ahh good old ISAM, yeah I wrote something similar on a commodore pet
system that I was tasked with writing a hospital lab data analysis suite
for in the early 80s. It had the ability to random access the FDD's so it
made sense to tag and allocate blocks rather than have to rewrite
sequential access files for even a minor field change in a record. Would
have run out of disk space to do that once you'd hit 50% on the data FDD
as well as taking far too long. You'd make a specific call to acquire a
random block and the OS would give you a block ID to use from free pool.
You could then direct access that block ID and once the record was written
update the index block you'd pre-allocated so you could go find it again
later.
Was an "interesting" experience doing that right at the start of my
programming career. Was written in basic on a 32KB machine and having to
still fit the actual application into the same memory ( no overlays etc).
Added fun was the graphic codes for the screen/form displays were
different to what the printer needed so any time someone wanted to print
the app would have to pull those codes in off FDD, overlaying the screen
ones and then revert once the print had completed. Not enough memory to
hold both sets of codes.
I later went on to messing about writing a pascal compiler for the Acorn
Atom (6502 CPU) I used to have, just for fun... after modding the mobo to
carry 32KB ram and upping clock speeds, oh the good old days, long gone
now its all surface mount and my eyes are not up to messing about at that
level now :-(
One odd thing about IBM is that some of the access method mechanisms
relied on hardware capabilities. For ISAM files, you'd write the file
data with key fields in each sector, and use the search for key match
feature in the disk drives (to find the matching sector so long as you
knew on which track to look). That seems to be pretty unusual, though
I've also seen it done by Electrologica in Holland in the mid 1960s.
paul
_______________________________________________
Simh mailing list
[email protected]
http://mailman.trailing-edge.com/mailman/listinfo/simh
--
_______________________________________________
Simh mailing list
[email protected]
http://mailman.trailing-edge.com/mailman/listinfo/simh
