Date: Sat, 8 Oct 1994 09:37:13 -0700 (PDT)
From: "Frederick W. Reimer" <fwreimer AT crl DOT com>
Subject: Re: 16/32 (was: djgpp and the 386SX)
To: Kimberley Burchett <OKRA AT max DOT tiac DOT net>
Cc: DJGPP Mailing List <djgpp AT sun DOT soe DOT clarkson DOT edu>

On Sat, 8 Oct 1994, Kimberley Burchett wrote:

> 
> On Fri, 7 Oct 1994, Frederick W. Reimer wrote:
> 
> > On Thu, 6 Oct 1994 terra AT diku DOT dk wrote:
> > 
> > > There is no execution time difference between 16-bit operations and
> > > 32-bit operations for i[34]86.
> > 
> > I'll have to assume that you also have the 386+ programmer's reference 
> > manual from Intel -- or at least have the instruction timings from some 
> > (supposedly) 32-bit compiler?  Thanks for setting the facts striaght!
> 
>   Well, I do have a reference (HelpPC to be exact) and it says that there 
> is a difference 'tween 16 and 32 bits when it comes to multiplication.  
> For 16 bit MULs, the timing is 13-26 cycles.  32 bit MULs are 13-42 
> cycles.  The actual time depends (I'm pretty sure) on how many bits are 
> set in the numbers you're multiplying.
>   So, the conclusion is that 32 bits CAN be slower, but only if you're 
> multiplying something that really needs 32bits to be represented.  The 
> thing is, if you tried to do the same multiplication using more than one 
> 16 bit MUL, it would take longer.  Depending on how you look at it, 32 
> bits is faster AND slower. :)
> 							Kim
> 

Well what we are talking about here is the way that compilers handle 
32-bit values for programs that require them.  Doesn't really matter what 
the C type is (int, short, or long), but that the 32-bit type, whatever 
it is, be implemented with a single entity in the compiled code.  If you 
don't need a 32-bit value, use a short (in DJGPP) or an int (in most DOS 
compilers).  The issue is that, when you NEED to use a 32-bit type, most 
DOS compilers break it into two parts and use 16-bit operations instead 
of true 32-bit compilers which use 32-bit operations on a single entity 
(whether it be memory or registers).  So, the conclusion is that 32-bit 
compilers ARE faster than 16-bit compilers, because 16-bit compilers 
would need to perform multiple operations while the 32-bit compiler would 
only need to perform one operation for 32-bit types.

Fred Reimer

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