Hệ điều hành - Chapter 13: Embedded systems

Chapter 13Embedded SystemsEmbedded SystemsCharacteristics of Embedded Operating SystemsEmbedded SystemA combination of hardware and software designed to perform a dedicated functionOften, embedded systems are part of a larger system or product, e.g., antilock braking system in a carEmbedded systems are tightly coupled to their environment  imposes real-time constraints by the need to interact with the environmentExamples of Embedded DevicesDifferences from typical computerA variety of interfacesenable the system to measure, manipulate, and interact with the external environmenthuman interface may be as simple as a flashing light or as complicated as real-time robotic visionUse of a diagnostic port for diagnosing the system Use of special purpose hardware to increase performance or safetyFixed-function or application-specific softwareRoadmapEmbedded SystemsCharacteristics of Embedded Operating SystemsCharacteristics of Embedded OSUsing a general-purpose OS for an embedded system may not be possibleconstraint of memory spaceconstraint of power consumptionreal-time requirementsSpecial-purpose OS designed for the embedded system environment is commonly used.Characteristics of Embedded OSReal-time operationcorrectness of computation depends, in part, on the time at which result is deliveredReactive operationneeds to consider worst-case conditions in execution in order to respond to external events that do not occur at predictable intervalsCharacteristics of Embedded OSConfigurabilitysupports flexible configuration so that only the functionality needed for a specific application and hardware suite is providede.g., allows to select only the necessary OS modules to loadI/O device flexibilityhandles devices by using special tasks instead of integrating their drives into the OS kernelCharacteristics of Embedded OSStreamlined protection mechanismsrequires limited protection because tested software can be assumed to be reliablee.g., I/O instructions need not be privileged instructions that trap to OS  tasks can directly perform their own I/Ono use of an OS service call  avoid overhead for saving and restoring the task contextCharacteristics of Embedded OSDirect use of interruptspermits user process to use interrupts directly no need to go through OS interrupt service routineshave efficient control over a variety of devicesDeveloping anEmbedded OSTwo general approachesTake an existing OS and adapt it for embedded purposesDesign a purpose-built OS solely for embedded useAdapting an Existing OSExamples include Windows, Linux familiar interfaces facilitate portability slower and less predictable than special purpose embedded OS not optimized for real-time and embedded applications  require considerable modification to achieve adequate performanceoptimizes for the average case rather than the worst case for schedulingassigns resources on demand ignores semantic information about an applicationAdapting an Existing OSNeed to add real-time capabilitystreamlining operationother specialized and necessary functionality for the given devicePurpose-Built Embedded OSMost OS are designed from the ground up for embedded applicationsTypical characteristics include:Has thread switch (fast and lightweight)Real time scheduling policy Small sizeResponds to external interrupts quickly (<10 s)Minimizes intervals during which interrupts are disabledPurpose-Built Embedded OSTypical characteristics include:Provides fixed or variable sized partitions for memory management and the ability to lock code and data in memoryProvides special sequential files that can accumulate data at a fast ratePurpose-Built Embedded OSTo deal with timing constraints, the kernel:provides bounded execution time for primitivesmaintains a real-time clockprovides for special alarms and timeoutssupports real-time queuing disciplines, e.g., EDF (earliest deadline first)provides primitives to delay processing and to suspend/resume execution