All ISAM was used at Florida Power for the customer master file before VSAM before DB2. Assembler subroutines were called by COBOL to access the file either sequentially or randomly and with or without update. The initial reason for using assembler was lack of variable length in COBOL initially, but it was a useful choice as it isolated the applicaiton from both software and hardware changes such as moving from data cell to pizza ovens etc. Interestingly the DB2 version implemented in 1995 used assembler compression/decompression to reduce network traffic by 50%
Anyway I remember studying the channel programs used by ISAM and although they were designed to maximize I/O speed, they were severely hampered by the lack of VSAM CA/CI indexing which resulted in exceedly long sequential chains of overflow records which could runs for hours if the file was not unloaded and reloaded to restore evenly distributed overflow track space. Don Higgins [email protected] ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to [email protected] with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
