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Re: n00b confusion regarding the implementation vs. specification of *

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On 3/20/2017 8:53 AM, Andrew Haley wrote:
> rickman <gnuarm@gmail.com> wrote:
>> On 3/20/2017 4:53 AM, Andrew Haley wrote:
>>> rickman <gnuarm@gmail.com> wrote:
>>>> On 3/19/2017 2:58 PM, Andrew Haley wrote:
>>>>>
>>>>> In two's complement representation, for every pair of arguments,
>>>>> signed or unsigned, the resulting bit pattern is correct when
>>>>> interpreted as either signed or unsigned.  This follows because
>>>>> signed and unsigned addition don't need special handling, and
>>>>> multiplication is no more than repeated addition.
>>>>
>>>> Try it sometime.  A multiplication routine doing shift and add of the
>>>> multiplicand using the bits in the multiplier as enable flags does not
>>>> work for a negative multiplier.  If your statement were correct, it
>>>> should work regardless.  Instead the multiplier has to be negated along
>>>> with the result.
>>>
>>> Please provide an example of a pair of numbers which demonstrates
>>> this.
>>
>> Any non-trivial negative number for a multiplier.  Try it, you'll see.
>> I wracked my brain trying to figure out a way to adjust the result to
>> make it work.  After all, negative numbers just have the sign bit
>> different, right?  Good luck!
>>
>> Albert, indicated that multiply works the same for unsigned and signed
>> numbers as long as the unsigned numbers don't overflow.  However if you
>> do a signed multiply using an unsigned number with the msb set, that
>> screws it up.  This is true even if the result is double precision.  So
>> you can't do an unsigned multiply on signed numbers in the general case.
>>
>>   0101  (5)  multiplicand
>> x 1010  (-6) multiplier
>
> The result is -30, which does not fit in a 4-bit signed integer.

So?  A multiply operation in general produces as many bits in the result 
as the total number of bits in the operands.  You can always toss any 
bits you choose, but that is not inherent in the multiply in any way.


> The question is about the Forth word *, which by definition takes two
> cell-size numbers and multiplies them together, producing a product of
> cell size.  The question is not about any other definition of
> multiplication, especially not one which has a double-length product.
>
> Mixed-precision multiplication needs M* and UM*, obviously.

Ok, if you limit multiplication to results that fit in the same word as 
the operands signed and unsigned may be the same.

-- 

Rick C

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n00b confusion regarding the implementation vs. specification of * Michael Barry <barrym95838@yahoo.com> - 2017-03-19 10:41 -0700
  Re: n00b confusion regarding the implementation vs. specification of * Andrew Haley <andrew29@littlepinkcloud.invalid> - 2017-03-19 13:58 -0500
    Re: n00b confusion regarding the implementation vs. specification of * rickman <gnuarm@gmail.com> - 2017-03-19 19:06 -0400
      Re: n00b confusion regarding the implementation vs. specification of * Andrew Haley <andrew29@littlepinkcloud.invalid> - 2017-03-20 03:53 -0500
        Re: n00b confusion regarding the implementation vs. specification of * rickman <gnuarm@gmail.com> - 2017-03-20 06:56 -0400
          Re: n00b confusion regarding the implementation vs. specification of * Andrew Haley <andrew29@littlepinkcloud.invalid> - 2017-03-20 07:53 -0500
            Re: n00b confusion regarding the implementation vs. specification of * rickman <gnuarm@gmail.com> - 2017-03-20 18:15 -0400
              Re: n00b confusion regarding the implementation vs. specification of * Coos Haak <htrofhc@gmail.com> - 2017-03-20 23:30 +0100
              Re: n00b confusion regarding the implementation vs. specification of * julian.fondren@endurance.com - 2017-03-20 15:34 -0700
              Re: n00b confusion regarding the implementation vs. specification of * "Elizabeth D. Rather" <erather@forth.com> - 2017-03-20 14:40 -1000
                Re: n00b confusion regarding the implementation vs. specification of * hughaguilar96@gmail.com - 2017-03-20 19:41 -0700
                Re: n00b confusion regarding the implementation vs. specification of * anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-03-21 17:18 +0000
          Re: n00b confusion regarding the implementation vs. specification of * anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-03-20 15:07 +0000
  Re: n00b confusion regarding the implementation vs. specification of * hughaguilar96@gmail.com - 2017-03-19 12:32 -0700
    Re: n00b confusion regarding the implementation vs. specification of * hughaguilar96@gmail.com - 2017-03-19 13:54 -0700
  Re: n00b confusion regarding the implementation vs. specification of * albert@cherry.spenarnc.xs4all.nl (Albert van der Horst) - 2017-03-19 21:21 +0100
    Re: n00b confusion regarding the implementation vs. specification of * Michael Barry <barrym95838@yahoo.com> - 2017-03-19 16:41 -0700
      Re: n00b confusion regarding the implementation vs. specification of * anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-03-21 17:56 +0000
  Re: n00b confusion regarding the implementation vs. specification of * anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2017-03-20 08:57 +0000

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