Re: the end of 18 bits, The joy of FORTRAN

According to John Ames  <commodorejohn@gmail.com>:

I suspect that, in the computer market of the early '60s, they
ended up as the awkward middle child  ... 

 
Mr. Ames's suspicion is unfounded.  The 18 bit systems were neither
awkward nor unsuccessful.

>
I yield to those with firsthand knowledge - but I do wonder about their
eventual abandonment (per Wikipedia, the last -15 was produced in 1979,)
when both the -8 and -10 were supported well into the early '80s.

When the PDP-4 was designed in 1962, scientific mainframes were mostly
36 bit word addressed, and characters were six bits.  The 18 bit PDP-1
sold reasonbly well at $120K, and the PDP-4 was a simpler design that
was half the price.  DEC's later 18 bit machines were all based on the
PDP-4, each one faster and cheaper with more memory.  I believe they
were mostly used in labs and for process automation, with a lot of
realtime code.  (Yes, I know about MUMPS but it was and is a niche.)

The 16 bit PDP-11 was wildly sucessful, I think somewhat to DEC's
surprise. It could do the things that the 18 bit machines could do,
could address more memory, and was easier to program particularly if
you had 8 bit data. Since DEC made so many of them they were cheaper,
too.

The PDP-8 hung on because they were used in a embedded applications,
like the 680 TTY controler and a very sucessful newspaper typesetting
system.  The PDP-10 was beloved in academia particularly once DEC
turned Tenex into TOPS-20.
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