[Info-vax] Y3K for PDP-11 Operating Systems

[vandys at vsta.org]

In alt.sys.pdp11 Johnny Billquist <bqt at softjar.se> wrote:
> I know that for some things, actually coding using more primitive 
> instructions on the (older) VAXen turned out to be faster than to use 
> the fancy instructions, but I would not have thought that was true for 
> things like MOVC or LOCC, but I might very well be wrong.

I'm pretty sure we ended up with a hand coded bcopy() back in the day.
It might've been that we could do optional initial odd, and then move
as words.  I think I heard that later Vaxen microcode would do that too.

>  (The big win, however, was 
> obviously the loads of actual code needed to do a context switch which 
> was reduced to just a few bytes on a VAX. :-) )

I'm pretty sure I remember that BSD on the Vax never ended up using
the high level task instructions.  I can remember trying to make it
line up, and ending up deciding it cost more than it saved.

No doubt VMS did, as otherwise, well, what was the point? :->

This all dribbles back to that old RISC versus CISC argument.  I think the
MIPS context switching I coded was faster than anything comparable on a x86
or Vax.  OTOH, MIPS was never able to beat the TLB miss performance of the
x86.  The ultimate (implied) instruction!

-- 
Andy Valencia
Home page: http://www.vsta.org/andy/
To contact me: http://www.vsta.org/contact/andy.html