Lpc2000

All,

The division by multiplication technique is well known, but does not
generate a remainder--you need to compute the remainder by multiplying
and subtracting.  In essence, you're multiplying by the reciprocal of 10
expressed (for instance) as 2^32/10 or 2^16/10, but it *does* require
that you can multiply two n-bit numbers to produce a 2n-bit product and
then take the high n bits of that product--and you need to account for
the error term.  Some processors don't do this in hardware.

Alternatively, Karl, try using the types div_t, ldiv_t and the functions
div and ldiv that deliver both quotient and remainder for you.  Note
that the operation of div_t is consistent across all implementations of
C, whereas / and % are *not*.

-- Paul.

> -----Original Message-----
> From: Karl Olsen [mailto:kro@...]
> Sent: 30 November 2004 08:01
> To: lpc2000@yahoogroups.com
> Subject: [lpc2000] Re: Slightly OT -- Fast Divide By 10
> 
> 
> 
> 
> --- In lpc2000@yahoogroups.com, "James Dabbs" <jdabbs@t...> wrote:
> > 
> > I'm trying to squeeze a few more cycles out of one my LPC, and I'm
> > looking for an optimized way to divide by 10 and leave the quotient 
> and
> > remainder.  I.e., a faster way to do this:
> > 
> > #define DIVIDE_BY_10(D, Q, R) Q=D/10; R=D%10;  // D is unsigned long
> 
> 
> Check out "How to optimize for the Pentium family of microprocessors"
> http://www.agner.org/assem/pentopt.pdf
> 
> The section "Division (all processors)" has a smart algorithm for 
> converting division by a constant to a cheaper multiplication and 
> shift.
> 
> Karl Olsen
> 
> 
> 
> 
> 
> 
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