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From: jones@pyrite.cs.uiowa.edu (Douglas W. Jones,201H MLH,3193350740,3193382879)
Newsgroups: alt.folklore.computers
Subject: Re: Y VAX? [was : TECO on a DEC-System 10]
Message-ID: <457@ns-mx.uiowa.edu>
Date: 16 Jan 90 22:47:45 GMT
References: <153.UUL1.3#5131@mvac23.UUCP>
Sender: news@ns-mx.uiowa.edu
Lines: 59

From article <153.UUL1.3#5131@mvac23.UUCP>,
by thomas@mvac23.UUCP (Thomas Lapp):
>
> Why did they name it VAX?  Well, as my .sig says it: VAX stands for
> Virtual Address eXtension.  I think it has to do with the fact that 
> it is an address extension to the PDP series...
>
It's not the PDP series, it's the PDP-11 series.  The PDP-11 was a 16
bit machine, the PDP-8 was an unrelated 8 bit machine, the PDP-10 was
an unrelated 36 bit machine, and the PDP-15 was an unrelated 18 bit
machine.  All were made by DEC, many used compatible hardware at some
level, but there was not one PDP series in any useful sense.

That aside, the PDP-11 went through many models.  The high-end line of
PDP-11's went as follows (in chronological order with parenthetic remarks
about siblings that play no part in this story):

    PDP-11/20  -- the ancestral machine (OEM'd as the 11/15?)
                    with no floating point, and no memory mapping.
                         (the 11/15 was the OEM version of the 11/20?)
    PDP-11/45  -- faster, with segmented memory address mapping
                    and a floating point unit that overlapped
                    floating point computation with scalar
                    computation.
                         (the 11/40 was the OEM version of the 11/45?)
                         (the 11/35 and 11/30 were introduced later?)
    PDP-11/70  -- as I understand it, this was basically a PDP-11/45
                    CPU with an expanded segmented memory mapping
                    architecture and something other than the UNIBUS
                    to connect the CPU and main memory.
                         (the 11/05 was at about the same time?)

Now for the folklore:

    PDP-11/78  -- a paper machine, intended to outperform the 11/70,
                    with improved support for 32 bit operands.

    PDP-11/78 VAX -- The 11/78 with an improved memory mapping system
                    allowing real demand paged virtual memory instead
                    of the interesting but ultimately crippling idea of
                    limiting each program to only a few segments of at
                    most 64K bytes each (Intel, take note).

    VAX-11/780  -- a new name for the PDP-11/78 project, coined when it
                    was realized that so many changes had been made to
                    the original PDP-11 architecture that the pretense
                    that the machine was upwards compatible was no longer
                    credible.

This is the story I heard in the 1970's, just after DEC lost its bid to
sell my group at Illinois an 11/45.  We got a MODCOMP IV, much more of a
real 32 bit machine than any of the PDP-11's, and as far as I know, this
story predates the delivery of the first VAX.

Would someone from DEC please post the definitive family tree of the PDP-11
family and hang the VAX from it correctly?

				Doug Jones
				jones@herky.cs.uiowa.edu