:Mark

Thank you for the pointer to the book.  I went out this AM and bought a=20
copy.  A little pricey, but there is a LOT
of stuff in there in additon to the divide info.  I'm wading thru it right=
=20
now (lots of choices).  I'll let the piclist
community know that I find out.

Regards
Joe Mickley

----- Original Message -----=20
From: "Mark Rages" <markrages@gmail.com>
To: "Microcontroller discussion list - Public." <piclist@mit.edu>
Sent: Wednesday, October 17, 2012 3:42 PM
Subject: Re: [PIC] Fast 32 bit by 32 bit divide in a dsPIC30F


> On Wed, Oct 17, 2012 at 12:56 PM, Joe Mickley <joemickley@comcast.net>=20
> wrote:
>> I am using a dsPIC30F3012 (writing in ASM, not C) on a project with very
>> fast loop execution (<20 uSec in some cases)
>> and need an effecient 32 bit by 32 bit unsigned divide algorithm.  The
>> dsPIC30F family has organic 16 by 16 divide
>> instruction (DIV.U and DIV.UD) and I would like to create a 32 by 32=20
>> using
>> that instruction set.
>>
>> The traditional 32 by 32 divide using a shift/subtract (restoring)
>> algorithms is slow (about 400 instruction cycles nominal). It
>> should be possible to break the 32 by 32 into a set of 16 by 16=20
>> operations,
>> then combine the partials back into a 32 by 32
>> result, but I lack the knowledge of how to do or drive this.  The=20
>> dsPIC30F
>> divide routine is fast (18 cycles, so
>> any use of itshould have a marked improvement)
>>
>> I need an unsigned divide, but could live with a signed divide as the
>> magnitude of numbers I am working with are less than
>> 2^30.  Remainder or lack of is not critical.  If I need it I could deriv=
e=20
>> a
>> remainder externally to the routine.
>
> This is discussed in Section 9-2 of Hacker's Delight, 2nd edition.  It
> is a page and a half of C code.
>
>>From the book: "The details are surprisingly complicated."
>
> You can read the code if go to the book's page at Amazon and "search
> inside this book" for "Multiword Division".  But buy the book if you
> can.
>
> --=20
> Regards,
> Mark
> markrages@gmail
> --=20
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