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What Does Real-Time Mean?
 Main difference to other computation: time
 time means that correctness of system depends
 - not only on logical results
 - but also on the time the results are produced
 real => reaction to external events must occur
during their evolution.
 system time ( internal time ) has to be measured
with same time scale as controlled environment
( external time )
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Foreground/Background.
 Systems which do not use an RTOS
 An application consist of an infinite loop which calls
application modules to perform the desired operations.
 The modules are executed sequentially (background)
with interrupt service routines (ISRs) handling
asynchronous events (foreground).
 Batch process
A process which executes without user interaction.
 Interactive process
A process which requires user interaction while
executing
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Kernel
l Kernel: the smallest portion of the operating system that
provides
l task scheduling, dispatching, and intertask communication.
l Kernel types
n Nanokernel - the dispatcher
n Microkernel - a nanokernel with task scheduling
n Kernel - a microkernel with intertask synchronization
n Executive - a kernel that includes privatized memory
blocks, I/O services, and other complex issues. Most
commercial real-time kernels are in this category.
n Operating system - an executive that also provides
generalized user interface, security, file management
system, etc
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What is RTOS?
 A real-time operating system (RTOS) that supports real-
time applications and embedded systems.
 Real-time applications have the requirement to meet task
deadlines in addition to the logical correctness of the
results.
– Multiple events handled by a single processor
– Events may occur simultaneously
– Processor must handle multiple, often competing
events
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Desirable Features of Real-Time Systems
 Timeliness
- OS has to provide kernel mechanisms for
- time management
- handling tasks with explicit time constraints
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Deterministic
Design for peak load
Predictability
Fault tolerance
Maintainability
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Multitasking
 Must provide mechanisms for scheduling and switching for
several user and kernel tasks
 Maximize CPU utilization
 Allow for managing of complex and real-time applications
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Categories
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Hard Real Time System:
failure to meet time constraints leads to system failure
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Firm Real Time System:
low occurrence of missing a deadline can be tolerated
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Soft Real Time System:
performance is degraded by failure to meet time
constraints
 An RTOS differs from common OS, in that the user
when using the former has the ability to directly access
the microprocessor and peripherals.
 Such an ability of the RTOS helps to meet deadlines.
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Real-Time Systems
RTOS is a multitasking system where multiple tasks run
concurrently
– system shifts from task to task
– must remember key registers of each task
• called its context
RTOS responsible for all activities related to a task:
 – scheduling and dispatching
 – intertask communication
 – memory system management
 – input/output system management
 – timing
 – error management
 – message management
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Basic requirements of an RTOS.
(i) Multi-threading and preemptibility
(ii) Thread priority
(iii) Thread synchronization mechanisms
(iv) Priority inheritance
(v) Predefined latencies
 Task switching latency: time to save the context of a
currently executing task and switch to another task..
 Interrupt latency: time elapsed between the execution of the
last instruction of the interrupted task and the first instruction
in the interrupt handler
 Interrupt dispatch latency: This is the time to go from the
last instruction in the interrupt handler to the next task
scheduled to run.
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Basic requirements of an RTOS.
priority inversion
occurs when a higher priority task must wait on a low
priority task to release a resource
 Priority Ceiling
 Each resource has an assigned priority
 Priority of thread is the highest of all priorities of the
resources it’s holding
 Priority Inheritance
 The thread holding a resource inherits the priority of the
thread blocked on that resource
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 Preemptive scheduling.
In a preemptive kernel, when an event makes a higher priority task
ready to run, the current task is immediately suspended and the
higher priority task is given control of the CPU.
 Reentrancy.
reentrant function : can be used by more than one task without fear of
data corruption.
non-reentrant function : cannot be shared by more than one task unless
mutual exclusion to the function is ensured by either using a
semaphore, by disabling interrupts during critical sections of code.
A reentrant function can be interrupted at any time and resumed at a
later time without loss of data.
Reentrant functions either use local variables (CPU registers or
variables on the stack) or protect their data when global variables are
used.
Compilers specifically designed for embedded software will generally
provide reentrant run-time libraries.
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Dynamic Memory Allocation
 RTOS uses abstract data types such as record, linked
list, and queue
 These data types normally use RAM dynamic memory
allocation techniques
 Data structures are created (allocated) on the fly during
program execution and destroyed when no longer
needed
– Requires large RAM memory
 Heap is portion of memory used for dynamic memory
allocation
 Must allocate separate RAM spaces for the Heap as
well as the Stack
Stack : Last-in-first-out (LIFO) data structure
 RTOS requires multiple stacks - one for each task
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Memory Management
 Two issues
 Heap management
 Stack management
 Heap management
 Classic heap
 Priority heap
 Fixed block heap
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Memory Management
 Classic heap
 The memory is collected into one giant heap and
partitioned according to the demand from tasks.
 There are several “fit” memory allocation algorithms, e.g.,
best-fit, first-fit, that also attempt to minimize the memory
fragmentation.
 Has a big management overhead so is not used in real-time
systems
 Priority heap
 partitions the memory along priority boundaries, e.g., a
high and a low priority partitions are created
 Fixed block heap
 partitions the memory into several pools of fixed block
length and upon a request, allocates a single block of
memory from the pool with size equal or larger than the
requested amount
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Stack management:
 When multiple tasks share a single processor, their contexts
(volatile information such as the contents of hardware registers,
memory-management registers, and the program counter)
need to be saved and restored so as to switch them.
This can be done using
 task-control block model OR one or more run-time
stacks
 Run-time stacks
- used to keep context
 may use only one run-time stack for all the tasks or one runtime stack in conjunction with several application stacks (or
private stacks), one for each task in memory
 Multiple stack case allows tasks to interrupt themselves,
 Stack size must be known a priori.
 Operating system manages the stacks
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Task and Task Control Blocks
 In RTOS program consists of
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independent,asynchronous, and interacting tasks
– Must have capability to store task context
Context is kept in the control block of the task.
Having multiple tasks means multiple control blocks,
which are maintained in a list
RTOS updates TCB when task is switched
best for full-featured real-time operating systems
 Device Control Block (DCB)
– tracks status of system associated devices
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Priorities
 Priority
An ordinal number which represents the relative
importance of a task.
 Static priority
A priority which is not automatically adjusted by the
system.
Static priority can typically be changed by user.
 Dynamic priority
A priority which is adjusted automatically by the system
according to task behavior and system loading.
Dynamic priority imposes an overhead on the system.
Dynamic priority can improve response times and
eliminate indefinite postponing
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Scheduling algorithms of RTOS
 The most commonly used static scheduling
algorithm is the Rate Monotonic (RM) scheduling
algorithm
 The RM algorithm assigns different priorities
proportional to the frequency of tasks.
 The task with the shortest period gets the highest
priority, and the task with the longest period gets
the lowest static priority.
 Rate monotonic algorithm is a dynamic
preemptive algorithm based on static priorities
 RM algorithm provides no support for dynamically
changing task periods and/or priorities and tasks
that may experience priority inversion.
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Rate Monotonic
 Priority inversion occurs in an RM system where
in order to enforce rate monotonicity, a noncritical task with a high frequency of execution is
assigned a higher priority than a critical task with
lower frequency of execution
 A priority ceiling protocol (PCP) can be used to
counter priority inversion, wherein a task blocking
a higher priority task inherits the higher priority
for the duration of the blocked task.
 The priority ceiling protocol is used to schedule a
set dependant periodic tasks that share resources
protected by semaphores
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Earliest deadline first
 Earliest deadline first (EDF) scheduling can be
used for both static and dynamic real-time
scheduling.
 a dynamic priority algorithm which uses the
deadline of a task as its priority.
 The task with the earliest deadline has the highest
priority
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Minimum Laxity First
 A variant of EDF is Minimum Laxity First (MLF)
scheduling where a laxity is assigned to each task in
the system and minimum laxity tasks are executed
first.
 Laxity : The difference between the time until a tasks
completion deadline and its remaining processing time
requirement.
 { the deadline by which
_
{ the amount of
the task must be completed }
computation
remaining to be
performed }
 MLF considers the execution time of a task, which EDF
does not
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Minimum Laxity First
 MLF assigns higher priority to a task with the
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least laxity
A task with zero laxity must be scheduled right
away and executed without preemption or it will
fail to meet its deadline.
The negative laxity indicates that the task will
miss the deadline, no matter when it is picked up
for execution.
A major problem with LLF algorithm is that it is
impractical to implement because laxity ties ( two
or more tasks have the same laxities ) result in
the frequent context switches among the
corresponding tasks.
This will cause the system performance to
remarkably degrade.
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Modified Least Laxity First
 MLLF schedules the task sets the same as LLF
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algorithm.
If the laxity tie occurs, the running task
continues to run with no preemption as far as the
deadlines of other tasks are not missed.
The MLLF algorithm defers the context switching
until necessary and it is safe even if the laxity tie
occurs.
That is, it allows the laxity inversion where a task
with the least laxity may not be scheduled
immediately.
Laxity inversion applies to the duration that the
currently running task can continue running with
no loss in schedulability
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Maximum Urgency First Algorithm
 solves the problem of unpredictability during a
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transient overload for EDF, LLF and MLLF
algorithms.
This algorithm is a combination of fixed and
dynamic priority scheduling, also called mixed
priority scheduling.
With this algorithm, each task is given an urgency
which is defined as a combination of two fixed
priorities (criticality and user priority) and a
dynamic priority that is inversely proportional to
the laxity.
The MUF algorithm assigns priorities in two
phases
Phase One concerns the assignment of static
priorities to tasks
Phase Two deals with the run-time behavior of the
MUF scheduler
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Maximum Urgency First Algorithm
 The first phase consists of these steps :
1) It sorts the tasks from the shortest period to the
longest period. Then it defines the critical set as
the first N tasks such that the total CPU load
factor does not exceed 100%. These tasks are
guaranteed not to fail even during a transient
overload.
2) All tasks in the critical set are assigned high
criticality.The remaining tasks are considered to
have low criticality.
3) Every task in the system is assigned an optional
unique user priority
{ CHIMERA II, a real-time operating system being used to
control sensor-based control systems }
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Maximum Urgency First Algorithm
 In the second phase, the MUF scheduler follows
an algorithm to select a task for execution.
 This algorithm is executed whenever a new task is
arrived to the ready queue.
 The algorithm is as follows:
1) If there is only one highly critical task, pick it up
and execute it.
2) If there are more than one highly critical task,
select the one with the highest dynamic priority.
Here, the task with the least laxity is considered to
be the one with the highest priority.
3) If there is more than one task with the same
laxity, select the one with the highest user priority.
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 In addition to basic scheduling and context switching, a
real-time kernel typically provides other valuable
services to applications such as:
 Time Delay
 System Time
 Inter-Process Communication (IPC)
 Synchronization —semaphores or flags
 Resource Protection - mutex
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 RTOS for small footprint, mobile and connected devices
Windows CE
(32 bit devices , minimum footprint of 400KB , 256 priority levels )
 RTOS for complex, hard real-time applications
LynxOS
(microkernel is 28 KB, 512 thread priority levels, supports memory
protection )
 General purpose RTOS in the embedded industry
VxWorks
(256 priority levels, multitasking, deterministic context switching,
preemptive and round robin scheduling, binary and counting
semaphores, mutual exclusion with inheritance, supports virtual
memory configuration )
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 RTOS for the Java Platform
Jbed RTOS package
( runs on 32-bit microprocessors and controllers. Current versions
support ARM7, 68k, PowerPC architectures, supports up to 10thread priority levels, EDF )
 Objected-oriented RTOS
pSOSystem
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Why Should I Use an RTOS?
 True that many or most applications can be written
without the support of an RTOS.
A few reasons to consider using an RTOS :
 The job of writing application software is generally
easier using an RTOS, because the use of a kernel
enforces certain disciplines in how your code is
structured.
 While the illusion of concurrency can be created
without the use of an RTOS (though not always), it
almost always results in a much more complex piece
of software.
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Disadvantages of Real-Time Kernels
 Extra cost of the kernel at Software
 More ROM/RAM
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 In addition to basic scheduling and context switching, a
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real-time kernel typically provides other valuable services
to applications such as:
Time Delays
System Time
Inter-Process Communication (IPC)
Synchronization
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