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OMSE 510: Computing Foundations5: The Operating System

Chris Gilmore <grimjack@cs.pdx.edu>

Portland State University/OMSE

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Today

The Operating System!

Back to talking about software!

Overview

Process/Kernel Abstraction

Timeslicing

Process Scheduling

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Computer System (Idealized)

CPUMemory

System Bus

Disk Controller

Disk

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What is an operating system?

Operating system --“is a program that controls the execution of application programs and acts as an interface between the user of a computer and the computer hardware”Narrow view

Traditional computer with applications running on it (e.g. PCs, Workstations, Servers)

Broad viewAnything that needs to manage resources (e.g. router OS,

embedded devices, pagers, ...)

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Two key OS functions

Abstract Machine Hide details of the underlying hardware Provide “common” API to applications and

services Simplifies application writing

Resource Manager Controls accesses to “shared” resources

CPU, memory, disks, network, ... Allows for “global” policies to be implemented

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Why is abstraction important?Without OSs and abstract interfaces application writers program all device access directly load device command codes into device registers handle initialization, recalibration, sensing, timing etc for physical

devices understand physical characteristics and layout control motors interpret return codes … etc

Applications suffer severe code bloat! very complicated maintenance and upgrading writing this code once, and sharing it, is how OS began!

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Providing abstraction via system calls

Operating System

Video CardCPU

Monitor PrinterDisk

MemoryNetwork

Application

Hardware

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Providing abstraction via system calls

Operating System

Video CardCPU

Monitor PrinterDisk

MemoryNetwork

Application

Hardware

System Calls: read(), open(), write(), mkdir(), kill() ...

Device MgmtFile System Network Comm.

Process Mgmt

Protection Security

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OS as a resource managerSharing resources among applications across space and timeschedulingallocation

Making efficient use of limited resources improving utilizationminimizing overhead improving throughput/good put

Protecting applications from each otherenforcement of boundaries

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Problems an OS must solve

Time sharing the CPU among applications

Space sharing the memory among applications

Space sharing the disk among users

Time sharing access to the disk

Time sharing access to the network

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More problems an OS must solve

Protectionof applications from each otherof user data from other usersof hardware/devicesof the OS itself!

The OS needs help from the hardware to accomplish these tasks!

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Overview of computer system layers

Hardware - CPU, memory, I/O devices - disk, network ...

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The OS is just a program!How can the OS cause application programs to run?

How can applications programs cause the OS to run?

How can the OS switch the CPU to run a different application and later resume the first one?

How can the OS maintain control?

In what ways can application code try to cheat?

And how can the OS stop the cheating?

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How can the OS invoke an application?

The computer boots and begins running the OS fetch/decode/execute OS instructions

OS can request user input to identify an application to runOS loads the address of the application’s starting

instruction into the PCCPU fetches/decodes/executes the application’s

instructions

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How can applications invoke the OS?

Trap instruction changes PC to point to an OS entry point instructionapplication calls a library procedure that

includes the appropriate trap instruction fetch/decode/execute cycle begins at a

specified OS entry point called a system call

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How can the OS run a new application?

To suspend execution of an application simply capture its memory state and processor statecopy values of all registers into a data

structure and save it to memorypreserve the memory values of this

application so it can be restarted later

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How can OS guarantee to regain control?

What if a running application doesn’t make a system call and hence hogs the CPU? timer interrupts!OS must register a future timer interrupt before it

hands control of the CPU over to an application

How can the OS avoid trampling on the processor state it wants to save?carefully written interrupt handlers!

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What if the application tries to cheat?

What stops the running application from disabling the future timer interrupt so that the OS can not regain control? the mode bit (in the PSW)!

Certain instructions can only be executed when the mode bit is setmanipulating timer interruptssetting the mode bit! ...

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What other ways are there to cheat?

What stops the running application from modifying the OS?Memory protection!can only be set with mode bit set ….

The OS must clear the mode bit before it hands control to an application! interrupts and trap instructions set the mode bit

and transfer control to specific locations (in the OS)

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Why its not quite that simple ...

Pipelined CPUs

Superscalar CPUs

Multi-level memory hierarchies

Virtual memory

Complexity of devices and buses

Heterogeneity of hardware

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Pipelined CPUs

Fetchunit

Decodeunit

Executeunit

Execution of current instruction performed in parallelwith decode of next instruction and fetch of the oneafter that

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Superscalar CPUs

Fetchunit

Decodeunit

Executeunit

Fetchunit

Decodeunit

Executeunit

Executeunit

Holdingbuffer

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What does this mean for the OS?

Pipelined CPUsmore complexity in capturing state of a running applicationmore expensive to suspend and resume applications

Superscalar CPUseven more complexity in capturing state of a running

applicationeven more expensive to suspend and resume applications

More details, but fundamentally the same task

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Terminology review - metric units

The metric prefixes

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The memory hierarchy2GHz processor 0.5 ns

Data/inst. cache 0.5ns – 10 ns, 64 kB- 1MB

(this is where the CPU looks first!)

Main memory 60 ns, 512 MB – 1GB

Magnetic disk 10 ms, 160 Gbytes

Tape Longer than you want, less than magnetic disk!

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Who manages the memory hierarchy?Movement of data from main memory to cache is under hardware control cache lines loaded on demand automatically replacement policy fixed by hardware

Movement of data from cache to main memory can be affected by OS instructions for “flushing” the cache can be used to maintain consistency of main memory

Movement of data among lower levels of the memory hierarchy is under direct control of the OS virtual memory page faults file system calls

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OS implications of a memory hierarchy?How do you keep the contents of memory consistent across layers of the hierarchy?

How do you allocate space at layers of the memory hierarchy “fairly” across different applications?

How do you hide the latency of the slower subsystems? Main memory… yikes! Disk Tape

How do you protect one application’s area of memory from other applications?

How do you relocate an application in memory?

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Summary/QuizHow does the OS solve these problems:

Time sharing the CPU among applications?Space sharing the memory among applications?Space sharing the disk among users?Time sharing access to the disk?Time sharing access to the network?Protection of applications from each other?Protection of user data from other users?Protection of hardware/devices?Protection of the OS itself?

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Section overviewOS-Related Hardware & Software

Memory protection and relocationVirtual memory & MMUsI/O & Interrupts

ProcessesProcess schedulingProcess statesProcess hierarchiesProcess system calls in Unix

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Memory protection and relocation ...

Memory protection (basic ideas) virtual vs physical addresses

address range in each application starts at 0

“base register” used to convert each virtual address to a physical address before main memory is accessed

address is compared to a “limit register” to keep memory references within bounds

Relocation by changing the base register value

Paged virtual memory same basic concept, but more powerful (and complex)

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Base & Limit Registers (single & multiple)

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Virtual memory and MMUsMemory management unit (MMU) hardware provided equivalent of base registers at the granularity of “pages” of memory, say 2kB, i.e., lots

of them! supports relocation at page granularity applications need not occupy contiguous physical memory

Memory protection limit registers don’t work in this context per-page and per-application protection registers

Relocation and protection occur at CPU speeds!

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What about I/O devices?Monitor

Bus

A simplified view of a computer system

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Structure of a large Pentium system

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How do programs interact with devices?

Devices vs device controllers vs device drivers device drivers are part of the OS programs call the OS which calls the device driver

Device drivers interact with device controllers either using special IO instructions or by reading/writing controller registers that

appear as memory locations

Why protect access to devices by accessing them indirectly via the OS?

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How do devices interact with programs?

Interrupts

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Different types of interruptsTimer interruptsAllows OS to maintain controlOne way to keep track of time

I/O interruptsKeyboard, mouse, disks, etc…

Hardware failures

Program generated (traps)Programming errors: seg. faults, divide by zero, etc.System calls like read(), write(), gettimeofday()

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Timer interrupts

OS can ask timer device to interrupt after a specified time period has elapse

Interrupt invokes timer interrupt handler which invokes OS “scheduler”

OS can take the opportunity to save the current application and restore a different one context switch

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Why use traps for system calls?The Operating System is just a program!It must have the privilege to manipulate the hardwareset base and limit registers for memory protectionaccess devicesset and clear mode bit to enable privilege

If user programs execute with the mode bit clear, and do not have privilege to set it, how can they invoke the OS so that it can run with the mode bit set?That’s what traps do … set the mode bit and begin

execution at a specific point in memory (in the OS!)

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System callsSystem calls are the mechanism by which programs communicate with the O.S.

Implemented via a TRAP instruction

Example UNIX system calls:open(), read(), write(), close()kill(), signal()fork(), wait(), exec(), getpid()link(), unlink(), mount(), chdir()setuid(), getuid(), chown()

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The inner workings of a system call

User-level code

Library code

Process usercode { ... read (file, buffer, n); ... }

Procedure read(file, buff, n) { ... read(file, buff, n) ... }

_read: LOAD r1, @SP+2 LOAD r2, @SP+4 LOAD r3, @SP+6 TRAP Read_Call

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Steps in making a system call

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What about disks and file storage?

Structure of a disk drive

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Disks and file storageManipulating the disk device is complicatedhide some of the complexity behind disk controller, disk device

driver

Disk blocks are not a very user-friendly abstraction for storagecontiguous allocation may be difficult for large data itemshow do you manage administrative information?

One application should not (automatically) be able to access another application’s storageOS needs to provide a “file system”

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File systems

File system - an abstraction above disk blocks

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What about networks?Network interfaces are just another kind of shared device/resource

Need to hide complexitysend and receive primitives, packets, interupts etcprotocol layers

Need to protect the deviceaccess via the OS

Need to allocate resources fairlypacket scheduling

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The Process ConceptProcess – a program in executionProgram

description of how to perform an activity instructions and static data values

Process a snapshot of a program in execution memory (program instructions, static and dynamic data values) CPU state (registers, PC, SP, etc) operating system state (open files, accounting statistics etc)

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Process address space

stack

text

dataAddress

space

Each process runs in its own virtual memory address space that consists of: Stack space – used for function and system calls Data space – variables (both initialized and uninitialized) Text – the program code (usually read only)

Invoking the same program multiple times results in the creation of multiple distinct address spaces

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Memory

ProgramCode

ProgramData

Running a process on a CPU

CPU

ALU

ADD R1, R2

SP PC

In its simplest form, a computer performs instructions on operands. Registers are used to hold values temporarily to speed things up

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Switching among multiple processes

Memory

ProgramCode

ProgramData

CPU

ALU

SP PC

ProgramState

Program 1 has CPU

Saving all the information about a process allows a process to be temporarily suspended

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Switching among multiple processes

Memory

ProgramCode

ProgramData

CPU

ALU

SP PC

ProgramState

ProgramCode

ProgramData

SUB R1, R2

Program 2 has CPU

Saving all the information about a process allows a process to be temporarily suspended

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Switching among multiple processes

Memory

ProgramCode

ProgramData

CPU

ALU

SP PC

ProgramState

ProgramCode

ProgramData

ProgramState

Program 2 has CPU

Saving all the information about a process allows a process to be temporarily suspended

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Switching among multiple processes

Memory

ProgramCode

ProgramData

CPU

ALU

SP PC

ProgramState

ProgramCode

ProgramData

ProgramState

Program 1 has CPU

Saving all the information about a process allows a process to be temporarily suspended

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Switching among multiple processes

Memory

ProgramCode

ProgramData

CPU

ALU

SP PC

ProgramState

ProgramCode

ProgramData

ProgramState

ADD R1, R3

Program 1 has CPU

Saving all the information about a process allows a process to be temporarily suspended

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Why use the process abstraction?

Multiprogramming of four programs in the same address spaceConceptual model of 4 independent, sequential processesOnly one program active at any instant

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The role of the scheduler

Lowest layer of process-structured OShandles interrupts & scheduling of processes

Above that layer are sequential processes

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Process states

Possible process states runningblocked ready

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Implementation of process switching

Skeleton of what the lowest levels of the OS do when an interrupt occurs

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How do processes get created?

Principal events that cause process creation

System initialization

Initiation of a batch job

User request to create a new process

Execution of a process creation system call from another process

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Process hierarchiesParent creates a child process, special system calls for communicating with and

waiting for child processeseach process is assigned a unique identifying number

or process ID (PID)

Child processes can create their own child processes Forms a hierarchyUNIX calls this a "process group"

Windows has no concept of process hierarchyall processes are created equal

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How do processes terminate?

Conditions which terminate processes

Normal exit (voluntary)

Error exit (voluntary)

Fatal error (involuntary)

Killed by another process (involuntary)

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Process creation in UNIXAll processes have a unique process id getpid(), getppid() allow processes to get their information

Process creation fork() creates a copy of a process with the lone exception

of the return value of fork() exec() replaces an address space with a new program system() like CreateProcess()

Process termination, signaling signal(), kill() allows a process to be terminated or have

specific signals sent to it

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Example: process creation in UNIX

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 22)

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Process creation in UNIX example

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 22)

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 24)

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Process creation in UNIX example

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 22)

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 24)

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Process creation in UNIX example

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 22)

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 24)

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Process creation in UNIX example

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pid = fork()if (pid == 0) { // child… … exec(); }else { // parent wait(); }…

csh (pid = 22)

//ls program

main(){

//look up dir

…

}

ls (pid = 24)

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What other process state does the OS manage?

Fields of a process table entry

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What about the OS?

Is the OS a process?

It is a program in execution, after all …

Does it need a process control block?

Who manages its state when its not running?