embedded and real time operating systems n.
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Embedded and Real-time Operating Systems

Embedded and Real-time Operating Systems

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Embedded and Real-time Operating Systems

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  1. Embedded and Real-time Operating Systems The suitability of Linux

  2. Operating Systems • Before looking at embedded and real-time it would be good to review the role and position of the OS • Started after BIOS • Controls all application interaction with hardware • Generally is the kernel • Role of file system(s), libraries, and applications • Is always memory resident in kernel space • Interface is through system calls for user processes

  3. Operating System

  4. Real-time requirements #1 • Real-time systems can be classed as hard and soft • Hard real-time systems must meet processing deadlines, data must/cannot be missed/ignored. The system is safety critical, i.e. medical systems, military systems. • Soft real-time systems have real-time data but can afford to miss some data, force re-transmissions and don’t need to be too strict about deadlines. For example audio or video streaming • Many systems are a mix of above – the data is real-time, but there are tolerances in the system. White goods equipment, engine management.

  5. Real-time requirements #2 • There are 2 major requirements for real-time systems: latency and jitter • Latency is the amount of time required to service a real-time task. The task must be dealt with immediately, complete and cannot be interrupted by the operating system. For example a bar code read. • Jitter is periodic but crucial data. The data rate can be very short and unpredictable, so data could be lost. An example could be any sensor reading in a real-time environment. • A system could involve both of these requirements

  6. Standards • The POSIX standard are for Portable Operating Systems. They are a set of graded standards for UNIX like OSs. They are supported by various governments (i.e. US), IEEE and ISO. • POSIX 1003.1d • Set of real-time extensions to UNIX like operating systems

  7. POSIX Standards

  8. Linux comparisons

  9. Review of commercial real-time systems

  10. Linux Version

  11. Real-time Linux? • From version 2.5.4 Robert Love introduced a patch that allowed pre-emption in the kernel. • This means that higher priority task can interrupt (pre-empt) lower priority ones. • This is now standard in the 2.6 kernels • It allows Linux, as standard, to run soft real-time tasks (e.g. streaming video) with a latency of 1 millisecond (100s MHz Pentiums).

  12. Pre-emptable Linux Kernel

  13. Micro kernels • The other versions of real-time Linux involve major patches to the existing systems • RTLinux and RTAI linux are both micro-kernel patches. • This means that the kernel is run as a low priority task from the micro-kernel • Thus the micro-kernel can interrupt the Linux kernel to run a real-time task. • These versions can run the POSIX 1003.13 version of real-time.

  14. Micro-kernel real-time architecture

  15. Nano-kernels • This was developed by ADEOS as an open source real-time Linux • The micro kernel approach has been patented. • This system runs various tasks and operating systems from a micro-kernel • It is rather similar to VMWare

  16. Nano kernel real-time architecture

  17. Resource kernel • This is a modification to the standard kernel that allows high speed pre-emption. • The resource kernel allows tasks to request and control resources (memory, CPU cycles) that a normal kernel would not allow. • Can’t reach hard real-time latencies.

  18. Resource kernel real-time architecture

  19. Performance comparison