[Info-vax] VMS defragmenters

[glen herrmannsfeldt]

Bob Koehler <koehler at eisner.nospam.encompasserve.org> wrote:
(snip)
 
>   I could never understand why IBM used absolute block addresses in
>   its ISAM, or why some MS-DOS DBMS vendors did the same.  VMS
>   keyed-indexed files don't.  C-ISAM and D-ISAM don't.

I don't know the details, but as I understand it, they used
self modifying channel programs.  There is a special emulation
in CMS needed to run them.

I presume it was needed to run faster on early small
S/360 hardware.  The problem with that, and others that
they did on early 360's, is that it doesn't scale with
increasing processor speed.
 
>   Having written some low level file access stuff with relative
>   addresses I never saw any advantage in using absolute addresses.

-- glen