NASA is studying a Mars Rover Sample Return (MRSR) mission to perform in-situ analysis, collection and return to Earth of Martian surface samples. The value of science return from this mission is critically dependent upon the ability of a robotic roving vehicle to negotiate the diverse geology of this planet without incurring accidental damage or vehicle entrapment. Legged locomotion offers the considerable advantages of stability, low power, and traversability over extremely rugged terrain, and legged vehicle design concepts are currently being developed under the MRSR1 and Pathfinder2 projects. Semi-autonomous operation of a walking planetary rover entails several unique technical challenges, and places a premium on the system architecture needed to coordinate and control the vehicle actions. A design framework for such a system is provided in the following paragraphs, and is intended for missions where a high degree of autonomy is dictated. It provides a logical computing architecture for rover mobility and local navigation subsystem design by defining a set of functional modules and interfaces to facilitate software and hardware specification.