When using the ODS-1 file system (RSX-11S, RSX-11M, RSX-11M-PLUS), (almost) all information about files is contained in the service file INDEXF.SYS, which contains (in addition to service information) file headers. The file header contains the name and extension of the file, information about the owner and protection of the file, as well as which blocks of the volume are occupied by this file.
MAXIMUM is the maximum number of file headers that can be created on a volume after it has been initialized. In fact, this is the maximum size (plus a certain constant) of INDEXF.SYS, in which these headers are located. Important facts to keep in mind:
- 5 headers belong to service files (including INDEXF.SYS itself)
- usually a file header is a single block in INDEXF.SYS, but - if the file is large enough or heavily fragmented - more blocks may be needed - or in filesystem terms - extension headers. Therefore, in reality (if there are many extension headers), fewer files can fit on the volume than the value specified in MAXIMUM
- as far as I remember, the theoretical limit for the number of headers (including service headers and extensions headers) is 65535 (or so)
When initializing a volume, a program that performs initialization (for example, INI or BRU) can immediately occupy a certain number of volume blocks for headers - this is the HEADERS parameter. If all such preallocated headers are already occupied by files - when creating a new file the file processor needs to expand INDEXF.SYS, but not more than the size specified in MAXIMUM.
And then the trade off begins - the goal is to allocate enough blocks for INDEXF.SYS so that it does not need to be expanded for some time - this is a costly operation and leads to fragmentation of INDEXF.SYS plus the speed of working with INDEXF.SYS decreases. But at the same time leave more space for regular files.
In principle, after initializing the volume, you can increase the maximum number of files, but, if memory serves me, you can’t add extension headers for INDEXF.SYS, so it’s not at all a fact that without a full reinitialization it will be possible to get the theoretical limit on the number of files.