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From: franka@mmintl.UUCP (Frank Adams)
Newsgroups: comp.lang.c
Subject: Re: MSC and stack manipulation
Message-ID: <2460@mmintl.UUCP>
Date: Wed, 7-Oct-87 16:34:45 EDT
Article-I.D.: mmintl.2460
Posted: Wed Oct  7 16:34:45 1987
Date-Received: Sun, 11-Oct-87 13:26:01 EDT
References: <325@picuxa.UUCP>
Reply-To: franka@mmintl.UUCP (Frank Adams)
Organization: Multimate International, E. Hartford, CT.
Lines: 25
Keywords: MSC

In article <325@picuxa.UUCP> gp@picuxa.UUCP (Greg Pasquariello X1190) writes:
>Here is a question for anyone who cares to throw his stack in the ring.
>When MSC 4.0 calls a function, it subtracts the amount of space needed
>by local variables from the stack.  Why?  The 80x86 instructions subtract
>two bytes from SP when a push is done anyway.

Because the program, in general, may use that local variable any number of
times.  Thus, when a value is put in it, it is *stored*, not pushed.  Also,
there may be other locals, and the order in which they are used may be
arbitrary.

So the routine has to adjust SP to leave space for the local variables.

It would be a possible optimization, if most or all of the locals had
initialization expressions, to compute and push them for the initial
allocation.  However, this is not necessarily an optimization.  I don't have
access to 8086 timing specs at the moment, but on many machines, stores take
less time than pushes, so for some number of arguments, the subtract and
store methodology is a win.

Again; the space left by the subtraction is used later in the program.
-- 

Frank Adams                           ihnp4!philabs!pwa-b!mmintl!franka
Ashton-Tate          52 Oakland Ave North         E. Hartford, CT 06108