1.1 Operating System Concepts Chapter 1: Introduction What is an operating system? Simple Batch Systems Multiprogramming Batched Systems Time-Sharing Systems

Operating System Concepts 1.1

Chapter 1: Introduction

• What is an operating system?

• Simple Batch Systems

• Multiprogramming Batched Systems

• Time-Sharing Systems

• Personal-Computer Systems

• Parallel Systems

• Distributed Systems

• Real -Time Systems


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.


Operating System Definitions

• Resource allocator – manages and allocates resources.

• Control program – controls the execution of user programs and operations of I/O devices .

• Kernel – the one program running at all times (all else being application programs).


Memory Layout for a Simple Batch System


Multiprogramming Batch Systems

Multiprogramming: several jobs are kept in main memory at the same time, and the CPU is multiplexed among them which requires memory management and protection.

Disk

Job Pool

Job Scheduling

CPU

Switching between jobs

CPU Scheduling


Multiprogramming Batch Systems

• The O.S. picks and begins to execute one job from memory. Once this job needs an I/O operation the O.S. switches to another job (CPU or O.S. always busy).

• The number of jobs in memory is less than the number of jobs in disk (Job Pool).

• If several jobs are ready to be brought into memory and there is not enough room for all of them, then the system choose jobs among them (Job Scheduling).

• If several jobs are ready to run at the same time, the system must choose among them (CPU Scheduling).

• Having several programs in memory at the same time requires memory management.

• In non-multiprogrammed system, CPU sit idle.

• In multiprogramming system, CPU will never be idle.


OS Features Needed for Multiprogramming

• I/O routine supplied by the system.

• Memory management – the system must allocate the memory to several jobs.

• Job scheduling – the system must choose among several jobs ready to run from Disk.

• CPU scheduling – the system must choose among several jobs ready to run in memory.

• Allocation of devices.

___________________________________________________

• Two main disadvantages of Multiprogrammed Batched Systems:– Users cannot interact with their jobs, while executing.– A programmer cannot modify a program as it executes to

study its behavior.


Time-Sharing Systems – Interactive Computing

A time-sharing system uses CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared computer.

O.S.

Job1

Job2

Job3

Multiprogramming

Memory Management

Main Memory

CPU

CPU Scheduling

Disk

User1

User2

User3

Interactive

I/O operations


Time-Sharing Systems–Interactive Computing

• The CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory).

• When a job needs an I/O operation, the CPU switches between jobs. Therefore, the CPU is always busy.

• A job is swapped in and out of memory to the disk which serves as a back up for main memory.

• On-line communication between the user and the system is provided; when the operating system finishes the execution of one command, it seeks the next “control statement” not from a card reader, but rather from the user’s keyboard.

• On-line system must be available for users to access data and code.


Time-Sharing Systems (Cont.)

• Time-Sharing Systems provide the following:– On-Line file system, where the files are on a

collection of disks. Therefore, disk management must be provided.

– A mechanism for concurrent execution, which requires CPU scheduling schemes.

– Mechanisms for job synchronization and communication to ensure orderly execution.

– A mechanism to avoid deadlock - a job waiting for another job forever.


Personal-Computer Systems

• Personal computers – computer system dedicated to a single user.

• I/O devices – keyboards, mice, display screens, small printers.

• PC operating systems were neither multi-user nor multi-tasking.

• The goal of PC operating systems were to maximize user convenience and responsiveness instead of maximizing CPU and I/O utilization.

• Examples: Microsoft Windows and Apple Macintosh


Parallel Systems

• Multiprocessor systems with more than one CPU in close communication.

• Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.

Storage Processor

I/O

Processor

I/O

Shared Memory

Single O.S.


Parallel Systems (Cont.)

• Advantages of parallel system (multiprocessor systems): – Increased throughput – number of processes that are

completed per time unit.– Economical (for large jobs) - no need to make copies of

data and distribute it among several processors.– Faster (for large jobs) – divide the work on all processors.– Increased reliability (fault – tolerant) – For example, if we

have 10 processors working together on a job and one processor failed, then the remaining 9 processors must pick up a share of the work of the failed processor. Thus, the entire system is still working but slower by 10%. Therefore, multiprocessor systems are reliable.



• Symmetric multiprocessing (SMP) model– Each processor runs an identical copy of the operating

system.– Most modern operating systems support SMP, such as

UNIX for Multimax computer.

Symmetric Multiprocessing Architecture



• Asymmetric multiprocessing model

– Each processor is assigned a specific task; master processor schedules and allocates work to slave processors.

StorageMaster

Processor

Slave

ProcessorI/O

Asymmetric Multiprocessing Architecture



• Asymmetric multiprocessing model (Cont.):

- Master performs I/O and computations.

- Only master may execute the O.S.

- Slave can execute only user programs.

- If master fails the system cannot perform I/O.

- If slave fails some computations are lost but still the system can function.

- More common in extremely large systems.


Distributed Systems

• Distribute the computation among several physical processors.

• Loosely coupled system – involves connecting 2 or more independent computer systems via communication link. So, each processor has its own O.S. and local memory; processors communicate with one another through various communications lines (message passing), such as high-speed buses or telephone lines.

Storage & O.S

Processor

I/O

Storage & O.S

Processor

I/O

Communication Link

Message Passing


Distributed Systems (Cont.)

• Advantages of distributed systems:– Resources Sharing – You can share files and

printers.– Computation speed up – A job can be partitioned

so that each processor can do a portion concurrently (load sharing).

– Reliability – If one processor failed the rest still can function with no problem.

– Communications – Such as electronic mail, ftp, etc.


Real-Time Systems

• Real – Time Systems are characterized by supplying immediate response. For example, sensors bring data to the computer.

• Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.

• Well-defined fixed-time constraints.

• Hard real-time system.– Secondary storage limited or absent, data stored in short-

term memory, or read-only memory (ROM)– Conflicts with time-sharing systems, not supported by

general-purpose operating systems.

• Soft real-time system– Limited utility in industrial control or robotics– Useful in applications (multimedia, virtual reality) requiring

advanced operating-system features.


Storage Structure

• Floppy disks consist of one platter and the head sits directly on the surface. Its inexpensive, less storage 1.4MB and slower compare it to the hard disk. Also, it is removable.

• The load instruction moves a word from main memory to an internal register, within the CPU for execution.

• The store instruction moves the content of a register to main memory.

• Can we store the programs and data in main memory permanently? The answer is no for two reasons:

– Main memory is small to store all programs and data.– Main memory (RAM) is volatile storage device that

loses its contents when power is turned off or otherwise lost.

• Therefore, to store all programs and data permanently you use secondary storage such as hard disk and diskette.


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.


Storage-Device Hierarchy

Size Small

Size Large

Fast and Expensive

Slow and

Cheap


Storage-Device Hierarchy

• Registers, cache, and main memory are volatile.

• All storage after main memory are non-volatile.

• Caching: Check first cache memory if data not there go to main memory and copy it into cache under the assumption that there is a high probability that it will be needed again.

• Data must be moved from secondary storage into main memory before use.

• Data transfer from cache to CPU and registers is usually a hardware function with no operating system control.

• Data transfer from disk to memory is usually controlled by the operating system.

• Cache Coherency and Consistency is the state that exists in a multiprocessor system, when any shared data is held by 2 or more caches, and no 2 caches hold different values of such a shared data simultaneously.


Hardware Protection

•Dual-Mode Operation

• I/O Protection

•Memory Protection

•CPU Protection


Dual-Mode Operation

• Protection is needed for any shared resource.

• 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 supervisor mode or system mode) – execution done on behalf of operating system.


Dual-Mode Operation (Cont.)

• At system boot time, the hardware starts in monitor mode. The O.S. is then loaded, and starts user processes in user mode.

• Whenever, an interrupt occurs, the hardware switches from user mode to monitor mode.

• Whenever, the O.S. gains control of the computer, it is in monitor mode.

• If you do not have dual mode then you can wipe or write over the O.S. Example: MS-DOS for 8088 architecture does not have a dual mode.

• MS-Widows NT and IBM OS/2 take advantage of dual mode feature and provide greater protection for the O.S.


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.

• Privileged instructions can be issued only in monitor mode.

monitor user

Interrupt/fault

set user 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).


Memory Protection

• We want to protect the O.S. from access by user programs, and to protect user programs from one another.

• 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.


A Base And A limit Register Define A Logical Address Space

Limit register = 300040 - 420940


Protection Hardware

• This protection is accomplished by the CPU hardware comparing every address generated in user mode with the registers.

• The base and limit registers can be loaded by only the O.S.


Operating-System Structures

• System Components

• Operating System Services

• System Calls

• System Programs

• System Structure

• Virtual Machines


Common System Components

• Process Management

• Main Memory Management

• File Management

• I/O System Management

• Secondary Storage Management

• Networking

• Protection System

• Command-Interpreter System


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 task.

• Processes can create sub-processes to execute concurrently.

• A program by itself is not a process; a program is a passive entity, whereas a process is an active entity.

• The execution of a process must progress in a sequential fashion. The CPU executes one instruction of the process after another until the process completes.

• Operating System processes: Those execute system code.

• User processes: Those that execute user code.


Process Management (Cont.)

• The operating system is responsible for the following activities in connection with process management.– Process creation and deletion.– Process suspension and resumption.– Provision of mechanisms for:

1) process synchronization

2) process communication– Deadlock handling


Main-Memory Management

• Memory is a large array of words or bytes, each with its own address. It is a repository (storage) of quickly accessible data shared by the CPU and I/O devices.

• Main memory is a volatile storage device. It loses its contents in the case of system failure.

• The operating system is responsible for the following activities in connections with memory management:

– Keep track of which parts of memory are currently being used and by whom.

– Decide which processes to load when memory space becomes available.

– Allocate and deallocate memory space as needed.


File Management

• A file is a collection of related information defined by its creator. Commonly, files represent programs (both source and object forms) and data.

• A file consists of a sequence of bits, bytes, lines, or records whose meanings are defined by their creators.

• The operating system is responsible for the following activities in connections with file management:

– File creation and deletion.– Directory creation and deletion.– Support of primitives for manipulating files and

directories.– Mapping files onto secondary storage.– File backup on stable (nonvolatile) storage media.


I/O System Management

• The I/O system consists of:

– A buffer-caching system

– A general device-driver interface

– Drivers for specific hardware devices

• The O.S. hides the peculiarities of specific hardware devices from the user.


Secondary-Storage Management

• Since main memory (primary storage) is volatile and too small to accommodate all data and programs permanently, the computer system must provide secondary storage to back up main memory.

• Most modern computer systems use disks as the principle on-line storage medium, for both programs and data.

• The operating system is responsible for the following activities in connection with disk management: – Free space management– Storage allocation– Disk scheduling


Networking (Distributed Systems)

• A distributed system is a collection of processors that do not share memory or a clock. Each processor has its own local memory and clock.

• The processors in the system are connected through a communication network.

• A distributed system provides user access to various system resources.

• Access to a shared resource allows:– Computation speed-up – Increased data availability– Enhanced reliability


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 usage.

– specify the controls to be imposed.

– provide a means of enforcement.


Command-Interpreter System

• Command-Interpreter system is a system program, which is the interface between the user and the operating system.

• Command-Interpreter system is known as the shell.

• Some operating systems provide a user-friendly interface (mouse-based window) such as, Macintosh and Microsoft Windows.

• Some operating systems provide text interface (commands are typed on keyboard) such as MS-DOS and Unix shells.


Command-Interpreter System (Cont.)

• Many commands are given to the operating system by control statements which deal with:

– I/O handling– process creation and management– secondary-storage management– main-memory management– file-system access – protection – networking

• The program that reads and interprets control statements is called variously:

– control-card interpreter– command-line interpreter– shell (in UNIX)

• Its function is to get and execute the next command statement.


Operating System Services

• Program execution – system capability to load a program into memory and to run it.

• I/O operations – since user programs cannot execute I/O operations directly, the operating system must provide some means to perform I/O.

• File-system manipulation – program capability to read, write, create, and delete files.

• Communications – exchange of information between processes executing either on the same computer or on different systems tied together by a network. Implemented via shared memory or message passing.

• Error detection – ensure correct computing by detecting errors in the CPU (such as power failure) and memory hardware, in I/O devices (such

as connection failure), or in user programs.


Additional Operating System Functions

Additional functions exist not for helping the user, but rather for ensuring efficient system operations.

• Resource allocation – allocating resources, such as CPU cycles, main memory, file storage, I/O devices, to multiple users or multiple jobs running at the same time.

• Accounting – keep track of and record which users use how much and what kinds of computer resources for account billing or for accumulating usage statistics.

• Protection – ensuring that all access to system resources is controlled.


System Calls

• System calls provide the interface between a running program and the operating system.

– Generally available as assembly-language instructions.– Languages defined to replace assembly language for

systems programming allow system calls to be made directly (e.g., C. Bliss, PL/360, PERL)

• Three general methods are used to pass parameters between a running program and the operating system.

– Pass parameters in registers.– Store the parameters in a table in memory, and the

table address is passed as a parameter in a register.– Push (store) the parameters onto the stack by the

program, and pop off the stack by operating system.


Passing of Parameters As A Table

X: Parameter

for callLoad

address x

System call 13

User Program

X

Register

Operating System

Code for

system call 13

Use parameters from table x


MS-DOS Execution

At System Start-up Running a Program

Free Memory

Command Interpreter

Kernel Kernel

Command Interpreter

Free Memory

Process

(a) (b)


UNIX Running Multiple Programs

Free Memory

Process C

Process B

Process D

Interpreter

Kernel


Communication Models

Message Passing Shared Memory

(a) (b)

Process B

Process A

Kernel

Process A

Process B

Kernel

Shared Memory


System Calls Categories

• System calls can be grouped into 5 categories:

1. Process Control: end, abort, load, execute, create process, terminate process, allocate and free memory.

2. File Manipulation: create file, delete file, open file, close file, read file, and write file.

3. Device Manipulation: request device, release device, read, write.

4. Information Maintenance: get time or date, set time or date, get process or file or device.

5. Communications: create or delete communication connection, send and receive messages.


System Programs

• System programs provide a convenient environment for program development and execution. They can be divided into several categories:

– File manipulation: create, delete, copy, rename, print files.– Status information: Some programs ask the system for date and time,

disk space, number of users.– File modification: Text editors to create and modify the content of files

stored on disk.– Programming language support: Compilers and assemblers are

provided to the user with the O.S. – Program loading and execution: After a program is assembled or

compiled, it must be loaded into memory to be executed. The system may provide loaders, linkage editors and debuggers.

– Communications: Programs provide mechanism for creating virtual connections among processes, users, and computer systems, such as sending messages and transferring files.


System Structure – Simple Approach

•MS-DOS – written to provide the most functionality in the least space

– Not divided into modules

– Although MS-DOS has some structure, its interfaces and levels of functionality are not well separated.

– No dual mode and no hardware protection (Intel 8088) in MS-DOS.


MS-DOS Layer Structure

Application Program

Resident System Program

MS-DOS Device Drivers

ROM BIOS Device Drivers


System Structure – Simple Approach (Cont.)

• UNIX – limited by hardware functionality, the original UNIX operating system had limited structuring. The UNIX OS consists of two separable parts.

– Systems programs– The kernel

1. Consists of everything below the system-call interface and above the physical hardware

2. Provides the file system, CPU scheduling, memory management, and other operating-system functions; a large number of functions for one level.


UNIX System Structure

Users

Shells and commands Compilers and interpreters

System Libraries

System – call interface to the kernel

Signals terminal handling character

I/O system terminal drivers

File system swapping I/O

system disk and tape drivers

CPU scheduling page replacement demand paging virtual memory

Kernel interface to the hardware

Terminal controllers

terminals Device controllers

disks and tapesMemory controllers

physical memoryHardware

Kernel


System Structure – Layered Approach

• The operating system is divided into a number of layers (levels), each built on top of lower layers. The bottom layer (layer 0), is the hardware; the highest (layer N) is the user interface.

• Main advantage of layered approach:– First layer can be debugged without any concern, because it uses

only basic hardware.– Once the first layer is debugged, its correct functioning while second

layer is worked on and so on.– If an error occur we know in which layer.– Each layer is implemented using those operations provided by

lower-level layers.– A layer does not need to know how the low-level operations are

implemented, it needs to know what these operations do.– Each layer hides the existence of data structures, operations, and

hardware from higher-level layer.


An Operating System Layer

Layer M

Layer M - 1Hidden operations

existing operations

new operations

It consists of data structures and a set of routines that can be invoked by higher-level layers.


Layered Structure of the THE OS

• A layered design was first used in THE operating system. Its six layers are as follows:

---------------------------------------------------------------------------Layer 5: user programs

---------------------------------------------------------------------------Layer 4: buffering for I/O devices

---------------------------------------------------------------------------Layer 3: operator-console device deriver

---------------------------------------------------------------------------Layer 2: memory management (virtual memory)

---------------------------------------------------------------------------Layer 1: CPU scheduling

---------------------------------------------------------------------------Layer 0: hardware

---------------------------------------------------------------------------


Venus Layer Structure

• It consists of 7 layers as follows:---------------------------------------------------------------------------

Layer 6: user programs---------------------------------------------------------------------------

Layer 5: device drivers and schedulers---------------------------------------------------------------------------

Layer 4: virtual memory---------------------------------------------------------------------------

Layer 3: I/O channel---------------------------------------------------------------------------

Layer 2: CPU scheduling---------------------------------------------------------------------------

Layer 1: instruction interpreter---------------------------------------------------------------------------

Layer 0: hardware---------------------------------------------------------------------------


Venus Layer Structure (Cont.)

•The low layers (layer 4 to layer 0) put into micro-code.

•Advantage: Additional speed of execution and clarity between micro-coded layers and higher layers.

•Order is important in layers.

•Each layer adds overhead to the system call, and the result is a system call takes longer than one does on a non-layered system.


OS/2 Layer Structure

•OS/2 is a descendant of MS-DOS that adds multitasking and dual mode operation.

•Access to low-level facilities directly by user is not allowed.

•OS/2 provides more control over the hardware and more knowledge of which resources each user program is using than the MS-DOS.


OS/2 Layer Structure (Cont.)

application application application

Application – programming interface

subsystems

subsystems

subsystems

System kernel

• memory management• task dispatching• device management

Device driver

Device driver

Device driver

Device driver


Virtual Machines (VM)

• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware.

• A virtual machine provides an interface identical to the underlying bare hardware.

• Example of disk systems in virtual machines: suppose you have 3 disks drives in physical machine and you want 7 disks drives in virtual machine. The solution is to provide virtual disks, which are identical in all respects except size, called minidisks in IBM’s VM. The sum of the sizes of all minidisks must be less than the actual amount of physical disk space available.


Virtual Machines (Cont.)

•The resources of the physical computer are shared to create the virtual machines.

• Implementation:

– Two modes for protection.

– In virtual machine, we must have a virtual user mode and a virtual monitor mode.

– Both modes run in a physical user mode.


System Models

Non-virtual Machine Virtual Machine

processes

processes

processes

processes

kernel

kernel kernelkernel

hardware hardware

Virtual machine

Programming interface

(a) (b)


Advantages/Disadvantages of 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.

• The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine.

• Sharing minidisk: Files can be shared.

• Sharing using a network of virtual machines, each of which can send information over the virtual communication networks. Note that virtual minidisks and communication networks are modeled after physical disks and communication networks. The virtual minidisks and communication networks are implemented in software.

• So, two ways for sharing in virtual: minidisks and communication networks.


Advantages/Disadvantages of Virtual Machines (Cont.)

•Modifying or changing an O.S. is difficult. Do it at night; the machine should be stopped, since the O.S. runs on and controls the entire machine.

•Virtual machines solves system compatibility problems: For example, MS-DOS programs on Intel CPU-based systems. Users like to use Sun Microsystems and DEC (faster processors). Solution is to create a virtual Intel machine on top of the Sun processors.


System Design Goals

•User goals – operating system should be convenient to use, easy to learn, reliable, safe, and fast.

•System goals – operating system should be easy to design, implement, and maintain, as well as flexible, reliable, error-free, and efficient.


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.

•An operating system is far easier to port (move to some other hardware) if it is written in a high-level language.

Documents

1.1 Operating System Concepts Chapter 1: Introduction What is an operating system? Simple Batch Systems Multiprogramming Batched Systems Time-Sharing Systems