1. A method for computing extrinsic likelihood values in a soft in soft out (SISO) decoder the method comprising: accepting a block of channel metrics and a priori values; dividing the block of channel metrics and a priori values into sub blocks accepting an nth channel metric and an nth a priori value into an alpha (&agr;) computer; computing a nth &agr; value from the nth channel metric and an nth a priori value using the nth &agr; value to compute the n&plus;1 alpha value; pushing &agr; values corresponding to the beginning of a sub block on a checkpoint stack until a beginning of the last sub block is reached; pushing each of the &agr; values of the final sub block on the checkpoint stack; popping each of the final &agr; values from the checkpoint stack and combining them with beta (&bgr;) values to form extrinsic likelihood values.