Hi,

I am trying to use the mod operator in my VHDL code, but my synthesis tool
only supports X MOD Y; where Y is a power of 2; X an integer.

What would be the equivalency between X MOD 255 and X MOD 256 ?
I would also like to find the equivalency between X MOD (2**m - 1) and
X MOD (2**m); where m is an integer.

Any help will be highly appreciated.

Thanks,
Michele

X= aY + r = Xn-1... X1 X0

X/Y = a
X mod Y = r    si Y=2**m   r=Xm-1...X1 X0,
that's why it's so trivial do perform X mod 2**m

X= a (Y-1+1) + r = a (Y-1) + (a+r)

=> X mod Y-1 = (X/Y + X mod Y ) mod Y-1 = Xn-1 ... Xm + Xm-1... X0 mod (Y-1)    
(if Y=2**m)

The advantage is that ^^ may be smaller than X. You can iterate the operation till
the result of the addition is less than Y-1. Note that it is easy to know the
maximum number of stages of addition you will need to perform the modulo.

Olivier

I had the same problem when I was trying to do multiply once, and the tool
required the second operand to be a power of 2.

-Bob

However your problem is simpler if the range of X is restricted.
For example, If as I suspect you're doing Galois-field
operations, you might need to take X mod 255 where X is in
the range 0 through 509.  In that case

    X mod 255  = X       (if X < 255)
                 X-255   (otherwise)

Hope this helps a bit --

Steve