[HECnet] This is probably been asked already but....

[Phil Mendelsohn]

On 12-07-02 09:13 AM, Johnny Billquist wrote:

To comment a tiny bit of what I know on RSX. RSX-11M was a clean
reimplementation of RSX by Dave Cutler.

To split hairs, -11M was a redo of -11D, which Cutler brought with him from DuPont.

One of the goals of 11M was to get something that could run on a really
small PDP-11 without an MMU, which 11M can. (I seriously doubt that
could ever be done with -11D.)
-11M+, which came later, was basically reimplementing some of the stuff
in -11D, since -11M+ had as the target the large PDP-11 systems.
Specifically the 11/70, as well as the never introduced 11/74. Which is
why -11M+ also have a very capable online reconfiguration tool (that
turned out to be useful in general, but it was specifically written for
the 11/74).

So -11M+ requires even more hardware than -11D, but does things
differently than -11D.

One of my favourite Cutler stories was that Sales was trying to differentiate the products by telling customers that -11M (the cheaper OS) wouldn't support as much memory as -11D.   No doubt this was a bastardized version of saying that -11M didn't *require* an MMU.   In any case, D cost more than M, by a lot, but it wasn't readily apparent why.

Cutler was not pleased with this scenario for various reasons.   He was offended that someone would spend more money to waste resources when they could have a tighter system cheaper, IIRC, but he was also mostly just pissed because it was not true that M wouldn't handle as much memory as D if you had it.   So he sent his *own* letter to customers, telling them that the price difference was not justified in terms of expansion.   Sales loved that<!>, but -11M has done fairly well since.

Cheers,
Phil M