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2.1.2 Areas of VSE and OS/390 Differences
In order to properly assess and size the magnitude of the migration project, it is
first necessary to understand some of the basic differences between the two
operating systems. Once these differences are understood a realistic or more
reasonable project outlook can be determined. The purpose of this section is to
put into perspective these differences.
Even though both VSE and OS/390 support the IBM S/390 architecture, there are
differences that must be considered at both the subsystem and application
program level. When migrating or converting application programs from VSE to
OS/390 it is important to identify these differences. The primary differences can
be categorized as follows:
1. Source Programs
2. Job Control Language (JCL)
3. Files
4. Operations
2.1.2.1 Source Programs
The significance of the differences when dealing with program source code can
vary by many factors. The primary determining factors involved in converting
source programs have to do with the interfaces which provide services to the
application programs. These application interfaces and corresponding protocols
for requesting supervisor services are different in VSE than in OS/390.
The factors involved in converting batch programs that interface directly to the
control program and programs that interface with application subsystems are
different. Consequently, the effort and the techniques used will vary.
Source Program Inventory
The first step in assessing the scope of any application program conversion is
determining the whereabouts of all of the program source code. This task must
not be overlooked and needs to be done early in the conversion project. You will
need to determine that all executable modules have associated source code and
that all source code has associated executable modules. Executable modules
missing source code, for example, will have to somehow be recreated or
alternate plans developed to provide the program function. Conversion tools are
available to assist in this task and are discussed later in this publication.
Customers who have completed or are in the process of Year 2000 compliance
are most likely aware of this issue.
The impact of source program conversion can be reduced by positioning the VSE
production system with source programs compatible with both VSE and OS/390.
For example, moving to the Language Environment for VSE will provide language
compiler compatibility (for COBOL , PL/I and C for VSE/ESA) between VSE and
OS/390.
Batch and Online Program Conversion
The conversion of batch applications must take into account differences in the
application interfaces provided by VSE and OS/390. The significance of the
changes required in the source programs depends a great deal on the source
program language and to some extent the I/O access methods used. This
14 VSE to OS/390 Migration Workbook