[Simh] Transferring the licence file to the VAX emulator

[Dave L]

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
>
>
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