judgement on whether or not there is an overflow

Altera_Forum · ‎07-28-2013

I have 3 numbers x, y, z to be added together, each with the format of s1.14. Then the sum will be in the format of s3.14. How to make judgement on whether or not there is an overflow? If it is judged based on the first 3 MSB and they are ALL either zeros or ones, is there is no overflow? why? Does the overflow in the middle cares (e.g. I have more than 3 addition)?

Altera_Forum · ‎07-28-2013

the rule is :

For an N bit number

N + N = N+1 bits required.

So N + N + N requires N+2 bits. Which is what you have, hence it will never overflow.

Altera_Forum · ‎08-01-2013

Sum is just an intermediate variable for the calculation, and the final output has the same number of bits as input. Besides, there is also an output (a flag) indicating whether there is an overflow in the final result. Then, how to make a judgement on whether or not there is an overflow, and why?

Altera_Forum · ‎08-01-2013

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Sum is just an intermediate variable for the calculation, and the final output has the same number of bits as input. Besides, there is also an output (a flag) indicating whether there is an overflow in the final result. Then, how to make a judgement on whether or not there is an overflow, and why?

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in general if your internal bitwidth is say 20 bits and you want only 16 LSBs out then you check bit(18:15) if it is different from sign bit then you clip output to either +32767 or -32767 and raise the flag, otherwise you pass the 16 bits direct out.


if data_int(19 downto 15) = "00000" or data_int(19 downto 15) = "11111" then
    dout <= data_int(15 downto 0);
    clip <= '0';
elsif if data_int(19) = '0' then
     dout <= 32767;
     clip <= '1';
else
     dout <= -32767;
     clip <= '1';
end if;

Altera_Forum · ‎08-01-2013

If the final result doesn't have an overflow, does an overflow in the middle of the calculation matters?

Altera_Forum · ‎08-01-2013

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If the final result doesn't have an overflow, does an overflow in the middle of the calculation matters?

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Yes it does.

Normally you should not allow that by applying enough width for internal bit growth.

If your application permits you may clip internally instead in the same way as my example but adjusted for relevant widths