Problem with length of register in multiplication op

Hello,

I'm working on a design which contains the following lines,

signal acc : unsigned (7 downto 0) ;

.........

..

process ()

....

if(i /= 7) then

acc <= acc * acc;

i <= i + 1;

.....

I am aware that in the above code, at each counter value the value of the acc is being doubled and is mismatching with the declared accumulator length and the compiler throws an error of mismatch. Is there any other way of getting through this type of situation ?

Thanks

Siva