Operating systems

Basics of Operating Systems

Prof. Anish Goel

What are operating systems? Operating systems are software environments that

provide a buffer between the user and the low levelinterfaces to the hardware within a system.

They provide a constant interface and a set of utilities toenable users to utilise the system quickly and efficiently.

They allow software to be moved from one system toanother and therefore can make application programshardware independent.

Many applications do not require any operating systemsupport at all and run direct on the hardware.

2 Operating Systems Anish Goel

Hardware independence through the use of an operating system


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 applicationsimmediately delegate these tasks to an operating system.

The delegation decreases software development time byproviding system calls to enable application softwareaccess to any of the I/O system facilities.

These calls are typically made by building a parameterblock, loading a specified register with its location andthen executing a software interrupt instruction


Typical system call mechanism for the M680x0 processor family


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 intosupervisor mode to execute the required function.

It effectively provides a communication path between theapplication and the operating system kernel.

The kernel is the heart of the operating system whichcontrols the hardware and deals with interrupts, memoryusage, I/O systems etc.

It locates a parameter block by using an address pointerstored in a predetermined address register

It takes the commands stored in a parameter block andexecutes them.


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 (basicdisk operating system) and BIOS (basic I/O system)

CP/M is a good example of a single tasking operatingsystem.

Only one task or application can be executed at any onetime 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.


Program overlays


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.


What is OS ?

Operating Systems Anish Goel10

Basic structure of OS


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 )

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 processA process which executes without user interaction.

Interactive processA process which requires user interaction while executing

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 schedulingn Kernel - a microkernel with intertask synchronizationn 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

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

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

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

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.

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

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.

Basic requirements of an RTOS


priority inversionoccurs 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


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.

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 StackStack : Last-in-first-out (LIFO) data structure

RTOS requires multiple stacks - one for each task

Memory Management


Two issues Heap management Stack management

Heap management Classic heap Priority heap Fixed block heap

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

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

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

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

Scheduling algorithms of RTOS


The most commonly used static schedulingalgorithm is the Rate MonotonicRate 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.

Operating systems

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