A multipurpose arithmetic functional unit selectively performs planar attribute interpolation, unary function approximation, double-precision arithmetic, and/or arbitrary filtering functions such as texture filtering, bilinear filtering, or anisotropic filtering by iterating through a multi-step multiplication operation with partial products (partial results) accumulated in an accumulation register. Shared multiplier and adder circuits are advantageously used to implement the product and sum operations for unary function approximation and planar interpolation; the same multipliers and adders are also leveraged to implement double-precision multiplication and addition.