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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 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.

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.

• Iguana Archived 2006-04-26 at the Wayback Machine
• L4 Based Operating Systems Archived 2018-06-14 at the Wayback Machine
• NICTA L4-embedded Kernel Archived 2014-02-27 at the Wayback Machine
• Qualcomm Corporate Site

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