gtps4m0a | System Generation |
Certain design activities must be completed before beginning the system generation procedures described in this publication. These activities include:
In standard IBM operating systems the substitution of real values for symbolic names (such as real addresses for symbolic labels) is postponed as long as possible to maintain flexibility for the programmers and the operators. An example is the process of locating a data set for use by an application program. Under MVS control, this data set may be defined at the time the application program is compiled (in the DCB), at JCL time (in the job control language), or during execution of the program (from the data set label during the open). While postponing substitution does increase flexibility, there is usually a penalty in terms of system performance.
In the TPF system, much of the substitution is done during system generation. By substituting during system generation, the system designer (and not the application programmer or the operations supervisor) makes the decisions that affect system performance. The design of TPF, with its heavy emphasis on performance, dictates that system resources should not be used for substitution that can be done by a thorough job of system design and planning.
Design decisions such as data set placement, database definition, and access method requirements are made during the system design phase. Front loading of the system design by TPF helps ensure that the system architects actually design the system and that errors during operation or application development will not affect the performance of the system. The entire TPF file structure must be designed before generating a TPF system. Data set placement, record sizes, organization, and file types must be decided during system design. This information is used during system generation to build internal tables that provide direct access to all programs and data records.
A TPF designer should allow for as much future growth as is feasible by specifying more at generation time than is required. This will compensate for the emphasis TPF places on system generation and allow additions to be made to the system as needed. Even so, changes in the file layout, communications network, or other areas may require the rerunning of steps in the generation process. System generation is usually not done just once in a TPF system. Figure 2 presents an overview of the design process.