REX OS

The REX Operating System (Real-time Executive Operating System) is a real-time operating system (RTOS) developed by Qualcomm for the ARM processor 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.

REX Operating System

Developer	Qualcomm
Written in	C[1]
OS family	L4A Pistachio and Iguana embedded operating system
Working state	Discontinued
Source model	Proprietary
Initial release	1999; 26 years ago (1999)[1]
Marketing target	Smartphones
Platforms	ARM
Kernel type	Microkernel RTOS
License	Proprietary

REX is a combination of two operating systems: L4Ka::Pistachio embedded microkernel and Iguana, with extensive modifications and extensions by Qualcomm and HTC.^[2]

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 software and requires a proprietary software license from Qualcomm for use.^[3]

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.

Features

REX is an RTOS with the following features:

• Preemptive multitasking
• Task management
• Task synchronization
• Exclusive lock
• Timer
• Interrupt management
• Uses less than 5K of ROM^[1]

REX provides no memory protection features but does have memory management abilities.