Subj : Re: Math error ??? doen't make sense!
To   : borland.public.cpp.borlandcpp
From : waking
Date : Sun Jul 04 2004 05:48 am

On Sat, 3 Jul 2004 23:12:17 -0400, "Stan DeGroff" <sdegroff@bright.net> wrote:

>My experience in assembly and/or PLC programming
>would have taken two 16 bit integers and expected a 32 bit result without
>having to type cast. I would only need to provide a 32 bit location/register
>for the result.  No mention of any intermediate storage considerations being
>required.

Some of the implementation details of the C/C++ languages can be
counter-intuitive to the uninitiated. (I programmed in 7 or 8 assembly
languages before moving to C, and can appreciate your perspective.)
Pay particular attention to the order of precedence for operators, and
the rules for evaluating expressions. With the assignment operator (=)
the expression to the right of the operator is evaluated independently
of the expression to the left. Thus, in the statement z=x*y; the x*y is
evaluated just as if it appeared all by itself: x*y;

One caveat: be careful when examining asm or obj code generated
by a compiler. Don't be too hasty to generalize based on what may
actually be implementation-specific details. It's often better to work
from the top down - using a good text or the official language spec -
rather than starting at the output of the compiler and trying to work
back to an inferred rule. Moving from the bottom up doesn't help
you understand *why* the compiler is handling something the way
that it is. It may be constrained by the requirements of the higher level
language.

That being said, if you remain curious, you can have the compiler
generate asm output from your C/C++ source code and browse to your 
heart's content. For example, here is the 16-bit asm code generated
by Turbo C++ 3.0. (BC++ 4.5x/5.0x would be similar if not identical.)

;        z=x*y;
;
     mov     ax,word ptr [bp-2]
     imul    word ptr [bp-4]
     cwd
     mov     word ptr [bp-6],dx
     mov     word ptr [bp-8],ax

;            z=x*(long)y;
;
     mov     ax,word ptr [bp-2]
     cwd
     push    ax
     mov     ax,word ptr [bp-4]
     push    dx
     cwd
     pop     cx
     pop     bx
     call    near ptr N_LXMUL@
     mov     word ptr [bp-6],dx
     mov     word ptr [bp-8],ax


--
Wayne A. King 
(waking@idirect.com, Wayne_A_King@compuserve.com)

.