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

[Simon Clubley]

On 2021-11-26, abrsvc <dansabrservices at yahoo.com> wrote:
> I find these kind of comments somewhat offensive since it is easy to criticize the decisions of people made 40 years ago using the context of knowledge today.  VMS was designed as a cooperative pairing of both hardware and software.  The use of R0 and R1 was for consistency across calls and had nothing to do with MACRO32 at all.  Bliss used the same register conventions.  If the VMS and VAX engineers knew in the late 70's what was known now, I suspect things would have been done differently.
>

Hello Dan,

The problem is not the preserving of R0/R1/PC/PS{L}, but the way in
which it was done. This information should be private to the AST
dispatcher that calls the AST routine. It should never be visible
to the called AST routine itself because that is an outright violation
of good modular design and that's as true back when VMS was designed
as it is now.

In case you are familiar with bare metal interrupt programming, you can
compare the calling of an AST routine with the way that an interrupt
handler is called when working with bare metal code or when implementing
an OS itself.

On more advanced MCUs, you can have an assembly language interrupt
dispatcher that calls the actual C language interrupt handler and you
can end up having to save more interrupt state than just the normal
registers in your assembly language interrupt dispatcher.

However, that information is always private to the interrupt dispatcher,
and is _never_ exposed to the interrupt handler itself and this is so
universally true, that I didn't even realise what the AST registers were
being used for until it was pointed out to me.

For example, in one bare metal assembly language interrupt dispatcher
I wrote a while back for an ARM processor and which I was looking at
recently, the dispatcher has to save what is called the priority limiter
register before programming a new value (this allows nested interrupts
to occur).

This is saved onto the stack by the dispatcher before calling the
C language interrupt handler and is restored by the interrupt dispatcher
upon return from the interrupt handler. This information is private
to the dispatcher, it is not visible to the interrupt handler, and
I would never design a system where it was, because that is an utter
violation of modular and good programming practice.

That priority limiter register can be compared to one of those private
AST registers and that's why I consider it so wrong that those private
registers are there in the AST call frame and hence visible to the
called routine as it's an utter violation of good modular design.

Simon.

-- 
Simon Clubley, clubley at remove_me.eisner.decus.org-Earth.UFP
Walking destinations on a map are further away than they appear.