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Chapter 2: Computer-System Structures. Computer System Operation I/O Structure Storage Structure Storage Hierarchy Hardware Protection General System Architecture. Computer-System Architecture. Computer-System Operation. I/O devices and the CPU can execute concurrently.

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Chapter 2: Computer-System Structures

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Chapter 2: Computer-System Structures

  • Computer System Operation

  • I/O Structure

  • Storage Structure

  • Storage Hierarchy

  • Hardware Protection

  • General System Architecture

Operating System Concepts

Computer-System Architecture

Operating System Concepts

Computer-System Operation

  • I/O devices and the CPU can execute concurrently.

  • Each device controller is in charge of a particular device type.

  • Each device controller has a local buffer.

  • CPU moves data from/to main memory to/from local buffers

  • I/O is from the device to local buffer of controller.

  • Device controller informs CPU that it has finished its operation by causing an interrupt.

Operating System Concepts

Common Functions of Interrupts

  • Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines.

  • Interrupt architecture must save the address of the interrupted instruction.

  • Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.

  • A trap is a software-generated interrupt caused either by an error or a user request.

  • An operating system is interrupt driven.

Operating System Concepts

Interrupt Handling

  • The operating system preserves the state of the CPU by storing registers and the program counter.

  • Determines which type of interrupt has occurred:

    • polling

    • vectored interrupt system

  • Separate segments of code determine what action should be taken for each type of interrupt

Operating System Concepts

Interrupt Time Line For a Single Process Doing Output

Operating System Concepts

I/O Structure

  • After I/O starts, control returns to user program only upon I/O completion.

    • Wait instruction idles the CPU until the next interrupt

    • Wait loop (contention for memory access).

    • At most one I/O request is outstanding at a time, no simultaneous I/O processing.

  • After I/O starts, control returns to user program without waiting for I/O completion.

    • System call – request to the operating system to allow user to wait for I/O completion.

    • 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.

Operating System Concepts

Two I/O Methods



Operating System Concepts

Device-Status Table

Operating System Concepts

Direct Memory Access Structure

  • Used for high-speed I/O devices able to transmit information at close to memory speeds.

  • Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention.

  • Only on interrupt is generated per block, rather than the one interrupt per byte.

Operating System Concepts

Storage Structure

  • Main memory – only large storage media that the CPU can access directly.

  • Secondary storage – extension of main memory that provides large nonvolatile storage capacity.

  • Magnetic disks – rigid metal or glass platters covered with magnetic recording material

    • Disk surface is logically divided into tracks, which are subdivided into sectors.

    • The disk controller determines the logical interaction between the device and the computer.

Operating System Concepts

Moving-Head Disk Mechanism

Operating System Concepts

Storage Hierarchy

  • Storage systems organized in hierarchy.

    • Speed

    • Cost

    • Volatility

  • Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage.

Operating System Concepts

Storage-Device Hierarchy

Operating System Concepts


  • Use of high-speed memory to hold recently-accessed data.

  • Requires a cache management policy.

  • Caching introduces another level in storage hierarchy. This requires data that is simultaneously stored in more than one level to be consistent.

Operating System Concepts

Migration of A From Disk to Register

Operating System Concepts

Hardware Protection

  • Dual-Mode Operation

  • I/O Protection

  • Memory Protection

  • CPU Protection

Operating System Concepts

Dual-Mode Operation

  • Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.

  • Provide hardware support to differentiate between at least two modes of operations.

    1.User mode – execution done on behalf of a user.

    2.Monitor mode (also kernel mode or system mode) – execution done on behalf of operating system.

Operating System Concepts

Dual-Mode Operation (Cont.)

  • Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1).

  • When an interrupt or fault occurs hardware switches to monitor mode.




set user mode

Privileged instructions can be issued only in monitor mode.

Operating System Concepts

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).

Operating System Concepts

Use of A System Call to Perform I/O

Operating System Concepts

Memory Protection

  • Must provide memory protection at least for the interrupt vector and the interrupt service routines.

  • 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.

Operating System Concepts

Use of A Base and Limit Register

Operating System Concepts

Hardware Address Protection

Operating System Concepts

Hardware Protection

  • When executing in monitor mode, the operating system has unrestricted access to both monitor and user’s memory.

  • The load instructions for the base and limit registers are privileged instructions.

Operating System Concepts

CPU Protection

  • Timer – interrupts computer after specified period to ensure operating system maintains control.

    • Timer is decremented every clock tick.

    • When timer reaches the value 0, an interrupt occurs.

  • Timer commonly used to implement time sharing.

  • Time also used to compute the current time.

  • Load-timer is a privileged instruction.

Operating System Concepts

Network Structure

  • Local Area Networks (LAN)

  • Wide Area Networks (WAN)

Operating System Concepts

Local Area Network Structure

Operating System Concepts

Wide Area Network Structure

Operating System Concepts

Layers of Computer System

Operating System Concepts

Services Provided by the Operating System

  • Program development

    • Editors and debuggers

  • Program execution

  • Access to I/O devices

  • Controlled access to files

  • System access

Operating System Concepts

Services Provided by the Operating System

  • Error detection and response

    • internal and external hardware errors

      • memory error

      • device failure

    • software errors

      • arithmetic overflow

      • access forbidden memory locations

    • operating system cannot grant request of application

Operating System Concepts

Services Provided by the Operating System

  • Accounting

    • collect statistics

    • monitor performance

    • used to anticipate future enhancements

    • used for billing users

Operating System Concepts

Operating System

  • Functions same way as ordinary computer software

    • It is program that is executed

  • Operating system relinquishes control of the processor to execute other programs

Operating System Concepts

Operating System Concepts


  • Portion of operating system that is in main memory

  • Contains most-frequently used functions

  • Also called the nucleus

Operating System Concepts


  • Processor must wait for I/O instruction to complete before preceding

Operating System Concepts


  • When one job needs to wait for I/O, the processor can switch to the other job

Operating System Concepts


Operating System Concepts

Major Achievements

  • Processes

  • Memory Management

  • Information protection and security

  • Scheduling and resource management

  • System structure

Operating System Concepts


  • A program in execution

  • An instance of a program running on a computer

  • The entity that can be assigned to and executed on a processor

  • A unit of activity characterized by a single sequential thread of execution, a current state, and an associated set of system resources

Operating System Concepts


  • Consists of three components

    • An executable program

    • Associated data needed by the program

    • Execution context of the program

      • All information the operating system needs to manage the process

Operating System Concepts


Operating System Concepts

Memory Management

  • Process isolation

  • Automatic allocation and management

  • Support for modular programming

  • Protection and access control

  • Long-term storage

Operating System Concepts

Virtual Memory

  • Allows programmers to address memory from a logical point of view

  • While one process is written out to secondary store and the successor process read in there in no hiatus

Operating System Concepts

File System

  • Implements long-term store

  • Information stored in named objects called files

Operating System Concepts


  • Allows process to be comprised of a number of fixed-size blocks, called pages

  • Virtual address is a page number and an offset within the page

  • Each page may be located any where in main memory

  • Real address or physical address in main memory

Operating System Concepts

Operating System Concepts

Virtual Memory Addressing

Operating System Concepts

Information Protection and Security

  • Access control

    • regulate user access to the system

  • Information flow control

    • regulate flow of data within the system and its delivery to users

  • Certification

    • proving that access and flow control perform according to specifications

Operating System Concepts

Scheduling and Resource Management

  • Fairness

    • give equal and fair access to all processes

  • Differential responsiveness

    • discriminate between different classes of jobs

  • Efficiency

    • maximize throughput, minimize response time, and accommodate as many uses as possible

Operating System Concepts

Major Elements ofOperating System

Operating System Concepts

System Structure

  • View the system as a series of levels

  • Each level performs a related subset of functions

  • Each level relies on the next lower level to perform more primitive functions

  • This decomposes a problem into a number of more manageable subproblems

Operating System Concepts

Operating System Design Hierarchy

LevelNameObjectsExample Operations

13ShellUser programmingStatements in shell language


12User processesUser processesQuit, kill, suspend, resume

11DirectoriesDirectoriesCreate, destroy, attach, detach,

search, list

10DevicesExternal devices, suchOpen, close,

as printer, displaysread, write

and keyboards

9File systemFilesCreate, destroy, open, close

read, write

8CommunicationsPipesCreate, destroy, open. close,

read, write

Operating System Concepts

Operating System Design Hierarchy

LevelNameObjectsExample Operations

7Virtual MemorySegments, pagesRead, write, fetch

6Local secondaryBlocks of data, deviceRead, write, allocate, free


5Primitive processesPrimitive process,Suspend, resume, wait, signal

semaphores, ready


Operating System Concepts

Operating System Design Hierarchy

  • LevelNameObjectsExample Operations

  • 4InterruptsInterrupt-handlingInvoke, mask, unmask, retry

  • programs

  • ProceduresProcedures, call stack, Mark stack, call, return

  • display

  • 2Instruction SetEvaluation stack, micro-Load, store, add, subtract

  • program interpreter,branch

  • scalar and array data

  • 1Electronic circuitsRegisters, gates, buses,Clear, transfer, activate,

  • etc.complement

Operating System Concepts

Characteristics of Modern Operating Systems

  • Microkernel architecture

    • assigns only a few essential functions to the kernel

      • address space

      • interprocess communication (IPC)

      • basic scheduling

Operating System Concepts

Characteristics of Modern Operating Systems

  • Multithreading

    • process is divided into threads that can run simultaneously

  • Thread

    • dispatchable unit of work

    • executes sequentially and is interruptable

  • Process is a collection of one or more threads

Operating System Concepts

Characteristics of Modern Operating Systems

  • Symmetric multiprocessing

    • there are multiple processors

    • these processors share same main memory and I/O facilities

    • All processors can perform the same functions

Operating System Concepts

Characteristics of Modern Operating Systems

  • Distributed operating systems

    • provides the illusion of a single main memory and single secondary memory space

    • used for distributed file system

Operating System Concepts

Characteristics of Modern Operating Systems

  • Object-oriented design

    • used for adding modular extensions to a small kernel

    • enables programmers to customize an operating system without disrupting system integrity

Operating System Concepts

Client/Server Model

  • Simplifies the Executive

    • possible to construct a variety of APIs

  • Improves reliability

    • each service runs as a separate process with its own partition of memory

    • clients cannot not directly access hardware

  • Provides a uniform means fro applications to communicate via LPC

  • Provides base for distributed computing

Operating System Concepts

Threads and SMP

  • Different routines can execute simultaneously on different processors

  • Multiple threads of execution within a single process may execute on different processors simultaneously

  • Server processes may use multiple threads

  • Share data and resources between process

Operating System Concepts

Operating System Concepts


  • Hardware is surrounded by the operating-system

  • Operating system is called the kernel

  • Comes with a number of user services and interfaces

    • shell

    • C compiler

Operating System Concepts


Operating System Concepts

Modern UNIX Systems

  • System V Release 4 (SVR4)

  • Solaris 2.x

  • 4.4BSD

  • Linux

Operating System Concepts

  • Login