ is an FSS/FSA defined to drive a 3800-3 or 3820 printer when operating in full-function mode.

Because an FSS is common to all FSAs connected to it, the FSS provides a common set of services to those applications. For example, HASPFSSM supports a number of functions to include acquiring data sets (GETDS) and releasing data sets (RELDS). These functions are invoked from the FSA and are used to communicate with JES2 for data set printing.

Functional subsystems communicate with both JES2 and the FSA that resides within the functional subsystem address space. JES2 communicates with the functional subsystem through the functional subsystem interface (FSI). JES2 supports AMODE(31) callers and data areas across the FSI. JES2 data areas reside in virtual storage, above 16 megabytes. The FSI is not a single definable piece of code; rather an FSI encompasses those individual pieces of code (resident within JES2 and the FSA) that provide the communications between JES2 and the FSA. Figure 1 depicts the relationship between JES2 and the functional subsystem components.