Abstract: A length decoder that rapidly calculates the group lengths of groups of variable length data words is provided. In accordance with one embodiment, a length decoder includes a length estimator and a length selector. The length estimator, estimates a length for each data word assuming the data word is the first member of a group. The length selector then selects the proper estimate based upon the actual length of the data word. Specifically, one embodiment of the length decoder can be used to calculate the length of instruction groups in a stack based computing system.