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Operating Systems: Components and Duties


Operating Systems: Components and Duties NOTE: this lecture and the next two lectures are intended to provide an overview of OS concepts important to the study of ... – PowerPoint PPT presentation

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Title: Operating Systems: Components and Duties

Operating Systems Components and Duties
NOTE this lecture and the next two lectures are
intended to provide an overview of OS concepts
important to the study of embedded systems. For
more comprehensive information, please visit the
website listed at the bottom of the slides and
the material accessible there.
What is an Operating System?
  • A program that acts as an intermediary between a
    user of a computer and the computer hardware.
  • Operating system goals
  • Execute user programs and make solving user
    problems easier.
  • Make the computer system convenient to use.
  • Use the computer hardware in an efficient manner.

System Components
Hides the complexity of machine language from
Instruction Set Architecture
Physical devices grouped together to form
functional units
Integrated circuit chips, power supply, CRT
  • The operating system runs in kernel or supervisor
    mode - protected from user tampering
  • Compilers, editors and application programs run
    in user mode

Functions of an OS
  • User Environment - OS layer transforms bare
    hardware machine into higher level abstractions
  • Execution environment - process management, file
    manipulation, interrupt handling, I/O operations,
  • Error detection and handling
  • Protection and security
  • Fault tolerance and failure recovery

Functions of an OS (continued)
  • Resource Management
  • Time management
  • CPU and disk transfer scheduling
  • Space management
  • main and secondary storage allocation
  • Synchronization and deadlock handling
  • IPC, critical section, coordination
  • Accounting and status information
  • resource usage tracking

Typical memory layout
Several jobs are kept in main memory at the same
time, and the CPU is multiplexed among them.
  • OS tasks
  • Memory management the system must allocate the
    memory to several jobs.
  • CPU scheduling the system must choose among
    several jobs ready to run.
  • Allocation of devices.

Real-Time Systems
  • Designed for a dedicated application such as
    scientific experiments or industrial processes,
    medical imaging and devices, consumer products,
    avionics, and automobiles
  • Responsibilities have well-defined, fixed time
  • Hard real-time
  • Secondary storage limited or absent, data stored
    in short term memory, or read-only memory (ROM)
  • Not supported by general-purpose operating
  • Firm real-time
    quality of service may degrade
  • a result is useless if it is provided after the
  • Soft real-time
  • The usefulness of a result and the quality of
    service degrade if the result is provided after
    the deadline
  • System may attempt to maximize the number of
    deadlines met or the number of high-priority
    tasks meeting their schedules or to minimize the
    lateness of tasks, for example

Computer-System Operation
  • Starting up a computer
  • Initial program, or bootstrap program, is run
  • Stored in ROM or EPROM within the computer
  • Initializes all aspects of the computer
    (registers, controllers, memory etc.)
  • Loads the operating system and executes it
  • Locates and loads the OS kernel
  • Once loaded, the OS waits for an event to occur
  • Events usually signaled by an interrupt from
    either the hardware or software
  • Hardware sends a signal to the CPU via the system
  • Software triggers an interrupt by executing a
    system call

Device Controller
  • A computer system typically consists of a CPU and
    multiple device controllers connected through a
    common bus
  • Each device controller is in charge of a specific
    device or multiple devices (e.g. SCSI controller)
  • Device controller maintains local buffer storage
    and a set of special purpose registers
  • The device controller moves data between the
    device it controls and its local buffer storage

  • Interrupts enable software to respond to signals
    from hardware
  • May be initiated by a running process
  • Interrupt is called a trap software generated
    caused by error or user request for an OS service
  • Dividing by zero or referencing protected memory
  • May be initiated by some event that may or may
    not be related to the running process
  • Key is pressed on a keyboard or a mouse is moved
  • Low overhead
  • Polling is an alternative approach
  • Processor repeatedly requests the status of each
  • Increases in overhead as the complexity of the
    system increases

Handling Interrupts
  • After receiving an interrupt, the processor
    completes execution of the current instruction,
    then pauses the current process
  • The processor will then transfer to a fixed
    location and executes the service routine for the
  • The interrupt handler determines how the system
    should respond
  • Interrupt handlers are stored in an array of
    pointers called the interrupt vector
  • To handle the interrupt quickly, a table of
    pointers is generally stored in low memory which
    hold the addresses of the ISR for the various
  • This array, or interrupt vector, of addresses is
    then indexed by a unique device number to provide
    the address of the ISR for the interrupting
  • After the interrupt handler completes, the
    interrupted process is restored and execution
    continues from the address of the interrupted
    instruction (stored on stack) or the next process
    is executed

Interrupt Classes
EXAMPLE Intel IA-32 exception classes
Interrupt Classes
EXAMPLE Intel IA-32 exception classes (continued)
I/O Interrupts
  • To start an I/O operation, the CPU loads the
    appropriate registers within the device
  • The device controller examines the values and
    determines what action to take (e.g. read, write)
  • When the transfer is complete, the device
    controller informs the CPU via an interrupt
  • The device driver returns control to the OS
  • Returns the data or pointer to the data if a read
    was done
  • For other operations it returns status

I/O Interrupts
  • Synchronous
  • After I/O starts, control returns to user program
    only upon I/O completion.
  • Wait instruction or tight loop (Loop jmp Loop)
    idles the CPU until the next interrupt
  • At most one I/O request is outstanding at a time,
    no simultaneous I/O processing.
  • Asynchronous
  • After I/O starts, control returns to user program
    without waiting for I/O completion.
  • Increased system efficiency by increasing CPU

Device-Status Table
  • Device-status table contains entry for each I/O
    device indicating its type, address, and state.
  • Operating system indexes into I/O device table to
    determine device status and to modify table entry
    to include interrupt.
  • If the device is busy with a request, the type of
    request and other parameters are stored in the
    table entry for that device
  • A queue will contain a list of all those requests
    waiting for a device

Hardware Protection
  • Dual-Mode Operation
  • I/O Protection
  • Memory Protection
  • CPU Protection

Dual-Mode Operation
  • Sharing system resources requires operating
    system to ensure that an incorrect program cannot
    cause other programs to execute incorrectly as
    well as to protect the OS from errant user
  • Provide hardware support to differentiate between
    at least two modes of operations.
  • User mode execution done on behalf of a user.
  • Monitor mode (also kernel mode or system mode)
    execution done on behalf of operating system.
  • MS-DOS for the Intel 8088 had no mode bit
  • User program can wipe out the OS
  • Multiple programs can write to a device

Dual-Mode Operation (Cont.)
  • Mode bit added to computer hardware to indicate
    the current mode monitor (0) or user (1).
  • When an interrupt or trap (program error) occurs
    hardware switches to monitor mode.

I/O Protection
  • All I/O instructions are privileged instructions.
  • Must ensure that a user program could never gain
    control of the computer in monitor mode (i.e., a
    user program that, as part of its execution,
    stores a new address in the interrupt vector).
  • System call - A privileged instruction provides a
    means for the user to interact with the OS to
    perform tasks that only the OS can do
  • Treated by the hardware as a software interrupt
  • Switch to monitor mode jumping to the address
    determined by the interrupt vector

Memory Protection
  • Must provide memory protection at least for the
    interrupt vector and the interrupt service
  • In order to have memory protection, add two
    registers that determine the range of legal
    addresses a program may access
  • Base register holds the smallest legal physical
    memory address.
  • Limit register contains the size of the range
  • Memory outside the defined range is protected.
  • Base and limit registers can be loaded only by
    the OS which uses a special privileged
    instruction (which can be executed only in
    monitor mode)

Use of A Base and Limit Register
Hardware Protection
  • CPU hardware compares every address generated in
    user mode with the registers
  • Any violation results in a trap to the monitor
    which treats it as a fatal error
  • When executing in monitor mode, the operating
    system has unrestricted access to both monitor
    and users memory.
  • The load instructions for the base and limit
    registers are privileged instructions.

CPU Protection
  • Timer interrupts computer after specified
    period to ensure operating system maintains
  • Timer is decremented every clock tick.
  • When timer reaches the value 0, an interrupt
  • Timer commonly used to implement time sharing
  • interrupt every N milliseconds (a time slice) at
    which time a context switch occurs
  • Performs housekeeping on the program just
    stopped, saves registers, internal variables,
    buffers etc. and prepares for the next program to
  • Load-timer is a privileged instruction.

Protection and Security
  • Protection controls the access of processes or
    users to the resources of the computer system
  • Distinguish between authorized and unauthorized
  • Security defends a system from external and
    internal attacks
  • Viruses, worms, denial of service attacks,
    identify theft

Operating-System Structures
  • System Components
  • Operating System Services
  • System Calls
  • System Programs
  • System Structure
  • Virtual Machines
  • System Design and Implementation

Common System Components
  • A system as large and complex as an OS can be
    created only by partitioning it into smaller
  • Process Management
  • Main Memory Management
  • File Management
  • I/O System Management
  • Secondary Management
  • Networking
  • Protection System
  • Command-Interpreter System
  • italicized topics will not be discussed here

Process Management
  • A process is a program in execution. A process
    needs certain resources, including CPU time,
    memory, files, and I/O devices, to accomplish its
  • A process is the unit of work in a system
  • The operating system is responsible for the
    following activities in connection with process
  • Process creation and deletion.
  • Process suspension and resumption.
  • Provision of mechanisms for
  • process synchronization
  • process communication

I/O System Management
  • Hide the peculiarities of specific hardware
    devices from the user
  • The I/O system consists of
  • A memory management component that includes
    buffering, caching and spooling
  • A general device-driver interface
  • Drivers for specific hardware devices

Protection System
  • Protection refers to a mechanism for controlling
    access by programs, processes, or users to both
    system and user resources.
  • The protection mechanism must
  • distinguish between authorized and unauthorized
  • specify the controls to be imposed.
  • provide a means of enforcement.

Command-Interpreter System
  • Serves as the interface between the user and the
  • User friendly, mouse based windows environment in
    the Macintosh and in Microsoft Windows
  • In MS-DOS and UNIX, commands are typed on a
    keyboard and displayed on a screen or printing
    terminal with the Enter or Return key indicating
    that a command is complete and ready to be
  • Many commands are given to the operating system
    by control statements which deal with
  • process creation and management
  • I/O handling
  • secondary-storage management
  • main-memory management
  • file-system access
  • protection
  • networking

Command-Interpreter System (Cont.)
  • The program that reads and interprets control
    statements is called variously
  • command-line interpreter
  • shell (in UNIX)
  • Its function is to get and execute the next
    command statement.

System Calls
  • System calls provide the interface between a
    running program and the operating system.
  • For example open input file, create output
    file, print message to console, terminate with
    error or normally
  • Generally available as routines written in C and
  • Certain low-level tasks (direct hardware access)
    may be written in assembly-language
  • Mostly accessed by programs via a high-level
    Application Program Interface (API) rather than
    direct system call use
  • Provides portability (underlying hardware handled
    by OS)
  • Hides the detail from the programmer
  • Three most common APIs are Win32 API for Windows,
    POSIX API for POSIX-based systems (including
    virtually all versions of UNIX, Linux, and Mac OS
    X), and Java API for the Java virtual machine

Example of Standard API
  • Consider the ReadFile() function in the
  • Win32 APIa function for reading from a file
  • A description of the parameters passed to
  • HANDLE filethe file to be read
  • LPVOID buffera buffer where the data will be
    read into and written from
  • DWORD bytesToReadthe number of bytes to be read
    into the buffer
  • LPDWORD bytesReadthe number of bytes read during
    the last read
  • LPOVERLAPPED ovlindicates if overlapped I/O is
    being used

System Call Implementation
  • Typically, a number associated with each system
  • System-call interface maintains a table indexed
    according to these numbers
  • The system call interface invokes intended system
    call in OS kernel and returns status of the
    system call and any return values
  • The caller need know nothing about how the system
    call is implemented
  • Just needs to obey API and understand what OS
    will do as a result call
  • Most details of OS interface hidden from
    programmer by API
  • Managed by run-time support library (set of
    functions built into libraries included with

API System Call OS Relationship
Standard C Library Example
  • C program invoking printf() library call, which
    calls write() system call

System Call Parameter Passing
  • Often, more information is required than simply
    identity of desired system call
  • Exact type and amount of information vary
    according to OS and call
  • Three general methods used to pass parameters to
    the OS
  • Simplest pass the parameters in registers
  • In some cases, may be more parameters than
  • Parameters stored in a block, or table, in
    memory, and address of block passed as a
    parameter in a register
  • This approach taken by Linux and Solaris
  • Parameters placed, or pushed, onto the stack by
    the program and popped off the stack by the
    operating system
  • Block and stack methods do not limit the number
    or length of parameters being passed

Passing Parameters as a Table
Types of System Calls
  • Process control
  • end, abort, load, execute, allocate/free memory
  • File management
  • Create/delete file, open, close, read, write
  • Device management
  • Request/release device, read, write
  • Information maintenance
  • Get/set date or time, get process, get/set system
  • Communications
  • Send/receive message, create/delete comm link

Example UNIX System Calls
UNIX Running Multiple Programs
  • FreeBSD is a multitasking system
  • Shell accepts a command to run a program but,
    unlike MS-DOS, continues to run while the other
    program is executing
  • To start a new process, the shell executes a fork
    system call which enables another program to be
    loaded into memory and executed
  • A process can run in the background but then it
    can not accept input from the keyboard (as it is
    dedicated to the shell) but can read and writes
  • The shell again waits for further commands run
    another program, monitor the progress of the
    running processes, change priorities, etc.
  • When a process terminates it issues an exit
    system call returning a status code

UNIX Running Multiple Programs
System Programs
  • System programs provide a convenient environment
    for program development and execution. The can
    be divided into
  • File manipulation
  • Status information (date,time, of users, free
    memory etc.)
  • File modification (text editors)
  • Programming language support (compilers,
    assemblers, etc)
  • Program loading and execution (loaders, linkage
  • Communications (create virtual connections among
    processes, users and different computer systems)
  • System utilities or application programs (web
    browsers, word processing, spreadsheets etc)
  • Most users view of the operating system is
    defined by system programs, not the actual system

OS Design Implementation
  • Mechanisms determine how to do something,
    policies decide what will be done.
  • The separation of policy from mechanism is a very
    important principle in designing an OS
  • It allows maximum flexibility if policy decisions
    are to be changed later.
  • The mechanism that implements the policy to give
    priority to I/O intensive processes over CPU
    intensive processes should be written in a
    general way so that if the policy is changed no
    or minimal change to the mechanism would be
  • The timer provides is a mechanism providing CPU
    protection. How long it runs for a particular
    user is a policy decision.

System Implementation
  • Traditionally written in assembly language,
    operating systems can now be written in
    higher-level languages.
  • Code written in a high-level language
  • can be written faster.
  • is more compact.
  • is easier to understand and debug.
  • Opponents to OS written high-level language claim
    slower performance and increased storage
  • Proponents argue
  • Modern compilers can produce highly optimized
  • Todays OSs run on highly complex hardware which
    can overwhelm the programmer with details
  • Better data structures and algorithms will truly
    improve the performance of OSs
  • Only a small amount of code is critical to high
    performance bottlenecks can be replaced with
    assembler code later on
  • An operating system is far easier to port (move
    to some other hardware) if it is written in a
    high-level language.

Operating System Structure MS-DOS
  • MS-DOS written to provide the most
    functionality in the least space
  • Not well divided into modules
  • Designed without realizing it was going to become
    so popular
  • Although MS-DOS has some structure, its
    interfaces and levels of functionality are not
    well separated
  • Vulnerable to system crashes when a user program
  • Written for Intel 8088 which had no dual mode or
    hardware protection

MS-DOS layer structure
Operating System Structure UNIX
  • UNIX the original UNIX operating system had
    limited structuring due to limited hardware
  • The UNIX OS consists of two separable parts.
  • Systems programs
  • The kernel series of interfaces and device
  • Consists of everything below the system-call
    interface and above the physical hardware
  • Provides the file system, CPU scheduling, memory
    management, and other operating-system functions
    a large number of functions for one level.
  • All this functionality in one level makes UNIX
    difficult to enhance difficult to determine
    impact of change in one part of the kernel on
    other parts

UNIX System Structure
Virtual Machines
  • A virtual machine treats hardware and the
    operating system kernel as though they were all
  • A virtual machine not provide any additional
    functionality but provides an interface identical
    to the underlying bare hardware. Each process is
    provided with a (virtual) copy of the underlying
  • The operating system creates the illusion that a
    process has its own processor with its own
    (virtual) memory.
  • The resources of the physical computer are shared
    to create the virtual machines.
  • CPU scheduling can create the appearance that
    users have their own processor.
  • Spooling and a file system can provide virtual
    card readers and virtual line printers.
  • A normal user time-sharing terminal serves as the
    virtual machine operators console.

System Models
Virtual Machine
Non-virtual Machine
Virtual Machines
  • The virtual-machine concept provides complete
    protection of system resources since each virtual
    machine is isolated from all other virtual
    machines. This isolation, however, permits no
    direct sharing of resources.
  • A virtual-machine system is a perfect vehicle for
    operating-systems research and development.
    System development is done on the virtual
    machine, instead of on a physical machine and so
    does not disrupt normal system operation.
  • The virtual machine concept is difficult to
    implement due to the effort required to provide
    an exact duplicate to the underlying machine
  • Virtual machines are a means to solve system
    compatibility problems
  • Windows applications to run on Linux-based
  • Intel instructions are translated into the native
    instruction set

Java Virtual Machine
  • A platform is the hardware or software
    environment in which a program runs. Some of the
    most popular platforms are Windows 2000, Linux,
    Solaris, and MacOS.
  • Most platforms can be described as a combination
    of the operating system and hardware. The Java
    platform differs from most other platforms in
    that it's a software-only platform that runs on
    top of other hardware-based platforms.
  • Compiled Java programs are platform-neutral
    bytecodes executed by a Java Virtual Machine
  • The JVM is specific for each system and it
    abstracts the system in a standard way to the
    Java program eliminating code changes when ported
    from one platform to another

Communication Models
  • Communication between processes may take place
    using either message passing or shared memory.
  • Message passing
  • Information is exchanged through an interprocess
    communication facility provided by the OS
  • Computers have host names, processes have process
    names for identification purposes
  • Useful when smaller number of data need to be
  • Easier to implement than shared memory
  • Shared memory
  • Processes use map memory system calls to gain
    access to regions of memory owned by other
  • Allows maximum speed and convenience of
  • Problems arise in the area of protection and

Communication Models
Message Passing
Shared Memory
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