Re: implementing software unsigned divide

[Stephen Leake <stephen.a.leake.1@gsfc.nasa.gov>]

Tucker Taft <stt@averstar.com> writes:

> Stephen Leake wrote:
> > 
> > I'm porting GNAT to a 16 bit microprocessor that does not provide a
> > hardware instruction for unsigned divide (it has multiply and signed
> > divide, both 16 and 32 bit). Can anyone provide a reference to a book
> > that discusses how to do this, and/or an algorithm that does it? I've
> > looked in Knuth, and did an initial web search, and haven't found
> > anything.
> 
> This is generally pretty straightforward using the
> signed hardware instructions, and some appropriate
> amount of fixups before and/or after the actual divide.
> I suggest you try some specific examples by hand where one or
> both of the values have the high bit on.  It is pretty
> easy to figure out what sort of fixup, if any, is
> necessary.

I thought of that, but the first example I came up with seemed
insoluble (lets assume 16 bit):

signed -1 / 2 => 0, but unsigned 16#FFFF# / 2 => 16#7FFF#

Maybe if I use the remainder, I can do something. I'll look at it
some more.

I'll also try the library, see if they have a "microprocessor design"
section or some such.

> I have done this myself in the past, but unfortunately I didn't
> write down the algorithms in a place that I can locate at the
> moment.
> 
> > 
> > --
> > -- Stephe
> 
> -- 
> -Tucker Taft   stt@avercom.net   http://www.averstar.com/~stt/
> Chief Technology Officer, AverCom Corporation (A Titan Company) 
> Burlington, MA  USA (AverCom was formerly the Commercial Division of AverStar:
> http://www.averstar.com/services/ebusiness_applications.html)

-- 
-- Stephe