Cs8493 unit 1

CS8493 OPERATING SYSTEMS
UNIT – I
Dr.A.Kathirvel, Professor &Head, Computer Science & Engg.
M N M Jain Engineering College, Chennai 1

Unit - I
OPERATING SYSTEMS OVERVIEW
Computer System Overview-Basic Elements, Instruction
Execution, Interrupts, Memory Hierarchy, Cache Memory,
Direct Memory Access, Multiprocessor and Multicore
Organization. Operating system overview-objectives and
functions, Evolution of Operating System.- Computer System
Organization Operating System Structure and Operations-
System Calls, System Programs, OS Generation and System
Boot.
Abraham Silberschatz, Peter Baer Galvin and Greg Gagne, “Operating System
Concepts”, 9 th Edition, John Wiley and Sons Inc., 2012. 2

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
3

Computer System Overview
Computer system can be divided into four components:
Hardware – provides basic computing resources
CPU, memory, I/O devices
Operating system
Controls and coordinates use of hardware among various applications
and users
Application programs – define the ways in which the system
resources are used to solve the computing problems of the
users
Word processors, compilers, web browsers, database systems, video
games
Users
People, machines, other computers
4

Four Components of a Computer System
5

What Operating Systems Do
 Depends on the point of view
 Users want convenience, ease of use and good performance
 Don’t care about resource utilization
 But shared computer such as mainframe or minicomputer must
keep all users happy
 Users of dedicate systems such as workstations have dedicated
resources but frequently use shared resources from servers
 Handheld computers are resource poor, optimized for usability
and battery life
 Some computers have little or no user interface, such as
embedded computers in devices and automobiles
6

Operating System Definition
 OS is a resource allocator
 Manages all resources
 Decides between conflicting requests for efficient and fair resource use
 OS is a control program
 Controls execution of programs to prevent errors and improper use of the
computer
 No universally accepted definition
 “Everything a vendor ships when you order an operating system” is a
good approximation
 But varies wildly
 “The one program running at all times on the computer” -the kernel.
 Everything else is either
 a system program (ships with the operating system) , or
 an application program. 7

Computer Startup
bootstrap program is loaded at power-up or
reboot
Typically stored in ROM or EPROM, generally
known as firmware
Initializes all aspects of system
Loads operating system kernel and starts
execution
8

Basic Elements
Computer-system operation
One or more CPUs, device controllers connect through
common bus providing access to shared memory
Concurrent execution of CPUs and devices competing
for memory cycles
9

Instruction Execution
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
10

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
A trap or exception is a software-generated
interrupt caused either by an error or a user
request
An operating system is interrupt driven
11

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
12

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 OS 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
OS indexes into I/O device table to determine device status and
to modify table entry to include interrupt
14

Storage Definitions and Notation Review
The basic unit of computer storage is the bit. A bit can contain one of two values, 0 and 1. All
other storage in a computer is based on collections of bits. Given enough bits, it is amazing how
many things a computer can represent: numbers, letters, images, movies, sounds, documents,
and programs, to name a few. A byte is 8 bits, and on most computers it is the smallest
convenient chunk of storage. For example, most computers don’t have an instruction to move a
bit but do have one to move a byte. A less common term is word, which is a given computer
architecture’s native unit of data. A word is made up of one or more bytes. For example, a
computer that has 64-bit registers and 64-bit memory addressing typically has 64-bit (8-byte)
words. A computer executes many operations in its native word size rather than a byte at a time.
Computer storage, along with most computer throughput, is generally measured and
manipulated in bytes and collections of bytes.
A kilobyte, or KB, is 1,024 bytes
a megabyte, or MB, is 1,0242 bytes
a gigabyte, or GB, is 1,0243 bytes
a terabyte, or TB, is 1,0244 bytes
a petabyte, or PB, is 1,0245 bytes
Computer manufacturers often round off these numbers and say that a megabyte is 1 million
bytes and a gigabyte is 1 billion bytes. Networking measurements are an exception to this
general rule; they are given in bits (because networks move data a bit at a time).
15

Storage Structure
 Main memory – only large storage media that the CPU can access
directly
Random access
Typically volatile
 Secondary storage – extension of main memory that provides large
nonvolatile storage capacity
 Hard 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
 Solid-state disks – faster than hard disks, nonvolatile
Various technologies
Becoming more popular
16

Memory Hierarchy
Storage systems organized in hierarchy
Speed
Cost
Volatility
Caching – copying information into faster storage
system; main memory can be viewed as a cache for
secondary storage
Device Driver for each device controller to manage
I/O
Provides uniform interface between controller and
kernel 17

Cache Memory
Important principle, performed at many levels in a
computer (in hardware, operating system, software)
Information in use copied from slower to faster storage
temporarily
Faster storage (cache) checked first to determine if
information is there
If it is, information used directly from the cache (fast)
If not, data copied to cache and used there
Cache smaller than storage being cached
Cache management important design problem
Cache size and replacement policy
19

Direct Memory Access
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 one interrupt is generated per block,
rather than the one interrupt per byte
20

How a Modern Computer Works
A von Neumann architecture
21

Multiprocessor Architecture
Most systems use a single general-purpose processor
Most systems have special-purpose processors as well
Multiprocessors systems growing in use and importance
Also known as parallel systems, tightly-coupled systems
Advantages include:
 Increased throughput
 Economy of scale
 Increased reliability – graceful degradation or fault tolerance
Two types:
 Asymmetric Multiprocessing – each processor is assigned a specie task.
 Symmetric Multiprocessing – each processor performs all tasks
22

Multiprocessing Architecture A Dual-Core Design
 Multi-chip and multicore
 Systems containing all chips
Chassis containing multiple separate systems
23

Clustered Systems
 Like multiprocessor systems, but multiple systems
working together
 Usually sharing storage via a storage-area network (SAN)
 Provides a high-availability service which survives failures
 Asymmetric clustering has one machine in hot-standby mode
 Symmetric clustering has multiple nodes running applications,
monitoring each other
 Some clusters are for high-performance computing (HPC)
 Applications must be written to
use parallelization
 Some have distributed lock manager
(DLM) to avoid conflicting operations
24

Operating System Overview
 Multiprogramming (Batch system) needed for efficiency
 Single user cannot keep CPU and I/O devices busy at all times
 Multiprogramming organizes jobs (code and data) so CPU always has one to
execute
 A subset of total jobs in system is kept in memory
 One job selected and run via job scheduling
 When it has to wait (for I/O for example), OS switches to another job
 Timesharing (multitasking) is logical extension in which CPU switches jobs so
frequently that users can interact with each job while it is running, creating
interactive computing
 Response time should be < 1 second
 Each user has at least one program executing in memory process
 If several jobs ready to run at the same time  CPU scheduling
 If processes don’t fit in memory, swapping moves them in and out to run
 Virtual memory allows execution of processes not completely in memory
25

Memory Layout for Multiprogrammed System
26

Operating-System Operations
 Interrupt driven (hardware and software)
 Hardware interrupt by one of the devices
 Software interrupt (exception or trap):
Software error (e.g., division by zero)
Request for operating system service
 Other process problems include infinite loop, processes modifying each other or
the operating system
 Dual-mode operation allows OS to protect itself and other system components
 User mode and kernel mode
 Mode bit provided by hardware
Provides ability to distinguish when system is running user code or kernel
code
Some instructions designated as privileged, only executable in kernel mode
System call changes mode to kernel, return from call resets it to user
 Increasingly CPUs support multi-mode operations
 i.e. virtual machine manager (VMM) mode for guest VMs 27

Transition from User to Kernel Mode
 Timer to prevent infinite loop / process hogging resources
Timer is set to interrupt the computer after some time period
Keep a counter that is decremented by the physical clock.
Operating system set the counter (privileged instruction)
When counter zero generate an interrupt
Set up before scheduling process to regain control or terminate
program that exceeds allotted time
28

Process Management
 process is a program in execution. It is a unit of work within the
system. Program is a passive entity, process is an active entity.
 Process needs resources to accomplish its task
CPU, memory, I/O, files
Initialization data
 Process termination requires reclaim of any reusable resources
 Single-threaded process has one program counter specifying
location of next instruction to execute
Process executes instructions sequentially, one at a time, until
completion
 Multi-threaded process has one program counter per thread
 Typically system has many processes, some user, some operating
system running concurrently on one or more CPUs
Concurrency by multiplexing the CPUs among the processes /
threads 29

Process Management Activities
Creating and deleting both user and system
processes
Suspending and resuming processes
Providing mechanisms for process
synchronization
Providing mechanisms for process communication
Providing mechanisms for deadlock handling
The operating system is responsible for the following
activities in connection with process management:
30

Memory Management
To execute a program all (or part) of the instructions must
be in memory
All (or part) of the data that is needed by the program
must be in memory.
Memory management determines what is in memory and
when
Optimizing CPU utilization and computer response to users
Memory management activities
Keeping track of which parts of memory are currently being used
and by whom
Deciding which processes (or parts thereof) and data to move into
and out of memory
Allocating and deallocating memory space as needed 31

Storage Management
 OS provides uniform, logical view of information storage
Abstracts physical properties to logical storage unit - file
Each medium is controlled by device (i.e., disk drive, tape drive)
Varying properties include access speed, capacity, data-
transfer rate, access method (sequential or random)
 File-System management
Files usually organized into directories
Access control on most systems to determine who can access
what
OS activities include
Creating and deleting files and directories
Primitives to manipulate files and directories
Mapping files onto secondary storage
Backup files onto stable (non-volatile) storage media 32

Mass-Storage Management
 Usually disks used to store data that does not fit in main memory or
data that must be kept for a “long” period of time
 Proper management is of central importance
 Entire speed of computer operation hinges on disk subsystem and its
algorithms
 OS activities
 Free-space management
 Storage allocation
 Disk scheduling
 Some storage need not be fast
 Tertiary storage includes optical storage, magnetic tape
 Still must be managed – by OS or applications
 Varies between WORM (write-once, read-many-times) and RW (read-write)
33

Performance of Various Levels of Storage
Movement
between
levels of
storage
hierarchy can
be explicit or
implicit
Migration of data
“A” from Disk to
Register
 Multitasking environments must be careful to use most recent value, no matter
where it is stored in the storage hierarchy. Multiprocessor environment must
provide cache coherency in hardware such that all CPUs have the most recent
value in their cache
 Distributed environment situation even more complex - Several copies of a datum
can exist
34

I/O Subsystem
One purpose of OS is to hide peculiarities of
hardware devices from the user
I/O subsystem responsible for
Memory management of I/O including buffering
(storing data temporarily while it is being transferred),
caching (storing parts of data in faster storage for
performance), spooling (the overlapping of output of
one job with input of other jobs)
General device-driver interface
Drivers for specific hardware devices
35

Protection and Security
 Protection – any mechanism for controlling access of processes or
users to resources defined by the OS
 Security – defense of the system against internal and external attacks
Huge range, including denial-of-service, worms, viruses, identity
theft, theft of service
 Systems generally first distinguish among users, to determine who
can do what
User identities (user IDs, security IDs) include name and
associated number, one per user
User ID then associated with all files, processes of that user to
determine access control
Group identifier (group ID) allows set of users to be defined and
controls managed, then also associated with each process, file
Privilege escalation allows user to change to effective ID with
more rights
36

Kernel Data Structures
Many similar to standard programming data structures
Singly linked list
Doubly linked list
Circular linked list
37

Kernel Data Structures
 Hash function can create a hash map
 Bitmap – string of n binary digits representing the status of n
items
 Linux data structures defined in
 include files <linux/list.h>,
<linux/kfifo.h>, <linux/rbtree.h>
38

Computing Environments - Traditional
Stand-alone general purpose machines
But blurred as most systems interconnect with others (i.e.,
the Internet)
Portals provide web access to internal systems
Network computers (thin clients) are like Web terminals
Mobile computers interconnect via wireless networks
Networking becoming ubiquitous – even home systems
use firewalls to protect home computers from Internet
attacks
39

Computing Environments - Mobile
Handheld smartphones, tablets, etc
What is the functional difference between them
and a “traditional” laptop?
Extra feature – more OS features (GPS, gyroscope)
Allows new types of apps like augmented reality
Use IEEE 802.11 wireless, or cellular data networks
for connectivity
Leaders are Apple iOS and Google Android
40

Computing Environments – Distributed
Distributed computiing
Collection of separate, possibly heterogeneous, systems
networked together
Network is a communications path, TCP/IP most common
Local Area Network (LAN)
Wide Area Network (WAN)
Metropolitan Area Network (MAN)
Personal Area Network (PAN)
Network Operating System provides features between
systems across network
Communication scheme allows systems to exchange messages
Illusion of a single system
41

Computing Environments: Client/Server
 Client-Server Computing
 Dumb terminals supplanted by smart PCs
 Many systems now servers, responding to requests generated by
clients
Compute-server system provides an interface to client to
request services (i.e., database)
File-server system provides interface for clients to store and
retrieve files
42

Computing Environments – Peer2Peer
Another model of distributed system
P2P does not distinguish clients and
servers
Instead all nodes are considered peers
May each act as client, server or both
Node must join P2P network
Registers its service with central lookup service on
network, or
Broadcast request for service and respond to
requests for service via discovery protocol
Examples include Napster and Gnutella,
Voice over IP (VoIP) such as Skype
43

Computing Environments-Virtualization
Allows operating systems to run applications within other
OS
Vast and growing industry
Emulation used when source CPU type different from
target type (i.e. PowerPC to Intel x86)
Generally slowest method
When computer language not compiled to native code –
Interpretation
Virtualization – OS natively compiled for CPU, running
guest OS also natively compiled
Consider VMware running WinXP guests, each running
applications, all on native WinXP host OS
VMM (virtual machine Manager) provides virtualization services44

Use cases involve laptops and desktops running multiple
OSes for exploration or compatibility
Apple laptop running Mac OS X host, Windows as a guest
Developing apps for multiple OSes without having multiple
systems
QA testing applications without having multiple systems
Executing and managing compute environments within data
centers
VMM can run natively, in which case they are also the
host
There is no general purpose host then (VMware ESX and Citrix
XenServer)
45

46

Computing Environments–Cloud Comp.
 Delivers computing, storage, even apps as a service across a network
 Logical extension of virtualization because it uses virtualization as the base for it
functionality.
 Amazon EC2 has thousands of servers, millions of virtual machines, petabytes
of storage available across the Internet, pay based on usage
 Many types
 Public cloud – available via Internet to anyone willing to pay
 Private cloud – run by a company for the company’s own use
 Hybrid cloud – includes both public and private cloud components
 Software as a Service (SaaS) – one or more applications available via the
Internet (i.e., word processor)
 Platform as a Service (PaaS) – software stack ready for application use via the
Internet (i.e., a database server)
 Infrastructure as a Service (IaaS) – servers or storage available over Internet
(i.e., storage available for backup use)
47

Cloud computing environments composed of
traditional OSes, plus VMMs, plus cloud
management tools
Internet connectivity requires
security like firewalls
Load balancers spread traffic
across multiple applications
Computing Environments–Cloud Comp.
48

Computing Environments – Real-Time Embedded
Systems
Real-time embedded systems most prevalent form of
computers
Vary considerable, special purpose, limited purpose OS, real-
time OS
Use expanding
Many other special computing environments as well
Some have OSes, some perform tasks without an OS
Real-time OS has well-defined fixed time constraints
Processing must be done within constraint
Correct operation only if constraints met
49

Open-Source Operating Systems
Operating systems made available in source-code format
rather than just binary closed-source
Counter to the copy protection and Digital Rights
Management (DRM) movement
Started by Free Software Foundation (FSF), which has
“copyleft” GNU Public License (GPL)
Examples include GNU/Linux and BSD UNIX (including
core of Mac OS X), and many more
Can use VMM like VMware Player (Free on Windows),
Virtualbox (open source and free on many platforms -
http://www.virtualbox.com)
Use to run guest operating systems for exploration
50

Operating System Services
 Operating systems provide an environment for execution of
programs and services to programs and users
 One set of operating-system services provides functions that are
helpful to the user:
User interface - Almost all operating systems have a user
interface (UI).
Varies between Command-Line (CLI), Graphics User Interface
(GUI), Batch
Program execution - The system must be able to load a program
into memory and to run that program, end execution, either
normally or abnormally (indicating error)
I/O operations - A running program may require I/O, which may
involve a file or an I/O device
51

Operating System Services (Cont.)
 One set of operating-system services provides functions that are helpful to the user
 File-system manipulation - The file system is of particular interest. Programs
need to read and write files and directories, create and delete them, search
them, list file Information, permission management.
 Communications – Processes may exchange information, on the same
computer or between computers over a network
Communications may be via shared memory or through message passing
(packets moved by the OS)
 Error detection – OS needs to be constantly aware of possible errors
May occur in the CPU and memory hardware, in I/O devices, in user
program
For each type of error, OS should take the appropriate action to ensure
correct and consistent computing
Debugging facilities can greatly enhance the user’s and programmer’s
abilities to efficiently use the system
52

Operating System Services (Cont.)
 Another set of OS functions exists for ensuring the efficient operation of
the system itself via resource sharing
Resource allocation - When multiple users or multiple jobs running
concurrently, resources must be allocated to each of them
Many types of resources - CPU cycles, main memory, file storage,
I/O devices.
Accounting - To keep track of which users use how much and what
kinds of computer resources
Protection and security - The owners of information stored in a
multiuser or networked computer system may want to control use of
that information, concurrent processes should not interfere with each
other
Protection involves ensuring that all access to system resources is
controlled
Security of the system from outsiders requires user authentication,
extends to defending external I/O devices from invalid access
attempts
53

A View of Operating System Services
54

User Operating System Interface - CLI
CLI or command interpreter allows direct
command entry
Sometimes implemented in kernel, sometimes by
systems program
Sometimes multiple flavors implemented – shells
Primarily fetches a command from user and executes it
Sometimes commands built-in, sometimes just names
of programs
If the latter, adding new features doesn’t require shell
modification
55

Bourne Shell Command Interpreter
56

User Operating System Interface - GUI
User-friendly desktop metaphor interface
Usually mouse, keyboard, and monitor
Icons represent files, programs, actions, etc
Various mouse buttons over objects in the interface cause
various actions (provide information, options, execute function,
open directory (known as a folder)
Invented at Xerox PARC
Many systems now include both CLI and GUI interfaces
Microsoft Windows is GUI with CLI “command” shell
Apple Mac OS X is “Aqua” GUI interface with UNIX kernel
underneath and shells available
Unix and Linux have CLI with optional GUI interfaces (CDE, KDE,
GNOME) 57

Touch screen Interfaces
Touch screen devices
require new interfaces
Mouse not possible
or not desired
Actions and selection
based on gestures
Virtual keyboard for
text entry
 Voice commands.
58

System Calls
Programming interface to the services provided by
the OS
Typically written in a high-level language (C or C++)
Mostly accessed by programs via a high-level
Application Programming Interface (API) rather
than direct system call use
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
(JVM)
60

Example of System Calls
 System call sequence to copy the contents of one file to another file
61

System Call Implementation
Typically, a number associated with each system call
System-call interface maintains a table indexed according to these
numbers
The system call interface invokes the 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 compiler)
62

API – System Call – OS Relationship
63

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 registers
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
64

Parameter Passing via Table
65

Types of System Calls
Process control
create process, terminate process
end, abort
load, execute
get process attributes, set process attributes
wait for time
wait event, signal event
allocate and free memory
Dump memory if error
Debugger for determining bugs, single step execution
Locks for managing access to shared data between processes
66

Types of System Calls
 File management
 create file, delete file
 open, close file
 read, write, reposition
 get and set file attributes
 Device management
 request device, release device
 read, write, reposition
 Get & set device attributes
 logically attach or detach devices
 Protection
 Control access to resources
 Get and set permissions
 Allow and deny user access
 Information maintenance
 get time or date, set time or date
 get system data, set system data
 get and set process, file, or device
attributes
 Communications
 create, delete communication
connection
 send, receive messages if message
passing model to host name or
process name
From client to server
 Shared-memory model create and
gain access to memory regions
 transfer status information
 attach and detach remote devices
67

Ex: Windows and Unix System Calls
Standard C Library Example
 C program invoking
printf() library call,
which calls write()
system call
68

Example: MS-DOS
 Single-tasking
 Shell invoked when system
booted
 Simple method to run
program
 No process created
 Single memory space
 Loads program into memory,
overwriting all but the kernel
 Program exit -> shell reloaded
At system startup running a program
69

Example: FreeBSD
 Unix variant
 Multitasking
 User login -> invoke user’s choice of shell
 Shell executes fork() system call to create
process
Executes exec() to load program into
process
Shell waits for process to terminate or
continues with user commands
 Process exits with:
 code = 0 – no error
 code > 0 – error code
70

System Programs
System programs provide a convenient environment for
program development and execution. They can be
divided into:
File manipulation
Status information sometimes stored in a File modification
Programming language support
Program loading and execution
Communications
Background services
Application programs
Most users’ view of the operation system is defined by
system programs, not the actual system calls
71

System Programs
 Provide a convenient environment for program development and
execution
Some of them are simply user interfaces to system calls; others
are considerably more complex
 File management - Create, delete, copy, rename, print, dump, list,
and generally manipulate files and directories
 Status information
Some ask the system for info - date, time, amount of available
memory, disk space, number of users
Others provide detailed performance, logging, and debugging
information
Typically, these programs format and print the output to the
terminal or other output devices
Some systems implement a registry - used to store and retrieve
configuration information
72

System Programs (Cont.)
 File modification
Text editors to create and modify files
Special commands to search contents of files or perform
transformations of the text
 Programming-language support - Compilers, assemblers, debuggers
and interpreters sometimes provided
 Program loading and execution- Absolute loaders, relocatable
loaders, linkage editors, and overlay-loaders, debugging systems for
higher-level and machine language
 Communications - Provide the mechanism for creating virtual
connections among processes, users, and computer systems
Allow users to send messages to one another’s screens, browse
web pages, send electronic-mail messages, log in remotely,
transfer files from one machine to another
73

System Programs (Cont.)
 Background Services
Launch at boot time
Some for system startup, then terminate
Some from system boot to shutdown
Provide facilities like disk checking, process scheduling, error
logging, printing
Run in user context not kernel context
Known as services, subsystems, daemons
 Application programs
Don’t pertain to system
Run by users
Not typically considered part of OS
Launched by command line, mouse click, finger poke
74

OS: Design and Implementation
 Design and Implementation of OS not “solvable”, but some approaches have
proven successful
 Internal structure of different Operating Systems can vary widely
 Start the design by defining goals and specifications
 Affected by choice of hardware, type of system
 User goals and System 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
 Important principle to separate
 Policy: What will be done? Mechanism: How to do it?
 Mechanisms determine how to do something, policies decide what will be done
 The separation of policy from mechanism is a very important principle, it allows
maximum flexibility if policy decisions are to be changed later (example – timer)
 Specifying and designing an OS is highly creative task of software engineering75

Implementation
Much variation
Early OSes in assembly language
Then system programming languages like Algol, PL/1
Now C, C++
Actually usually a mix of languages
Lowest levels in assembly
Main body in C
Systems programs in C, C++, scripting languages like PERL, Python,
shell scripts
More high-level language easier to port to other hardware
But slower
Emulation can allow an OS to run on non-native hardware
76

Operating System Structure
General-purpose OS is very large
program
Various ways to structure ones
Simple structure – MS-DOS
More complex -- UNIX
Layered – an abstrcation
Microkernel -Mach
77

Simple Structure -- MS-DOS
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
78

Non Simple Structure -- UNIX
 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
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
79

Traditional UNIX System Structure
Beyond simple but not fully layered
80

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.
 With modularity, layers are
selected such that each uses
functions (operations) and
services of only lower-level
layers
81

Microkernel System Structure
 Moves as much from the kernel into user space
 Mach example of microkernel
Mac OS X kernel (Darwin) partly based on Mach
 Communication takes place between user modules using message
passing
 Benefits:
Easier to extend a microkernel
Easier to port the operating system to new architectures
More reliable (less code is running in kernel mode)
More secure
 Detriments:
Performance overhead of user space to kernel space
communication 82

Microkernel System Structure
Application
Program
File
System
Device
Driver
Interprocess
Communication
memory
managment
CPU
scheduling
messagesmessages
microkernel
hardware
user
mode
kernel
mode
83

Modules
Many modern operating systems implement loadable
kernel modules
Uses object-oriented approach
Each core component is separate
Each talks to the others over known interfaces
Each is loadable as needed within the kernel
Overall, similar to layers but with more flexible
Linux, Solaris, etc
Solaris Modular Approach
84

Hybrid Systems
Most modern operating systems are actually not one pure
model
Hybrid combines multiple approaches to address performance,
security, usability needs
Linux and Solaris kernels in kernel address space, so monolithic,
plus modular for dynamic loading of functionality
Windows mostly monolithic, plus microkernel for different
subsystem personalities
Apple Mac OS X hybrid, layered, Aqua UI plus Cocoa
programming environment
Below is kernel consisting of Mach microkernel and BSD Unix
parts, plus I/O kit and dynamically loadable modules (called kernel
extensions)
85

Mac OS X Structure
graphical user interface
Aqua
application environments and services
kernel environment
Java Cocoa Quicktime BSD
Mach
I/O kit kernel extensions
BSD
86

iOS
Apple mobile OS for iPhone, iPad
Structured on Mac OS X, added functionality
Does not run OS X applications natively
Also runs on different CPU architecture (ARM vs. Intel)
Cocoa Touch Objective-C API for developing apps
Media services layer for graphics, audio, video
Core services provides cloud computing,
databases
Core operating system, based on Mac OS X kernel
87

Android
 Developed by Open Handset Alliance (mostly Google)
Open Source
 Similar stack to IOS
 Based on Linux kernel but modified
Provides process, memory, device-driver management
Adds power management
 Runtime environment includes core set of libraries and Dalvik virtual
machine
Apps developed in Java plus Android API
Java class files compiled to Java bytecode then translated to
executable than runs in Dalvik VM
 Libraries include frameworks for web browser (webkit), database
(SQLite), multimedia, smaller lib c
88

Android Architecture
Application Framework
Android runtime
Core Libraries
Dalvik
virtual machine
Libraries
SQLite openGL
surface
manager
webkit libc
media
framework
89

Operating-System Debugging
 Debugging is finding and fixing errors, or bugs
 OS generate log files containing error information
 Failure of an application can generate core dump file capturing
memory of the process
 Operating system failure can generate crash dump file containing
kernel memory
 Beyond crashes, performance tuning can optimize system
performance
Sometimes using trace listings of activities, recorded for analysis
Profiling is periodic sampling of instruction pointer to look for
statistical trends
 Kernighan’s Law: “Debugging is twice as hard as writing the code in
the first place. Therefore, if you write the code as cleverly as possible,
you are, by definition, not smart enough to debug it.” 90

Performance Tuning
Improve performance
by removing
bottlenecks
OS must provide means
of computing and
displaying measures of
system behavior
For example, “top”
program or Windows
Task Manager
91

DTrace
 DTrace tool in Solaris,
FreeBSD, Mac OS X allows
live instrumentation on
production systems
 Probes fire when code is
executed within a provider,
capturing state data and
sending it to consumers of
those probes
 Example of following
XEventsQueued system call
move from libc library to
kernel and back
92

Dtrace (Cont.)
 DTrace code to record amount of
time each process with UserID 101
is in running mode (on CPU) in
nanoseconds
93

Operating System Generation
Operating systems are designed to run on any of
a class of machines; the system must be
configured for each specific computer site
SYSGEN program obtains information concerning
the specific configuration of the hardware system
Used to build system-specific compiled kernel or
system-tuned
Can general more efficient code than one general
kernel
94

System Boot
When power initialized on system, execution starts at a
fixed memory location
Firmware ROM used to hold initial boot code
Operating system must be made available to hardware so
hardware can start it
Small piece of code – bootstrap loader, stored in ROM or
EEPROM locates the kernel, loads it into memory, and starts it
Sometimes two-step process where boot block at fixed location
loaded by ROM code, which loads bootstrap loader from disk
Common bootstrap loader, GRUB, allows selection of
kernel from multiple disks, versions, kernel options
Kernel loads and system is then running
95

Cs8493 unit 1

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Cs8493 unit 1