NETW 3005

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NETW 3005
System Structure and Processes
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Reading

• For this lecture, you should have read Chapter 2
  (Sections 1-9).

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Last lecture

• History and types of operating systems
• batch systems, multiprogramming systems, time
  sharing systems, etc.
• Operating system tasks
• process management, storage management, I/O
  device management, user interface.

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This lecture

• Hierarchical Structure in Operating Systems
• System calls and interrupts
• Representing processes in Operating Systems
• Overview of process scheduling.

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Hierarchical structure in OS

• An operating system, like any complex computer
  system, must be carefully designed.
• One central requirement is modularity.
• Distinction between system programs and
  application programs.

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Application programs

• Ones that ordinary users interact with
• Word-processors
• Database packages
• Web browsers
• Compilers, editors, IDEs, etc
• . . .

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System programs

• Provide a general-purpose lower-level function.
  System functions include
• file manipulation create, delete, copy etc.
• status info date, available memory.
• program loading and execution.
• communication between processes.
• Command interpreters (a.k.a. shell programs). The
  set of system programs de?ne the user interface.

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Other system functions

• Not strictly part of the OS, but often packaged
  with it.
• Programs to modify files (text editors, search,
  transform).
• Compilers, assemblers and interpreters.

The set of system programs defines the user
interface.
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Degrees of modularity

• Different operating systems enforce different
  degrees of modularity.
• Ideally you want to oblige all system and
  application programs to talk to the hardware via
  the kernel.

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MS-DOS
Application programs
System programs
Kernel operations
Device drivers
Terminal drivers
Memory manager
COSC 243 (Operating Systems)
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Talking to the kernel system calls
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System calls

• Written in the same language as kernel (typically
  C).
• Available to Assembler and (at least) some HLLs
  C, Perl, etc.
• The set of system calls is termed the programmer
  interface to the system.

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A simple system program  copy

• cp file1 file2
• Open file1 Create file2.
• Loop Read from file1 Write to file2.
• Close file1 Close file2.

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Types of system call (1)

• Process control
• create, terminate, suspend, resume, abort.
• File manipulation
• open, close, read, write
• Device manipulation
• request, release, read, write.

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Types of system call (2)

• Housekeeping
• get/set time or date
• get/set attributes (process, device, file)
• Communications
• set up link,
• send/receive message

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InterruptsHow system calls are implemented

• CPU responds to interrupts no matter what else it
  happens to be doing.
• An interrupt transfers control to an appropriate
  module in the kernel.
• A system call transfers control to the kernel by
  generating an interrupt (sometimes called a trap
  in this context).

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Responding to an interrupt

• Effectively a Jump to Subroutine
• current instruction address (PC) is saved
• control transferred to fixed address, depending
  on the interrupt.
• The interrupt vector is an array of locations
  that hold the addresses of these routines,
  usually held in low memory.

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Implementation issues

• How do we guarantee that the interrupt-handing
  routine wont affect the interrupted process?
• What happens if an interrupt occurs while an
  interrupt-handling routine is executing?

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Virtual machines

• System calls allow the OS to hide the low-level
  hardware from application programs.
• In a virtual machine these system calls are
  executed by a program which emulates the
  hardware.
• This hardware may, or may not be the same as the
  actual hardware.

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processes
kernel
hardware
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Benefits of virtual machines

• Protection users arent even aware there are
  other users.
• Good for operating systems R D. (No down-times
  just give a system programmer her own virtual
  machine.)
• A way of solving system-compatibility problems.

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Problems with virtual machines

• Speed virtual machines are slower.
• Implementation is very difficult, e.g. resource
  allocation (particularly disc).

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Java

• Compiled Java code is called byte-code.
• It is designed to run on the Java Virtual Machine
  (JVM).
• Think about the command-line process of compiling
  and running a Java program as opposed to
  compiling and running a C program.

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Processes

• Recall a process is not just a program it is a
  dynamic entity.
• A given program (e.g. emacs) could be executing
  many times on a given machine the machine must
  represent each execution as a separate process.

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Components of a process (1)

• Code section the program code itself
• Data section any global variables used by the
  program
• Process stack any local variables currently
  being used (subroutine parameters, return
  addresses, etc.)
• Program counter a pointer to some place in the
  program code.

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Components of a process (2)

• Contents of CPU registers.
• Memory management information.
• Accounting information who owns the process, how
  long its been running, how much CPU time its
  used so far, etc.

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Process state

• The operating system keeps track of the state of
  each process.
• A process can be in any of the following states
• new
• running
• waiting/blocked
• ready
• terminated

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An Example from MacOS X

• oucs1046 chandley ps ax

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Macos X processes (1)
PID TTY TIME CMD 1 ?? 126.15
/sbin/launchd 10 ?? 003.03 /usr/libexec/kextd 1
1 ?? 106.37 /usr/sbin/DirectoryService 12 ??
027.57 /usr/sbin/notifyd 13 ?? 1511.87
/usr/sbin/syslogd 14 ?? 208.74
/usr/sbin/configd 15 ?? 018.55
/usr/sbin/distnoted 16 ?? 2523.28
/usr/sbin/mDNSResponder launchd 20 ?? 003.56
/usr/sbin/securityd -i
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Macos X processes (2)
24 ?? 124.42 /usr/sbin/ntpd -n -g -p 25 ??
002.00 /usr/sbin/cron 26 ?? 1758.14
/usr/sbin/update 27 ?? 000.01
/sbin/SystemStarter 31 ?? 000.03
/System/Library/CoreServices/
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Macos X processes (3)

• Produced 75 different processes.
• Several distinct classes
• Daemons security, cron, update, etc.
• Core Services Dock, Finder, etc.
• Network and communications.
• Application programs Preview, Power-Point,
  Word, Adobe Acrobat, etc.

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Scheduling an overview
Disk
Job2, Job2, Job3, Job4,
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Types of scheduler

• Long-term scheduler (batch systems) decides
  which jobs loaded onto the disk should be moved
  to main memory.
• Short-term scheduler (a.k.a. CPU scheduler)
  chooses how to allocate the CPU between the
  processes which are ready to execute.

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Scheduling queues

• The relationships between processes are
  represented by the O/S as queues.
• Job queue all the processes in the system
  (including those on disk).
• Ready queue all the processes which are ready to
  execute.
• Device queue all the processes waiting to use a
  particular device. (One queue per device.)

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The Flow of Processes in an OS
ready queue
I/O request
fork
interrupt mechanism
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Process creation

• A process is created (spawned) by another
  process we talk of parent and child processes.
• When you launch an application, the terminal
  process spawns the application process.
• Processes are able to call other processes,
  just as a program can call functions.

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Differences

• A child process can (and often does) run
  concurrently with its parent.
• A child process neednt be constrained to use the
  resources of its parent (although it may be, and
  often is).

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A process tree (1)
root
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A process tree (2)

• Note that users are treated as processes by the
  operating system.
• How does that work?
• Answer users are really represented to the
  system as shell processes.
• An important notion the root user.

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Homework

• The Unix command ps displays information about
  processes on the system. Read the man page for ps
  (i.e. do man ps), and try out some of the
  options.

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Next Lecture
Threads and Data Sharing Chapter 4 (Sections 1-4)