Microsoft KB Archive/37899

Definition of IBM VSAM, Compared to Microsoft COBOL 3.0 ISAM

PSS ID Number: Q37899 Article last modified on 04-20-1993

3.00 | 3.00 MS-DOS | OS/2

Summary: Microsoft COBOL Version 3.0 (and earlier versions) does not support VSAM (Virtual Storage Access Method). VSAM is IBM’s mechanism for the support of indexed files on their mainframe systems, such as the IBM System/370 series computers. In the early 1970s, IBM replaced ISAM (Indexed Sequential Access Method, their original implementation for indexed files) with VSAM. There are several different implementations of ISAM and VSAM in the market today, and they are not limited to IBM computer systems.

More Information: VSAM has several valued features over ISAM. They are as follows: 1. One or more alternate keys (IBM’s ISAM only supports one primary key, and one alternate key, which is referenced through WORKING-STORAGE SECTION.) 2. DELETE clause supported 3. FILE STATUS value and “nominal key” for COBOL 4. ALTERNATE KEY field in the file description section 5. The following three access methods are available per alternate key file (based on expected usage for optimum performance): a. ACCESS MODE IS SEQUENTIAL sequential access b. ACCESS MODE IS RANDOM keyed access c. ACCESS MODE IS DYNAMIC sequential and keyed access Microsoft’s ISAM is a combination of the original ISAM and VSAM developed by IBM, and contains features from both of these file access methods. The ISAM in Microsoft COBOL Version 3.0 supports the following features: 1. From 1 to 80 alternate keys supported 2. DELETE clause supported 3. FILE STATUS value and “nominal key” for COBOL 4. ALTERNATE KEY field in the file description section 5. Three access methods are available to the ISAM file in general: a. ACCESS MODE IS SEQUENTIAL sequential access b. ACCESS MODE IS RANDOM keyed access c. ACCESS MODE IS DYNAMIC sequential and keyed access The Microsoft COBOL Version 3.0 ISAM file structure stores all the alternate keys and the primary key in one file. When the file is opened using one of the three access methods, then any key used to access the file will use that method. In contrast, IBM VSAM stores each alternate key in a separate file, and you can specify the access method structure of the file when building the VSAM file structure. This allows the programmer to setup the each alternate key file for optimum access performance, by predefining the most expected access method using a specific key.

Additional reference words: 3.00 Copyright Microsoft Corporation 1993.