What is Real Time ?
• “ Real time in operating systems:
The ability of the operating system to provide a
required level of service in a bounded response
WHAT IS RTOS.
• It responds to inputs immediately(Real-Time).
• Here the task is completed within a specified time delay.
• In real life situations like controlling traffic signal or a
nuclear reactor or an aircraft.
• The operating system has to respond quickly.
• Necessary signaling between interrupt routines and task
code is handled by RTOS.
• RTOS can suspend one task code subroutine in the middle
of its processing in order to run another.
What a RTOS is
• Real time computing is equivalent to fast computing.
• Real time systems operate in a static environment
• Real time programming involves assembly coding, priority
interrupt programming, writing device drivers.
SELECTING AN ARCHITECTURE
• Select the simplest architecture that will meet
your response requirements.
• If response requirements not possible, RTOS
can be used.
INTRODUCTION TO RTOS
– A more complex software architecture is needed to handle
multiple tasks, coordination, communication, and interrupt
handling – an RTOS architecture
• RTOS – OS and embedded software are integrated, ES
starts and activates the OS – both run in the same
• RTOS includes only service routines needed by the ES
• Desirable RTOS properties: use less memory,
application programming interface, debugging tools,
support for variety of microprocessors, already-
debugged network drivers
CHARACTERISTICS OF RTOS.
TASK AND TASK STATES
• A task – a simple subroutine
• ES application makes calls to the RTOS functions to start
tasks, passing to the OS, start address, stack pointers, etc. of
• Task States:
– Running (Executing task)
– Ready (possibly: waiting, dormant, delayed)
– Blocked (possibly: suspended, pended)
Pended Ready Delayed
• Each task/Process can belong to one and only one
• The Scheduler only operates on the processes in the
• There is a single process in the Run/current state at
• Transitions to and from the Ready queue are affected
as a part of the execution of the RTOS
resource/object services or as a result of timing
Typical Task Operations
• creating and deleting tasks,
• controlling task scheduling, and
• obtaining task information.
– Scheduler – keeps track of the state of each task and
decides which one task should go to running state.
– schedules/shuffles tasks between Running and Ready
– Blocking is self-blocking by tasks, and moved to Running
state via other tasks‟ interrupt signaling (when block-factor
– When a task is unblocked with a higher priority over the
„running‟ task, the scheduler „switches‟ context
immediately (for all pre-emptive RTOSs)
Preemptive and Non preemptive
Stops low priority task as soon as high
priority task unblocked
Non preemptive RTOS
Takes microprocessor away from the
lower priority task when that task blocks
Tasks and Data
– Each tasks has its won context - not shared, private
registers, stack, etc.
– In addition, several tasks share common data (via
global data declaration; use of „extern‟ in one task to
point to another task that declares the shared data
– Shared data caused the „shared-data problem‟
without solutions discussed in Chp4 or use of
„Reentrancy‟ characterization of functions
Semaphores and Shared Data
• A new tool for atomicity
– Semaphore – a variable/lock/flag used to control access to
shared resource (to avoid shared-data problems in RTOS)
– Protection at the start is via primitive function, called take,
indexed by the semaphore
– Protection at the end is via a primitive function, called
release, also indexed similarly
– Simple semaphores – Binary semaphores are often
adequate for shared data problems in RTOS
RTOS Semaphores & Initializing Semaphores
– Using binary semaphores to solve the „tank monitoring‟
– The nuclear reactor system: The issue of initializing the
semaphore variable in a dedicated task (not in a
„competing‟ task) before initializing the OS – timing of
tasks and priority overrides, which can undermine the
effect of the semaphores
– Solution: Call OSSemInit() before OSInit()
Semaphores and Shared Data
– Reentrancy, Semaphores, Multiple Semaphores, Device
– Each shared data (resource/device) requires a separate
semaphore for individual protection, allowing multiple tasks
and data/resources/devices to be shared exclusively, while
allowing efficient implementation and response time
– example of a printer device signaled by a report-buffering
task, via semaphore signaling, on each print of lines
constituting the formatted and buffered report
The initial values of semaphores – when not set properly
or at the wrong place
• The „symmetry‟ of takes and releases – must match or
correspond – each „take‟ must have a corresponding
„release‟ somewhere in the ES application
• „Taking‟ the wrong semaphore unintentionally (issue
with multiple semaphores)
• Holding a semaphore for too long can cause „waiting‟
tasks‟ deadline to be missed
• Priorities could be „inverted‟ and usually solved by
Semaphores and Shared Data
• Binary semaphores – single resource, one-at-a time,
alternating in use (also for resources)
• Counting semaphores – multiple instances of resources,
increase/decrease of integer semaphore variable
• Mutex – protects data shared while dealing with priority
– Summary – Protecting shared data in RTOS
• Disabling/Enabling interrupts (for task code and interrupt
• Taking/Releasing semaphores (can‟t use them in interrupt
routines), slower, affecting response times of those tasks that
need the semaphore
• Disabling task switches (no effect on interrupt routines),
holds all other tasks‟ response