Path: utzoo!mnetor!uunet!husc6!think!ames!oliveb!pyramid!voder!cullsj!jayz
From: jayz@cullsj.UUCP (Jay Zorzy)
Newsgroups: comp.os.vms
Subject: Re: 2 C questions
Message-ID: <282@cullsj.UUCP>
Date: 13 Apr 88 00:42:15 GMT
References: <281@cullsj.UUCP>
Organization: Cullinet Software, San Jose, CA
Lines: 38

From article <281@cullsj.UUCP>, by jayz@cullsj.UUCP (Jay Zorzy):
> It depends on the size of the variables.  Dynamic variables are allocated
> from the stack, so if you've got huge arrays, VMS will obviously have more
> work to do to allocate them on the stack.  Another point to consider, is
> that static variables are allocated during image activation in R/W, copy-
> on-reference (CRF) sections.  Depending on the frequency these sections
> are accessed, you may have paging activity to consider.

I've taken a lot of heat on this one.  Yes, it's true that all that's 
required to expand the stack is a simple SUBL instruction to reset the stack
pointer to a lower address in P1 space.  Where you pay a penalty is when this
address is beyond any previously allocated P1 space; the next instruction that
attempts to access this address will incur an access violation.  The access
violation is then handled by an exception service routine which, if possible,
must allocate additional virtual memory in P1 space.  

So there is some dynamic memory allocation overhead involved in some cases, 
particularly in recursive routines that have large local data segments.  The
difference is the memory is allocated during execution instead of image 
activation.

By the way, if the aforementioned exception service routine fails to allocate
the needed memory (insufficient PGFLQUO/VIRTUALPAGECNT/etc), then the access
violation is passed on to user or system condition handlers.  So an ACCVIO
with a reason mask bit 0 set and virtual address in P1 space (i.e. 3FFFFFFF
< VA < 80000000) is really an insufficient memory condition.

Speaking of stacks, here's a common gotcha to watch out for:  If you're 
issuing any asynchronous $QIOs (no wait, I/O completion handled by AST
routine), make sure to declare your IOSB variables as static.  Otherwise,
once the calling routine returns, the stack is reset; an IOSB on the stack
may now point to never-never land.

Hope this information is useful...

Jay Zorzy
Cullinet Software
San Jose, CA