Current day general purpose processors have been enhanced with what is called ``media instruction set'' to achieve performance gains in applications that are media processing intensive. The instruction set that have been added exploit the fact that media applications have small native datatypes and have widths much less than that supported by commercial processors and the plethora of data-parallelism in such applications. Current processors enhanced with the ``media instruction set'' support arithmetic on sub-datatypes of only 8-bit, 16-bit, 32-bit and 64-bit precision. In this paper we motivate the need for arbitrary precision packed arithmetic wherein the width of the sub-datatypes are programmable by the user and propose an implementation for arithmetic on such packed datatypes. The proposed scheme has marginal hardware overhead over conventional implementations of arithmetic on processors incorporating a multimedia extended instruction set.