The Tandem NonStop series ran a custom operating system which was significantly different from Unix or HP 3000's MPE. It was initially called T/TOS (Tandem Transactional Operating System) but soon named Guardian for its ability to protect all data from machine faults or software faults. In contrast to all other commercial operating systems, Guardian was based on message passing as the basic way for all processes to interact, without shared memory, regardless of where the processes were running. This approach easily scaled to multiple-computer clusters and helped isolate corrupted data before it propagated.