Recently, difference between a Massively Parallel Processor and a network of workstations (NOW) almost disappears from hardware point of view. However, current performance of NOWs is still much lower than that of MPPs because of huge overheads of operating systems. Furthermore, both MPPs and current NOWs are not general-purpose because they have not succeeded in giving users (1) a single system image with flexible resource allocation, or (2) multi-tasking environments with shared memory spaces. SSS-CORE is a general purpose scalable operating system for NUMA parallel distributed systems. It provides very efficient multi-tasking environment with timesharing and space partitioning. Furthermore, it tries to allow each parallel application to achieve maximum performance by cooperation of user and kernel level resource allocation and scheduling and by offering low-latency high-throughput memory based communication facilities. SSS-CORE also provides a low-overhead mechanism that allows information transfer between kernel and user level.