[SOLVED] How to find odd or even without using any compare operator ?

[mr_vasanth]

How to find out a number is odd or even without using "if based check" in verilog or VHDL language ?

 

[RetroTechie]

If you have the number in binary form, its least significant bit tells you odd (lsb = 1) or even (lsb = 0). You could reference that bit directly or use a logical operator (AND) to produce a zero / non-zero result.

 

[mr_vasanth]

Hi RetroTechie,

Thanks for your help. Won't we need a compare operator to check the zero / non-zero result of the AND operator (gate) ?

 

[mohi@608]

well you can use modulus operator (%2) for this . and i am not able to figure out why you want to use only compare ???

 

[jeffrey samuel]

try resolving the no to factors if the no has odd no of odd factors then it is eventually odd

this is a least prefered soln but if you are not comparing then this may do it

 

[mr_vasanth]

Hi mohi, the output of the modulus operator will be either 0 or 1 right ? Without checking whether the output is 0 or 1, how is it possible to know whether number is odd or even ? Please correct me if I am missing to understand somethign here.

 

[FvM]

The question has been completely answered in post #2.
You can e.g. write in VHDL odd <= my_number(0);

Obviously the expression works without a compare operator. If you are looking for something special, please tell what it is.

 

[mr_vasanth]

I think I have over complicated this topic. Thanks to all who helped me here.
I have just googled and found out all the different ways in stackoverflow website (using VHDL). Just pasting them here for boarders reference.

Method1:
if (A mod 2) = 0 then
-- it's even
else
-- it's odd
end if;

Method2:
if (A(0)) then
-- it's odd
else
-- it's even

Method3:
function is_even(val : integer) return boolean is
begin
return val mod 2 = 0;
end;

Method4:
function is_even(val : integer) return boolean is
constant vec: signed(31 downto 0) := to_signed(val, 32);
begin
return vec(0) = '0';
end;

I agree that Method1 and Method3 (similarly Method2 and Method4) are one and the same. But they are just two different coding styles.
But the one difference I think is, Method3 and Method4 are not synthesizable. Can someone please confirm this ?

 

[FvM]

I think I have over complicated this topic.

You're hitting the point.

Seriously speaking, I still don't understand what's the problem with a "compare operator". In my view, methods 1, 3 and 4 are using a "=" compare operator. Why not?

All 4 methods are basically doing the same. If syntactically correct, they'll synthesize to the same gate level logic, the said XNOR gate.