The REX Operating System (Real-time Executive Operating System) is a real-time operating system (RTOS) developed by Qualcomm for the ARMprocessor based mobile phone Dual-Mode Subscriber Station (DMSS) or Advanced Mode Subscriber Software (AMSS) development. As of 2007, most Korean cell phones ran on REX.
It seems not to be actively developed or marketed as of 2012, but is used in currently or recently sold low end cell phones, typically for use by pay-as-you-go customers, like the Samsung SPH-M300.
^Asif, Saad Zaman (2011-01-11). Next Generation Mobile Communications Ecosystem: Technology Management for Mobile Communications. John Wiley and Sons. p. 299.