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RTOSes target safety-critical designs

Posted: 19 Nov 2008     Print Version  Bookmark and Share

Keywords:embedded operating system  RTOS  microkernel  rate monotonic 

According to Morris the just released Deos makes use of a processor's memory management unit to build a firewall between the kernel and user tasks, thereby preventing errant or malicious code from corrupting other user tasks and the kernel.

"To guarantee CPU and memory access for critical tasks, Deos uses time and space partitioning," he said. "In the space domain, Deos enables programmers to assign each address space a fixed amount of physical memory, which prevents tasks from exhausting system memory and depriving other tasks.

"In the time domain, Deos allows programmers to assign each task a fixed percentage of CPU time, which prevents priority inversion and keeps tasks from hogging the CPU."

The RTOS employs rate monotonic (RMA) scheduling, which Morris claims provides the most efficient scheduling mechanism of any time- and space-partitioned COTS RTOS.

To further enhance scheduling efficiency, Deos utilises slack scheduling, taking advantage of an algorithm the company has patented, to enhance performance by recycling worst-case time budgeted for time-critical tasks and reallocating it for "anytime" algorithms.

"In a conventional RTOS, large amounts of CPU time have to be reserved to accommodate worst-case requirements for time-critical tasks, even if the tasks utilise only a fraction of that time in practice," said Morris. "Slack scheduling frees up this unused time and makes it available to non-critical 'any time' tasks such as displays and GPS.

"This approach greatly reduces idle time, making it possible to achieve 100% CPU utilisation. Deos also boosts performance by supporting ultra-fast, interrupt-driven threads, which enable it to respond in microseconds to external events."

The DDC-I RTOS will be delivered in a modular, executable binary format that streamlines the test and certification process. Unlike competitive RTOSes, which are delivered as source code, said Morris, the RTOS will not have to be recompiled, relinked, and retested (including structural coverage) in order to certify the application.

Deos also features an XML configuration tool that generates verifiable configuration profiles for each software module's time and space quotas and budgets. This verifiable output eliminates the need to perform configuration checks by hand.

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