1. Basics of Operating Systems
Prof. Anish Goel

2. What are operating systems?
 Operating systems are software environments that
provide a buffer between the user and the low level
interfaces to the hardware within a system.
 They provide a constant interface and a set of utilities to
enable users to utilise the system quickly and efficiently.
 They allow software to be moved from one system to
another and therefore can make application programs
hardware independent.
 Many applications do not require any operating system
support at all and run direct on the hardware.
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3. Hardware independence through the use of
an operating system
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4. Non OS systems
 Such software includes its own I/O routines, for example,
to drive serial and parallel ports.
 With the addition of mass storage and the complexities
of disk access and file structures, most applications
immediately delegate these tasks to an operating system.
 The delegation decreases software development time by
providing system calls to enable application software
access to any of the I/O system facilities.
 These calls are typically made by building a parameter
block, loading a specified register with its location and
then executing a software interrupt instruction
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5. Typical system call mechanism for the
M680x0 processor family
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6. Typical system call mechanism for the
M680x0 processor family…
 The TRAP instruction is the MC68000 family equivalent
of the software interrupt and switches the processor into
supervisor mode to execute the required function.
 It effectively provides a communication path between the
application and the operating system kernel.
 The kernel is the heart of the operating system which
controls the hardware and deals with interrupts, memory
usage, I/O systems etc.
 It locates a parameter block by using an address pointer
stored in a predetermined address register
 It takes the commands stored in a parameter block and
executes them.
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7. Operating system internals
 The first widely used operating system was CP/M,
developed for the Intel 8080 microprocessor and 8"
floppy disk systems.
 It supported I/O calls by two jump tables — BDOS (basic
disk operating system) and BIOS (basic I/O system)
 CP/M is a good example of a single tasking operating
system.
 Only one task or application can be executed at any one
time and therefore it only supports one user at a time.
 When an application is loaded, it provides the user-
defined part of the total ‘CP/M’ program.
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8. Program overlays
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9. Program overlays
 Any application program has to be complete and
therefore the available memory often becomes the
limiting factor
 Program overlays are often used to solve this problem.
 Parts of the complete program are stored separately on
disk and retrieved and loaded over an unused code area
when needed
 This allows applications larger than the available memory
to run, but it places the control responsibility on the
application.
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10. What is OS ?
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11. Basic structure of OS
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12. RTOS Concepts
 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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13. 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 with interrupt
service routines (ISRs) handling asynchronous events .
 Batch process
A process which executes without user interaction.
 Interactive process
A process which requires user interaction while
executing
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14. 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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15. 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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16. Desirable Features of Real-Time Systems
 Timeliness
- OS has to provide kernel mechanisms for
- time management
- handling tasks with explicit time constraints
 Deterministic
 Design for peak load
 Predictability
 Fault tolerance
 Maintainability
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17. 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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18. Categories
 Hard Real Time System:
failure to meet time constraints leads to system failure
 Firm Real Time System:
low occurrence of missing a deadline can be tolerated
 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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19. 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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20. 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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21. 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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22.  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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23. 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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24. Memory Management
 Two issues
 Heap management
 Stack management
 Heap management
 Classic heap
 Priority heap
 Fixed block heap
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25. 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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26. 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 run-time
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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27. Task and Task Control Blocks
 In RTOS program consists of
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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28. 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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29. 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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