A computationally tractable unified finite word length closed-loop stability measure is derived which is applicable to fixed-point, floating-point and block-floating-point representation schemes. Both the dynamic range and precision of an arithmetic scheme are-considered in this new unified measure. For each arithmetic scheme, the optimal controller realization problem is defined and a numerical optimization approach is adopted to solve it. Numerical examples are used to illustrate the design procedure and to compare the optimal controller realizations in different representation schemes.