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REAL TIME OPERATINGREAL TIME OPERATING

SYSTEMSSYSTEMS

LessonLesson--2:2:PROCESS MANAGEMENTPROCESS MANAGEMENT

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1. Process Creation1. Process Creation

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Step 1: At the reset of the processor. in aStep 1: At the reset of the processor. in a

computer system, an OS is initialized firstcomputer system, an OS is initialized first

enabling the use of the OS functions, whichenabling the use of the OS functions, which

includes the function to create the processesincludes the function to create the processes

Step 2: Using OS process creation function, aStep 2: Using OS process creation function, a

process, which can be called initial process, isprocess, which can be called initial process, is

created.created.Step 3: OS started and that calls the initialStep 3: OS started and that calls the initial

process to run.process to run.

Process creationProcess creation

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Process creationProcess creation

Step 4: When the initial process runs, it

creates subsequent processes.

Processes can be created hierarchically.

OS schedules the threads and provide forcontext switching between the threads (or

tasks).

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(a) Context

(ii) Process stack pointer

(iii) Current state [Is it created, activated or

spawned? Is it running? Is it blocked?] (iv) Addresses that are allocated and that are

presently in use

PCBPCB

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(v) Pointer for the parent process (vi) Pointer to a list of daughter processes.

(vii) Pointer to a list of resources, which areusable (consumed) only once. Forexamples, input data, memory buffer orpipe, mailbox message, semaphore

(viii) Pointer to a list of resources, which

are usable (consumed) only once

PCBPCB

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(ix) Pointer to queue of messages.

(x) Pointer to Access-permissions

descriptor

(xi)ID

PCBPCB

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2. Example of Process Creation2. Example of Process Creation

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OS function creates a process usingOS_Task_Create ( ) functionTask_Send_Card_Info in the mai

Task_Send_Card_Info task creates twoother tasks, Task_Send_Port_Output andTask_Read_Port_Input.

OS then controls the context switching

between the processes

OS_Task_Create ( ) functionOS_Task_Create ( ) function

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OS Process Creation functionOS Process Creation function

OS function first creates theDisplay_process.Display_process creates

Display_Time_DateThread

Display_BatteryThread Display_SignalThread

Display_ProfileThread

Display_MessageThread Display_Call StatusThread

Display_MenuThread

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3. Message passing and System call to3. Message passing and System call to

OS by ProcessesOS by Processes

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Message Passing by process running inMessage Passing by process running in

user modeuser mode Generates and puts (sends) a message

OS lets the requested resource (for example, input

from a device or from a queue) use or run an OS

service function (for example, define a delayperiod after which process needs to be run again).

A message can be sent for the OS to let the LCD

display be used by a task or thread for sending the

output. An ISR sends a message to a waiting thread to

start on return from the ISR

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System call by process running in userSystem call by process running in user

modemode Call to a function defined at the OS.

For example, OSTaskCreate ( )to create a task.

First an SWI instruction is issued to trap the

processor and switch to supervisory mode. OS then executes a function like a library function

Processor on finishing the instructions of a called

function, the processor again switches back to user

mode and lets the calling process run further

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4. Process Manager Functions4. Process Manager Functions

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(i) makes it feasible to let for a process tosequentially execute or block when needinga resource and to resume when it becomesavailable,

(ii) implements the logical link to theresource manager for resourcesmanagement (including scheduling ofprocess on the CPU),

Process managerProcess manager

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(iii) allows specific resources sharingbetween specified processes only,

(iiiv) allocates the resources as per the

resource allocation- mechanism of thesystem and

(iv) manages the processes and resources ofthe given system.

Process managerProcess manager

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SummarySummary

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We learntWe learnt

Process manager has functions to create theprocesses,

allocates a PCB to each process,

manages access to the resources andfacilitates the switching from one process

state to another.

The PCB defines the process structure for aprocess state.

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We learntWe learnt

Process can send a message and make asystem call to enable OS actions and run OS

function

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End of Lesson 2 of Chapter 8End of Lesson 2 of Chapter 8