Why use U* over VMS

[Kevin Quick, Simpact Assoc., Inc.]

In article <12234 at medusa.cs.purdue.edu>, smb at cs.purdue.EDU (Scott M. Ballew) writes:
> In article <1809.272c3135 at dcs.simpact.com> kquick at dcs.simpact.com (Kevin Quick, Simpact Assoc., Inc.) writes:
>
>>4. The VMS synchronization is much more specific and explicit, and in
>>   some cases much better than Unix synchronization, but that is partially
>>   because VMS is specific to Digital machines, whereas Unix is forced
>>   to be much more general.
>
> Actually, since Unix was originally designed for DEC machines, this is
> not a valid statement.  The difference really lies in the philosophy
> underlying the systems' designs.  See Bach's book on the design of
> Unix for a discussion.
>
> Scott Ballew
> Cypress Network Operations Center
> Purdue University Department of Computer Sciences

I would agree with the position of Mr. Ballew's statement with regards to the
origins of the Unix OS, but later developments in computer hardware have
caused a divergence between Unix and VMS; Unix, while initially developed
in a DEC environment, attempts to maintain a portability focus as much as
possible, and therefore the synchronization mechanisms are limited to that
subset that exists most broadly throughout the many architectures that today
support the Unix environment.  VMS, on the other hand, has been free to use
and "feature" all of the hardware developments available in the DEC environment
because it is not required to be portable outside that environment.

VMS implements 32 discrete interrupt priority levels with OS tasks being
specifically performed at the various levels;  the recent addition of
multiple CPUs in the SMP environment overlay a ranked spinlock structure
onto this interrupt scheme to provide inter-cpu synchronization as well
as intra-cpu syncronization.

Unix provides 8 interrupt levels, accessed via standard spl routines which
may or may not provide that many discrete interrupt lockout levels as
determined by the underlying hardware.  There are various spinlock additions
to support SMP environments, although my limited experience with these
indicates that they are not as complex (well developed?) as the VMS
counterparts.

Anyhow, I digress in defense of my original statement, and perhaps this topic
is becoming more apropo to the comp.unix.internals newsgroup.

--
	-- Kevin Quick,  Simpact Associates, Inc.,  San Diego, CA.
	   Internet: simpact!kquick at crash.cts.com