Subj : Re: puzzle little closer to machine.
To   : comp.programming
From : CBFalconer
Date : Mon Jul 04 2005 07:01 pm

Jack Klein wrote:
> unknown@this-world.com wrote in comp.programming:
> 
>> Can somebody solve the following puzzle ?
>>
>> Suppose I have a 32 bit machine with 32 bit 32 registers.
>> How to multiply two integers in such a machine with taking
>> care of all the overflows. Also MUL or such multiplication
>> operands are not available in such a machine.
> 
> I don't need luck, I know how to do it.  In fact, I've written 32
> x 32 multiply routines that produced correct 64 bit results on 8
> and 16 bit machines.
> 
> The basic binary multiplication algorithm requires only shifts and
> adds, and I assume your 32 bit processor provides both of those.

In addition, the 32*32 -> 64 multiplication can NEVER create an
overflow.  Any possible problems arise later, when trying to stuff
a 64 bit entity into a 32 bit pot.  If it doesn't fit you must act
on it.

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