Module 1: Introduction

1
Module 1 Introduction

• What is an operating system?
• Multiprogramming Systems
• Time-Sharing Systems
• Parallel Systems
• Distributed Systems
• Real -Time Systems
• System Calls and APIs

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 Components

• 1. Hardware provides basic computing resources
  (CPU, memory, I/O devices).
• 2. Operating system controls and coordinates
  the use of the hardware among the various
  application programs for the various users.
• 3. Applications programs define the ways in
  which the system resources are used to solve the
  computing problems of the users (compilers,
  database systems, video games, business
  programs).
• 4. Users (people, machines, other computers).

4
Abstract View of System Components
5
Multiprogramming and Timesharing

• Multiprogramming CPU is multiplexed (shared)
  among a number of jobs -- while one job waiting
  for I/O, another can use CPU.
• Advantages
• CPU is kept busy.
• Disadvantages
• Hardware and O.S. became significantly more
  complex for handling and scheduling multiple
  jobs.
• Timesharing switch CPU among jobs for
  pre-defined time interval.
• Most O.S. issues arise from trying to support
  multiprogramming -- CPU scheduling, deadlock,
  protection, memory management, virtual memory,
  etc.

6
Why need multiprogramming? I/O Times vs. CPU times

• 400 MHz Pentium II 400 million cycles/second
• 10 cycles/instruction 40 million
  instructions/second
• Read 1 disk block 20 msec
• CPU can do (40 x 106) / 103 40,000
  instructions/msec
• Thus
• In time to read one disk block, CPU can do 20
  40,000 800,000 instructions !!

7
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.
• Advantages of parallel system
• Increased throughput
• Economical
• Increased reliability
• graceful degradation or fault-tolerant
• The ability to continue providing service
  proportional to the level of surviving hardware.

8
Parallel Systems (Cont.)

• Symmetric multiprocessing (SMP)
• Each processor runs an identical copy of the
  operating system.
• Many processes can run at once without
  performance deterioration.
• Most modern operating systems support SMP
• Asymmetric multiprocessing
• Each processor is assigned a specific task
  master processor schedules and allocates work to
  slave processors.
• More common in extremely large systems
• Also used in PS/3 and Sbox
• PS/3 has 8 CPUs with one CPU acting as a master

9
Symmetric Multiprocessing Architecture
10
Distributed Systems

• Distribute the computation among several physical
  processors.
• Loosely coupled system each processor has its
  own local memory processors communicate with one
  another through various communications lines,
  such as high-speed buses or telephone lines.
• Advantages of distributed systems.
• Resource sharing
• sharing and printing files at remote sites
• processing information in a distributed database
• using remote specialized hardware devices
• Computation speedup load sharing
• Reliability detect and recover from site
  failure, function transfer, reintegrate failed
  site
• Communication message passing

11
Network vs. Distributed OS

• Network OS
• A configuration in which there is a network of
  application machines, typically a workstations
  with multiple server machines.
• Server machines can be file servers, printer
  servers, mail, etc.
• Each computer has its own OS. The user must be
  aware that there are multiple independent machine
  and must deal with them explicitly.
• NW OS allows machines to interact with each other
  by having a common communication architecture.
• Distributed OS
• A common OS shared by a network of computers
• Offers the illusion of a unified system image,
  i.e. single system image
• i.e, a pool of interconnected computers appears
  as a single unified computing resource
• can say that these machines have a Single System
  Image (SSI) Buyya vol.1, 1999.
• It provides the user with transparent access to
  the resources of multiple machines
• Therefore
• less autonomy between computers
• gives the impression there is a single operating
  system controlling the network.
• Research vehicle
• Examples Bell Labs Inferno Plan 9, Mach,
  Amobea by Tanenbaum, Chorus by CMU

12
Computing Architectures

• Vector Computers (VC) - proprietary system
• provided the breakthrough needed for the
  emergence of computational science, buy they were
  only a partial answer.
• Massively Parallel Processors (MPP) -proprietary
  systems
• high cost and a low performance/price ratio.
• Symmetric Multiprocessors (SMP)
• suffers from scalability
• Distributed Systems
• difficult to use and hard to extract parallel
  performance.
• Clusters - gaining popularity
• High Performance Computing - Commodity
  Supercomputing
• High Availability Computing - Mission Critical
  Applications
• Grid Computing
• Cloud Computing

13
Cloud Computing

• http//www.youtube.com/watch?v8RMWO9JxZjA
  http//www.youtube.com/watch?vhplXnFUlPmgfeature
  fvw
• beta.cloudos.com
• eyeos.info eyeos.mobi (has and an internet
  browser)
• oos.cc

14
Scalability vs. SSI
15
Real-Time Systems

• 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. Deadline support
• 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. No deadline support
• Limited utility in industrial control or robotics
• Useful in applications (multimedia, virtual
  reality) requiring advanced operating-system
  features.

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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 and memory hardware,
  in I/O devices, or in user programs.

17
System Programs

• System programs provide a convenient environment
  for program development and execution. The can
  be divided into
• File manipulation
• Status information
• File modification
• Programming language support
• Program loading and execution
• Communications
• Application programs
• Most users view of the operation system is
  defined by system programs, not the actual system
  calls.

18
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
• 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.

19
System Calls

• Provide "direct access" to operating system
  services (e.g., file system, I/O routines, memory
  allocate free routines) by user programs.
• System calls execute instructions that control
  the resources of the computer system, e.g., I/O
  instructions for devices.
• We want such privileged instructions to be
  executed only by a system routine, under the
  control of the OS!
• As we will see, system calls are special, and in
  fact, are treated as a special case of
  interrupts.
• Programs that make system calls were
  traditionally called "system programs" and were
  traditionally implemented in assembly language.

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System Call Scenario (cont.)

• File system

• Userprogram

• I/Odevices

• memory

• Operating System

• User program is confined!

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

22
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 supervisor mode or system
  mode) execution done on behalf of operating
  system.

23
Dual-Mode Operation (Cont.)

• Mode bit added to computer hardware to indicate
  the current mode monitor (0) or user (1).
• Part of EFLAG in x86 architecture
• Part of PSW in Motorola architecture
• When an interrupt or fault occurs hardware
  switches to monitor mode.

Interrupt/fault
monitor
user
set user mode

• Privileged instructions can be issued only in
  monitor mode.
• 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,
  traps if it executes a privileged instruction).

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API vs. System Calls

• Mostly accessed by programs via a high-level
  Application Program 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)
• Why use APIs rather than system calls?

25
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
  ReadFile()
• 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

26
API System Call OS Relationship
27
Standard C Library Example

• C program invoking printf() library call, which
  calls write() system call

28
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 intended system
  call in OS kernel and returns status of the
  system call and any return values
• The callers 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)

29
System Calls (cont.)

• Now, system calls can be made from high-level
  languages, such as C and Modula-2 (to a degree).
• Unix has about 32 system calls read(), write(),
  open(), close(), fork(),
• using trap instructions
• i read(fd,80, buffer)
• push buffer
• push 80
• push fd
• trap read
• pop i
• Each system call had a particular number.
  Instruction set has a special instruction for
  making system calls
• SVC (IBM 360/370)
• trap (PDP 11)
• tw (PowerPC) - trap word
• tcc (Sparc)
• break (MIPS)

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User vs. System Mode

• case

• i-call

• System(or kernel)memory

• traptoO.S.

• l

• n

• code for read

• User Program
• (text)

• trap n

• Special mode-bit set in PSW register
• mode-bit 0 gt user program executing
• mode-bit 1 gt system routine executing
• Privileged instructions possible only when
  mode-bit 1!

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System Call Scenario

• User program executing (mode-bit 0)
• User makes a system call
• hardware sets mode-bit to 1
• system saves state of user process
• branch to case statement in system code
• branch to code for system routine based on system
  call number
• copy parameters from user stack
• execute system call (using privileged
  instructions)
• restore state of user program
• hardware resets mode-bit
• return to user process