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From: bjorn@alberta.UUCP (Bjorn R. Bjornsson)
Newsgroups: comp.arch
Subject: Re: Architecture upgrade
Message-ID: <166@pembina.alberta.UUCP>
Date: Mon, 22-Dec-86 23:06:14 EST
Article-I.D.: pembina.166
Posted: Mon Dec 22 23:06:14 1986
Date-Received: Tue, 23-Dec-86 22:36:16 EST
References: <112@bnr-vpa.UUCP> <9200002@rmi.UUCP> <193@drilex.UUCP> <653@instable.UUCP>
Organization: U. of Alberta, Edmonton, AB
Lines: 48
Summary: The brilliance of the VAX design team, was expediency!

In article <653@instable.UUCP>, amos@instable.UUCP (Amos Shapir) writes:
> In article <7426@utzoo.UUCP> henry@utzoo.UUCP (Henry Spencer) writes:
> >Remember when a 780's
> >official designation was a "VAX-11/780"?
> 
> Actually, DEC had made a brilliant (may be unintentional) upgrade,

It could have been brilliant, but was greatly marred by
flaws like the following:

	RTI and RTT ignore the mode bits in the PSW image
	on the stack, and set the mode to user without
	trapping.

If RTI / RTT had trapped mode changes, or simply been left
unimplemented, you could have written or ported real PDP-11
operating systems to run on the VAX.
As it turned out, the AME (pseudo RSX-11M on a VAX), was
useless except for trivial RSX applications, this because
they left out socalled PLAS directives.  What it did do
was allow DEC to push the VAX through the door without any
software.  My suspicion has always been that DEC included
compatibility mode in the early VAXen for this reason and
this reason only.  After the fashion of computer manufacturers,
you then proceed advertise it as a feature.  When you take
it away, you can again advertise that as a feature, this
time saying it's not taking up any resources, and users
don't have to pay for something they don't need or want.

Remember VMS versions 1 through 2.  Almost utilities and compilers
ran in compatibility mode.  Yeah, DCL translated COPY commands
to RSX PIP invocations, ditto for RENAME, DELETE, ad nauseum.
The Fortran compiler was a modestly hacked version of PDP-11
Fortran-IV-Plus, and on and on and on.  After version 2, more
and more things started showing up in native mode.  Until
finally DEC was in a position to release VAXen without a PDP-11
mode, which is basically when they turfed it.

It's easy to imagine that DEC would have been at least a year
later in introducing the VAX without compatibility mode.  Makes
you wonder what the world would look like today, if that had
been the case.

		Don't ascribe to brilliance, that which
		can be explained by simple expediency,

			Bjorn R. Bjornsson
			alberta!bjorn