Programmer System Interaction 1

1
Programmer System Interaction -1

• Objective To develop test Application
  Programs written in some High Level Language.
• Requirements A computer System with
• a) Requisite Peripherals.
• b) Operating System with Proper I/O device
  driver programs installed and resident on Disk.
• c) The corresponding Translator Program as
  well as System Libraries available in Disk.

2
Programmer System Interaction- 2

• Assumptions
• a) The Programmer develops the Program
  without knowing anything about the Computer
  System where it is to be executed.
• b) The Computer System in Question is already
  having an Operating System along with other
  relevant System Programs resident on Disk and Non
  Volatile Main memory BIOS ROM/EPROM

3
Summarized Steps from Program Development to
Execution

• Develop the Program.
• Search for an Installation having a Computer
  System with all desired , compatible System
  Softwares installed and available on disk.
• Get a Log In there.
• Key in the developed program using some Editor A
  System Program via keyboard to create a High
  Level Source Code With the Aid of the concerned
  O.S. through the relevant Device Driver(s) .
• Translate that Source Code using the existing
  Translators Another System Programs and then
  Link up the used Libraries to create an
  Executable Module.
• Run execute it by the help of the O.S.

4
Steps involved in Program Development - 1

• Identify Input Output Specifications i.e. what
  are given and what is to be produced.
• Develop an algorithm.
• Design/Specify a Data Structure (if needed) i.e.
  specify an Abstract Data Type (ADT).
• Select a High level programming language (HLL).

5
Steps involved in Program Development - 2

•    A) Implement the specified algorithm by a
  program (Source Code) using the selected HLL.
• B) Design and implement the ADT using the
  Data type(s) of the selected HLL.

6
Program Development Steps Example - 1

• Ex. Given two integers A B , Compute the
  quotient Q A/B and remainder R A modulo B .
• Check for non zero B to ensure feasibility of
  division

7
Program Development Steps Example - 2

• Algorithm (in English like Steps)
• 1. Input value of A
• 2. Input value of B
• 3. If (B is Equal to 0) then
• Print B is invalid Input Again go to step 2.
• 4. (Here B is Non Zero) proceed with Division
• Q A/B
• 5. R A mod B
• 6. Output (Print A,B,Q,R)
• 7. Stop

8
The Implementing HLL (C ) Program - 1

• include ltstdio.hgt / C Library Linkage
  Directive/
• main ( ) / Start/
• / begin main/
• int I_A, I_B, I_Q, I_R / Variable
  declaration and
  storage reservation Directives . Non
  Executable Data Segment . /
• / Code Segment Starts Here . Reserve Variable
  Spaces on STACK/
• scanf ("d", I_A) / Input A , a System
  Call, Data Movement Activity /
• scanf ("d", I_B) / Input B , another
  System Call/
• while (I_B 0) / if B 0 , Control Flow/
  
• /begin while/
• printf ("B is invalid Input Again
  \n") / System Call/
• scanf ("d", I_B) / Again input
  B (System Call)/
• //end while

9
The implementing HLL (C) Program - 2

• / Here B is non Zero/
• I_Q I_A / I_B / Data Processing
  Activity/
• I_R I_A I_B / Data Processing
  Activity/
• printf ("A d \f Bd\f Qd\f Rd\n",
  I_A, I_B, I_Q, I_R) / System Call/
• / end main /

10
The Actual Boot Process of a Typical Computer
System

• In computing, booting is a bootstrapping process
  that starts the O.S. when the user powers on any
  computer system.
• A boot sequence is the set of operations the
  computer performs when it is switched on to load
  an operating system and subsequently hand over
  control to it.
• Booting , in a way, readies a Computer System
  for use.

11
Towards Program Execution on a PC ( The BOOT UP
Sequence - 1)

• Assumed that the High Level Source Code has
  already been developed Off Line.
• The System is Powered on.
• The internal power supply of the System
  subsequently turns on and initializes. Before it
  stabilizes , the chipset will generate a reset
  signal to the processor until it receives the
  Power Good signal from the power supply.
• Receipt of Power Good Signal , signifies that the
  processor is ready to execute program stored in
  Memory provided it knows where to start.

12
From Power on to Boot up on a PC - 2

• 5. In order to achieve uniformity , the processor
  always look at the same place in the system non
  volatile Memory CMOS Flash / BIOS ROM for the
  very first machine Instruction to execute.
  This is normally location FFFFFFF0h, right at the
  end of the system memory.
• This location is chosen because since there
  are only 16 bytes left from there to the end of
  conventional memory (FFFFFFFF H) , this location
  just contains a JUMP lt Start Addressgt "
  instruction telling the processor where to go to
  find the real START Up program.
• This way enables one to keep the Flexibility
  so that the size of the Starting Program can be
  changed without creating compatibility problems.

13
From Power on to Boot up on a PC - 3

• 3. This start up program next performs the
  power-on self test (POST). The POST is a built-in
  diagnostic program that checks the system
  hardware to ensure that everything is present and
  functioning properly, so that the BOOT process
  can proceed smoothly later.
• This involves testing the following Devices in
  turn
• a) The Video Card.
• b) The Keyboard Mouse.
• c) The Main Memory.
• d) The BOOT Devices namely the CD/DVD Drives ,
  FLASH Drives (if connected) followed by the
  internal Hard Disk Drives.
• If there are any fatal errors during this
  testing phase , the boot process stops with a
  beep. Because of absence of O.S. it is difficult
  to isolate the problem area in case of any error.
  

14
From Power on to Boot up on a PC - 8

• 9. If the BIOS supports the Plug and Play
  standard, it will detect and configure Plug and
  Play devices at this time and display a message
  on the screen for each one it finds.
• 10. The BIOS will display a summary screen
  about the system's configuration like Processor ,
  Memory, CACHE, Disk interfaces etc.

15
From Power on to Boot up on a PC - 9

• 11. The BIOS begins the search for the Initial
  Program Loading Location (IPL) / drive to boot
  from based on an User alterable boot sequence
  BIOS setting

16
IPL Device Options in PCs

• In a modern BIOS, the user can select one of
  several interfaces from which to boot. These
  include hard disk, floppy, SCSI, CDROM, Zip,
  LS-120, a network interface card using PXE, or
  USB (USB-FDD, USB-ZIP, USB-CDROM, USB-HDD).
• For example, one can install Microsoft Windows on
  the first hard disk and Linux on the second. By
  changing the BIOS boot device, the user can
  select the operating system to load.

17
The Computer Start Up Problem- 1

• Most computer systems can only execute code
  found in the memory (ROM or RAM/ RWM).
• Modern operating systems are mostly stored on
  hard disks (occasionally on CDs/DVDs, USB flash
  drives, and the like).
• Just after a computer has been turned on, it
  doesn't have an operating system in memory.

18
The Computer Start Up Problem - 2

• 4. The computer's Start up Program is ill
  equipped to be able to perform complicated
  actions of the operating system, such as loading
  a program from disk.
• 5. A seemingly irresolvable paradox is created
  to load the operating system into memory, one
  appears to need to have an operating system
  already installed in the Memory.
• 6. Any Operating System, normally, is too big a
  program to be fitted into the Non Volatile
  portion of the main memory.

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The BOOTSTRAP Loader

• The solution to the paradox involves using a
  special small program, called a bootstrap loader
  or boot loader.
• This program doesn't have the full functionality
  of an operating system, but is tailor-made to
  load enough other software for the operating
  system to start.
• Often, multiple-stage boot loaders are used, in
  which several small programs summon each other,
  until the last of them loads the operating system
  from the Bootable Device / Initial Program
  Location (IPL).
• The name bootstrap loader comes from the image of
  one pulling oneself up by one's bootstraps
  (bootstrapping).

20
From Power on to Boot up on a PC - 10

• 12. Having identified its target boot drive, the
  BIOS looks for this BOOT STRAP Loader . If it is
  searching a hard disk, it looks for the master
  boot record at cylinder 0, head 0, sector 1 (the
  first sector on the disk) if it is searching
  some other Media like a Floppy/ CD/DVD , it looks
  at the same address on the disk for a volume
  boot sector.

21
The Master Boot Record (MBR)

• Every Bootable Device ( usually hard disk
  partition C ) must have a consistent "starting
  point" where key information is stored about the
  disk, such as how many partitions it has, what
  sort of partitions they are, etc. There also
  needs to be somewhere that the BIOS can load the
  initial boot program that starts the process of
  loading the operating system. The place where
  this information is stored is called the master
  boot record (MBR). It is also sometimes called
  the master boot sector or even just the boot
  sector. (Though the master boot sector should not
  be confused with volume boot sectors, which are
  different.)
• The master boot record is always located at
  cylinder 0, head 0, and sector 1, the first
  sector on the disk This is the consistent
  "starting point" that the disk always uses. When
  the BIOS boots the machine, it will look here for
  instructions and information on how to boot the
  disk and load the operating system.

22
Disk Partition - 1

• Master Partition Table This small table contains
  the descriptions of the partitions that are
  contained on the hard disk. There is only room in
  the master partition table for the information
  describing four partitions. Therefore, a hard
  disk can have only four true partitions, also
  called primary partitions. Any additional
  partitions are logical partitions that are linked
  to one of the primary partitions. One of the
  partitions is marked as active, indicating that
  it is the one that the computer should use for
  booting up.

23
Disk Partitions Key Observations - 1

• What is a Partition ? A certain area or amount
  of space on a hard drive, be it 1 or 100 of the
  drives total capacity.
• Primary Partitions A partition that is used to
  start an operating system, although you can use
  primary partitions that don't contain the
  operating system
• Logical Partitions A partition that contains
  Data.
• Active Partitions Any primary partition that
  has an operating system installed on it may be
  designated as the Active partition simply for the
  sake of convenience in making it the System
  partition. Active partition and System partition
  mean the same thing.
• The boot files reside on the System partition    
  ---   The system files reside on the Boot
  partition
• The BOOT partition usually contains the Master
  Boot Record / MBR .

24
Disk Partitions Key Observations - 2

• There can be up to a maximum of four primary
  partitions on a single basic disk.
• The Primary partitions do not have to come before
  Extended partitions.
• A drive is not required to have any Primary
  partitions .
• Any CD-ROM Drive , while it contains a blank CD,
  is not shown as containing a partition because
  there is no data contained on the CD.

25
Typical Structure of an MBR - 2

• Master Boot Code / Boot Loader Code The master
  boot record contains the small initial boot
  program that the BIOS loads and executes to start
  the boot process. This program eventually
  transfers control to the boot program stored on
  whichever partition is used for booting the PC.

26
Importance of the MBR

• Any corruption / erasure / change in MBR may make
  the whole Disk UNUSABLE.

27
From Power on to Boot up on a PC - 11

• 13. The BOOT LOADER code , that is a part of the
  Master Boot Record (MBR) of the Active Primary
  Partition of the chosen IPL /BOOT DEVICE , takes
  over CONTROL from the BIOS.

28
Boot Loader Structure

• Boot loaders are small piece of code that helps
  to load the Operating System.
• These may face peculiar constraints, especially
  in size for instance on the IBM PC and
  compatibles, the first stage of boot loaders must
  fit into the first 446 bytes of the Master Boot
  Record, in order to leave room for the 64-byte
  partition table and the 2-byte AA55h 'signature',
  which the BIOS requires for a proper boot loader.

29
Second Stage Boot Loader ( Multiple O.S.)

• The MBR , in this case, contains a
  second-stage boot loader, such as NTLDR, LILO or
  GRUB.
• This second stage BOOT LOADER will then be able
  to load the operating system proper depending on
  the users response , and finally transfer
  execution to it.
• Subsequently the system will initialize itself,
  and may load device drivers and other programs
  that are needed for the normal operation of the
  OS.

30
BOOT.ini File

• The Bootable O.S. Selector Menu for WINDOWS
  environment.
• Stored in the very first Primary Active
  Partition ( C for a Hard Disk Drive).

31
Cold Start vs. Warm Start

• This process is called a "cold boot" (since the
  machine was off, or cold, when it started). A
  "warm boot" is the same thing except it occurs
  when the machine is rebooted using
  CtrlAltDelete or similar. In this case
  the POST is skipped and the boot process
  continues roughly from the step 8 onwards.

32
Boot Process Time

• The boot process is considered complete when the
  computer is ready to interact with the user or
  the operating system is capable of running
  ordinary applications. Typical modern PCs boot in
  about a minute (of which about 15 seconds are
  taken by the preliminary boot loaders, and the
  rest by loading the operating system), while
  large servers may take several minutes to boot
  and to start all services - to ensure high
  availability, they bring up some services before
  others.
• Most embedded systems must boot almost instantly
  -- for instance, waiting a minute for the
  television to come up is not acceptable.
  Therefore they have their whole operating system
  in ROM or flash memory, so it can be executed
  directly.

33
The BOOT Process Summary - 1

• On power on CPU is directed to a specific/fixed
  address ( BIOS ROM address) which usually
  stores a Power On Self Test (POST) Program..
• After executing the POST stored in BIOS ROM, the
  CPU searches for the designated Boot Device in a
  pre specified Order as stored in the Set-Up
  information ( Non Volatile but User Alterable
  Area of main Memory) .
• After locating a pre-fixed valid Boot Device, the
  CPU is directed to a specific Area of that BOOT
  Device (usually the 0th Sector of the Primary
  Hard Disk).
• That area contains the BOOT Loader Code within
  the MASTER BOOT RECORD (MBR).

34
The BOOT Process Summary - 2

• 5. This MBR Code is read into the Main Memory
  and executed.
• 6. On Execution this MBR Code may load the a
  2nd Level BOOT Loader ( For Multi Boot System)
  stored in the neighborhood place into Main
  Memory.
• 7. This 2nd Level Boot Loader (like LILO or
  GRUB) contains entry points to the various
  Operating System along with provision for the
  User to choose the designated Operating System as
  well as a default O.S. that will start after a
  specified Time Interval.
• 8. Relevant Portion of the Chosen / Selected
  Operating System is then LOADED into the main
  memory.

35
Post Boot Up Scenario A

• The overall system control is taken over by
  the main memory resident Operating System(O.S.).
  
• 1. The O.S. Command Interpreter / Graphic User
  Interface starts executing. At this point only
  the KERNEL / O.S. / Supervisor Process is running
  , which first loads the User Code / PASSWD File
  into the Kernel Data Area in the Electronic Read
  Write Memory . Then it starts WAITING for user
  response via the Log In process.
• 2. User selects Log In Option either through
• Keyboard or Mouse that sends an Interrupt
  to the System in response the System displays the
  USERCODE Screen. Still O.S. process is executing
  waiting for User to type in the User Code.
• 3. User starts typing in the user code via
  keyboard . Each key press sends an Interrupt to
  the system that causes the following.

36
Post Boot Up Scenario B

• 5. The running O.S. / Kernel / Supervisor Process
  recognizes and services that interrupt in the
  following manner
• a) Receives the Key Code from the Keyboard
  Interface Buffer into the KERNEL Area of the Read
  Write Memory.
• b) Locates the Display Font corresponding to
  the stored key code .
• c) Sends that Display Font to the Display
  Interface for displaying on the screen.
• d) All these steps a), b) c) are repeated
  for each key press till user presses ltEntergt /
  ltReturngt Key. On pressing Return Key the
  following events take place.

37
Post Boot Up Scenario C

• 6. The running O.S. / Kernel / Supervisor Process
  recognizes and services that interrupt caused by
  User pressing the RETURN / ENTER Key in the
  following manner
• a) Receives the Key Code from the Keyboard
  Interface Buffer into the KERNEL Area of the Read
  Write Memory.
• b) Composes all the Previous Characters
  pressed by the User as user code (stored in the
  Kernel Area of Read Write Memory) into a String .
• c) Searches for a match for that string in the
  User code list stored as a part of User code /
  Password Block in the KERNEL Data Area.
• d) If it DOESNOT find a match then displays
  the message like INVALID USERCODE TRY AGAIN and
  WAITS for new user code to be typed in .
• e) On finding a match / getting a valid User
  code the resident running O.S. process asks for
  PASSWD . It accepts and handles user typed in
  password in exactly the same manner as in the
  case of user code.

38
Post User Log In

• The very first User Process gets created . This
  becomes the PARENT user process.
• B. This parent user process needs to create
  other processes to proceed further.
• C. The User Process is running currently.
• D. The user can now create other threads by
  opening new WINDOW.
• E. Next user types in the Command to invoke the
  EDITOR . This command is handled in exactly the
  same way as the earlier User code and Pass Word
  till the PARSING ( Command Recognition) phase.
• F. Subsequently it loads the Editor Code from the
  Disk to Memory , creates the EDITOR process , and
  sets it running as a CHILD Process to the user
  process.

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Editor Process Running

• 1. User types in the Source Code via Keyboard
  which gets stored in the EDITOR Buffer
• ( Read Write Memory) .
• 2. Subsequently this Buffer can be saved as a
  Source file on Secondary Media (like DISK) based
  on User command. The EDITOR process uses File
  System commands which in turn employs Disk Write
  Routine to achieve this.
• 3. Editor Process is still running can be
  closed / put to background by the User.
• N.B Secondary Media already contains the
  relevant translator program (Compiler) for the
  selected HLL in Executable (.EXE ) form.
• 4. The User creates another Window and gives
  COMPILE Command.

40
Post Compilation Command - 1

• 5. Compiler and Source File brought into main
  memory (in parts if needed) from secondary store
  (Disk) by the KERNEL / O.S. running as a service
  within the logged in user process employing
  memory management File System Calls Disk Read
  Routine in response to the compile command.
• 6. The compilation process is created as
  ANOTHER Child to the User Process, The EDITOR
  process if running, gets pre-empted from Running
  State to Ready State to make way for the
  Compilation process.
• 7. The Compilation Process executes with source
  file as input to produce the translated assembly
  language equivalent of source. It stores this
  translated code in the Memory OR in the DISK as
  .ASM /.S file in parts within swap area if
  needed using memory management File System
  Calls Disk Write Routine. On completion
  Compilation Process is terminated i.e. it no
  longer occupies memory.
• .

41
The Assembly Process

• 8. Assembler executable program (Assembly
  language translator) available on disk and .ASM /
  .S file are loaded into main memory (in parts if
  needed) from secondary store (Disk) by the O.S.
  using memory management File System Call Disk
  Read Routine based on assemble directive.
• 9. Assembler program gets executed i.e. Assembler
  Process starts running as yet another Child
  Process to convert the assembly language source
  .ASM file) to equivalent machine coded version
  (object .OBJ file) on Disk in parts if needed
  using memory management File System Call Disk
  Write Routine . On completion it also gets
  terminated.
• .

42
Linking

• 10. Linker module (existing on Disk) and machine
  code object (.OBJ file generated by the
  Assembler) are loaded into main memory from disk
  (in parts if needed) by the O.S. using memory
  management File System Call Disk Read Routine
  based on link command.
• 11. Execute linker program i.e. Create Linker
  process as another child of the parent user
  process to resolve references to already existing
  system library modules and user modules. This
  converts .OBJ file to executable (.EXE)
  version/command (.COM) version and copies it onto
  disk in parts if needed using memory
  management File System Call Disk Write
  Routine . The linked libraries may / may not
  form part of the executable module ( STATIC /
  DYNAMIC Linking).
• .

43
Loading Execution

• 12.Bring loader module existing in disk into main
  memory.
• 13. Execute loader module to load some portion of
  the executable version (.EXE file) into main
  memory using memory management File System Call
  Disk Read Routine . Currently however linking
  and loading phases are done together by the same
  module ( Linking Loader / Linkage Editor that
  performs dynamic linking , loading relocation).
• 14. Execute the loaded linked program portion
  as another Child process of the User (this will
  cause the memory management part of the Operating
  System to automatically load copy back the
  remaining portions of the .EXE file from disk
  into main memory on demand using the File System
  Call Disk Read / Write routine to obtain
  result.

44
Various Execution Environment

• In most cases the steps starting from
  compilation of the Source File to loading the
  executable file into main memory are either
  combined in a BATCH File to form a single step.
  This is known as compile and go / translate and
  go.
• In LINUX / UNIX the steps of COMPILATION (
  TRANSLATING High Level Source to ASSEMBLY Code) ,
  ASSEMBLING ( ASSEMBLY Code to Equivalent Machine
  Coded Version Object File ) , LOADING Machine
  Object File DYNAMICALLY LINKING libraries are
  combined in a Batch File.
• When the executable code is brought into main
  memory it is in effect brings in the code first,
  then subsequently during execution it
  occupies/uses some more memory locations to store
  data and status , creates uses Stack , as well
  as employs some CPU registers and also may use
  some other resource (peripheral/file). This
  executable image of any program is termed as a
  process.

45
The Next Phase

• What Code gets Executed ? The Machine Code.
• High Level Code ? Assembly Level Code ? Machine
  Level Code. EXAMPLE ?