The Real-Time Message Passing Interface (MPI/RT) is a communication layer middleware standard that is aimed at providing guaranteed Quality of Service for data transfers on high performance networks. It poses "middleout" requirements both on applications and on the operating system. In this paper, we consider the "middledown" issues by modifying a POSIX compliant operating system in order to support hard real time scheduling, a feature needed for efficient MPI/RT on real-time Networks of Workstations (NOWs). This paper describes the evolution of Turtle, a variant of RT-Linux, that will later support a prototype implementation of time-driven MPI/RT. Analysis, design approach and results for Turtle are discussed.