[Info-vax] Example of random access by record number on an RMS fixed record size, relative organization file?

[gah4]

On Thursday, September 21, 2023 at 5:19:50 AM UTC-7, bill wrote:

(snip)

> While ISAM may be less efficient relative files can waste a 
> lot of space. Space is allocated for empty records and back 
> in the good ole days disk space was premium and very expensive.

Relative files are convenient if your numbering system is already
not sparse.

Otherwise, it is usually to make it a hash table, and hash the
index before using it to read/write a record.  

ISAM is convenient, in that it does some of the work for you,
that you could otherwise do yourself. 

As far as I know, ISAM first appeared in OS/360.  Among others,
it uses features of the S/360 disk controllers for finding records.
That minimized memory use at the time.  For one,
it puts absolute disk addresses in the index, not relative.
The data sets have the U (unmovable) attribute.
There are utility programs that know how to back them
up, and restore them to different disk addresses.

They also use self-modifying channel programs. 
Pretty much the equivalent of self-modifying code,
but at the I/O channel level.

There is special code in VM/CMS for processing the self-modifying
channel programs from ISAM.  But much of that was later fixed
by VSAM instead, which also has ISAM emulation.