 The PEs are arranged in a 3-dimensional torus topology, with 96 PEs in each cabinet.The XT3 runs an operating system called UNICOS/lc that partitions the machine into three sections, the largest comprising the "Compute nodes", and two smaller sections for "Service nodes" and "IO nodes".In UNICOS/lc 1.x, the "Compute" PEs run a Sandia developed microkernel called Catamount, which is descended from the SUNMOS OS of the Intel Paragon; in UNICOS/lc 2.0, Catamount was replaced by a specially tuned version of Linux called Compute Node Linux (CNL).