REX OS

The REX Operating System (Real-Time Executive Operating System) is an RTOS developed by Qualcomm for the ARM-based mobile phone Dual-Mode Subscriber Station (DMSS) or Advanced Mode Subscriber Software (AMSS) development. As of 2007 most Korean cell phones run on REX.
REX is an L4A embedded micro-kernel and Iguana operating system combination and has extensive Qualcomm and HTC modifications and extensions.
Originally developed for the Intel 80186 it was then ported to ARM and x86 architectures are no longer supported. REX is not POSIX compliant or open source and requires a license from Qualcomm for use.
It does not seem 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.
Features.
REX is a Real-time operating system with the following features:
REX does not provide any memory protection features but does have memory management capabilities.