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Lecture Topics

• Today: Processes and Threads

(Silberschatz, 3.1-3.3 and 4.1-4.4)

• Next: continued

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Announcements

• Self-Study Exercise #3 posted

• Score for Project #1 sent via email today

• Computer Project #2 – due 1/23

• Computer Project #3 – due 1/30

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Processes

• A process is a program in execution

• OS must support multitasking (multiple

processes in RAM, executing concurrently)

• OS interleaves execution of multiple

processes in order to maximize processor

utilization, yet still provide reasonable

response time

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Process Elements

• A process consists of (at least):

– code and data for executing program

– program counter (PC) and status register (PSW)

– general-purpose registers

– set of OS resources (open files, signals received,

network connections, …)

• In other words, everything the OS needs to

run the program -- or to re-start it, if it’s

interrupted at some point

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Example

Assume no virtual

memory: dispatcher

and three processes

are fully loaded into

memory

Program counter

currently points to an

address in Process B

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Example (cont)

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4

Example (cont)

Assume timer interrupt

after 6 clock cycles

Assume dispatcher is

at addresses 100-105

Assume dispatcher

selects next process

using "round robin"

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Process Execution

• Typical execution sequence:

– created (PCB assigned)

– wait to use CPU

– execute machine language instructions on CPU

– wait for I/O to complete

– wait to use CPU

– execute machine language instructions on CPU

– wait for I/O to complete

– wait to use CPU

– execute machine language instructions on CPU

– terminated (PCB deassigned)8

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Five-State Process Model

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Process States

• New – created (waiting to be admitted to pool

of executable processes)

• Ready – waiting to be assigned to processor

• Running – using the processor

• Blocked – waiting for event (such as I/O

completion or signal)

• Exit – terminated (removed from pool of

executable processes)10

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State Transitions

• Admit – OS moves process to pool of

executable processes

• Dispatch – OS allows process to use CPU

• Timeout – OS responds to timer interrupt

• Event wait – process issues system call

• Event occurs – OS responds to interrupt

• Release – Process issues “exit” system call

(or halts abnormally) 11

Previous Example

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Process Creation

• User logs onto system

• Submission of a batch job

• OS creates process to provide a service

(ex: control print job)

• One process creates another process

(ex: parent process spawns child process)

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Process Termination

• User logs off system

• Batch job terminates

• Process halts normally

• Process halts abnormally (ex: segmentation

fault, divide by zero)

• Parent process terminates child process

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Process Execution

• Once process is in the Running state, the OS

can only regain control through an interrupt:

– process issues system call for I/O or similar task

(which may block the process)

– process issues system call to terminate

– process generates exception

– interrupt from timer

– interrupt from I/O device (not related to process in

the Running state) 15

Process Execution

• After the OS has regained control and is

using the CPU, it processes the interrupt,

then gives up control of the CPU

• If the interrupted process was moved out of

the Running state, the OS selects and

dispatches a different process

• If the interrupt was due to an event for a

different process (I/O completion), the OS

resumes the interrupted process16

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Scenario #1

• Assume the process in the Running state

issues a system call to exit

– System call generates a software interrupt; kernel

saves context of the current process

– Kernel handles the system call and moves the

current process to the Exit state

– Kernel selects a different process from the pool of

processes in the Ready state and moves it to the

Running state, then restores its context

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Scenario #2

• Assume the process in the Running state

generates an exception (fault)

– Exception generates a software interrupt; kernel

saves context of the current process

– Kernel handles the exception and moves the

current process to the Exit state

– Kernel selects a different process from the pool of

processes in the Ready state and moves it to the

Running state, then restores its context

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Scenario #3

• Assume the process in the Running state

issues a system call to perform I/O

– System call generates a software interrupt; kernel

saves context of the current process

– Kernel handles the system call and moves the

current process to the Blocked state

– Kernel selects a different process from the pool of

processes in the Ready state and moves it to the

Running state, then restores its context

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Scenario #4

• Assume the process in the Running state

issues a system call to access information in

its PCB

– System call generates a software interrupt; kernel

saves context of the current process

– Kernel handles the system call (extracts info from

PCB of current process)

– Kernel restores the context of the current process

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Scenario #5

• Assume the time quantum for the process in

the Running state expires

– Timer generates a hardware interrupt; kernel

saves context of the current process

– Kernel handles the interrupt and moves the

current process to the Ready state

– Kernel selects a different process from the pool of

processes in the Ready state and moves it to the

Running state, then restores its context

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Scenario #6

• Assume the process in the Running state is

interrupted by the completion of an I/O

operation for a different process

– I/O module generates a hardware interrupt; kernel

saves context of the current process

– Kernel handles the interrupt and moves the other

process from the Blocked state to the Ready state

– Kernel restores the context of the current process

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Summary

• The OS gives up control of the CPU when it

moves a process from the Ready state to the

Running state (and restores its context)

• The OS regains control of the CPU whenever

there is an interrupt (hardware interrupt or

software interrupt)

• The OS consumes resources (CPU cycles

and memory)

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Mode Switch

• User mode to kernel mode

– interrupt: during execution of current instruction;

reaction to asynchronous external event

– exception: caused by execution of current

instruction; reaction to exceptional condition

– trap (system call): explicit request by current

instruction; request service from OS

• Kernel mode to user mode

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Mode Switch

• System call (other than exit):

• Processing similar for interrupts (although

control may return to a different process)

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Process Switch

• Process currently in Running state

– save context (PC, other registers)

– move PCB to appropriate queue

– update PCB (state, accounting info)

• Process selected by OS

– update PCB (state, accounting info)

– move PCB out of Ready queue

– restore context (PC, registers)

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Process Switch

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Operating System Data Structures

• Information about

the current status

of each process

and resource

• Table for each

resource the OS

manages

• Data structures in

kernel memory

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Process Management

• OS allocates a PCB (process control block)

when a process is created – a data structure

to hold information about that process

– Process ID number

– Current state

– Copy of control registers

– Copy of regular registers

– Accounting information

– Resource information

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Process Control Block

• PCB assigned to

process when created

• OS maintains info in

PCB while process is

in existence

• OS moves PCBs

between queues to

manage system

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Queueing Models: Single Queue

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Queueing Models: Multiple Queues

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