On Fri, 5 Nov 2010 09:03:35 -0400, Lizette Koehler wrote: >what in the PDS structure reduced the pds member name to a maximum >of 8 chars to start with.
A PDS directory has an 8 byte key and the member name in the directory is found using the Search Key High or Equal channel command. Each block is 256 bytes the records are of variable size. The key for the block is the same as the key for the last record in the block, with the key of the last used block being set to all ones (X'FF...FF'). All of the records in the PDS directory are maintained in sequence. I don't know why eight bytes was selected for the length of the key, but can only guess that it seemed a good compromise considering the high cost of DASD at the time and the amount of processing needed when a member had to be inserted or deleted. When a member was deleted, removing that member from the directory block, the first record on the next block would be examined to see if it would fit in the empty space at the end of the block. If it did, it would be moved and the process would be repeated on the next block, keeping the directory free of usable gas. When a member is inserted, the reverse process occurred when the last record in the block was pushed off the end. BTW, this same structure was used for CVOL catalogs, and that is the origin of eight character nodes for cataloged data set names. The same routines were used to perform the manipulation of the entries the blocks for PDS directories and catalogs. A major difference was that in a CVOL there were different some entries were "Index" entries, which pointed to the track that contained the next level of the catalog. Thus, for example, the high level index would contain an entry for "SYS1" that would point to the track where the SYS1 index could be found. Then, within that part of the the catalog, there was a record for "LINKLIB", containing the unit and volume information for SYS1.LINKLIB. Of course, there could be index entries in that level of the catalog as well. -- Tom Marchant ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@bama.ua.edu with the message: GET IBM-MAIN INFO Search the archives at http://bama.ua.edu/archives/ibm-main.html
