PASM is a concept for a parallel processing system that allows experimentation with different architectural design alternatives. PASM is dynamically reconfigurable along three dimensions: partitionability into independent or communicating submachines, variable interprocessor connections, and mixed-mode SIMD/MIMD parallelism. With mixed-mode parallelism, a program can switch between SIMD (synchronous) and MIMD (asynchronous) parallelism at instruction-level granularity, allowing the use of both modes in a single machine. The PASM concept is presented, showing the ways in which reconfiguration can be accomplished. Trade-offs among SIMD/MIMD, and mixed-mode parallelism are explored. The small-scale PASM prototype with 16 processing elements is described. The ELP mixed-mode programming language used on the prototype is discussed. An example of a prototype-based study that demonstrates the potential of mixed-mode parallelism is given.