[Info-vax] VMS internals design, was: Re: BASIC and AST routines

[Arne Vajhøj]

On 11/26/2021 2:41 PM, Simon Clubley wrote:
> On 2021-11-26, Félim Doyle <felim.doyle at gmail.com> wrote:
>> On Thursday, 25 November 2021 at 15:48:37 UTC, Dave Froble wrote:
>>> VMS was designed and implemented for VAX, not generic computers.
>> 
>> As I remember it, VAX/VMS was designed by DEC to be its best ever
>> OS then the VAX hardware was designed and built to run it not the
>> other way around. There were probably some mistakes made,
>> unforeseen implementation issues and some miscommunications during
>> parallel development of the hardware and software but the
>> facilities that this combination provided, especially in comparison
>> to the price range of other systems at the time, was
>> revolutionary.
> 
> One of the biggest mistakes made is that DEC went to the trouble of 
> implementing a 4-mode architecture and then completely blew how it
> was used.
> 
> That 4-mode architecture could have provided some really truly
> radical internal security separation within VMS, but once you are in
> any of the 3 inner modes, you can get to any of the other inner modes
> so all those extra modes were wasted from a security isolation point
> of view.

When that design was done then it was a good protection against
accidental S and E trashing something.

20-40 years later it was realized that it could have been
good if it also protected against malicious code that
tried to switch to an inner mode.

But what so?

It would also have been great if they had foreseen:
* future HW changes and had done a HAL
* the need for later switching from 32 to 64 bit
* etc.etc.

But looking back that way is a rather futile exercise.

There is an old quote:

"A man who never makes a mistake will never make anything."

Nobody can foresee the future and mistakes are made.

Looking back and figuring out what decisions should have been
made is pointless (unless there is lesson learned for
decisions to be made now).

Arne