Each LU has an INDEX, whose header block resides at RDA 1.
It is a Random structured file. No zeros are allowed in the header’s RDA list of blocks (except after the last RDA).
The initial file size (for LU’s other than zero) is determined by the system when the LU is first created (INSTALL AND CLEAR).
The system views INDEX as a continuous set of ‘records’ each of which is 10 words (8 dec) long, identifying a file on the LU.
The first 7 words of each ‘record’ allow for a 14-character (dec) filename, followed by the 8th word, an integer which is the RDA of the file header.
A small sample of INDEX might look like the following:
ADR: words 0-6 word 7 0: INDEX 1 10: ACCOUNTS 3 20: DMAP 14 30: USERFILE1 2057 40: LIBR 102 50: (not used) 60: etc.....
Records may or may not be in use. If word 0 of a record is zeroed out, the record is considered empty and the rest of the record is ignored. This can happen either because the record has never been written in, or because a file has been deleted.