Structure of Processes

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Structure of Processes

• Chapter 6

Process State and Transition Data Structure for
Process Layout of System Memory
THE DESIGN OF THE UNIX OPERATING SYSTEM
Maurice J. bach Prentice Hall
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Process image in virtural memory
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Process Creation

• Assign a unique pid to the new process.
• Allocate space for all the elements of the
  process image. How much?
• The process control block is initialized. Inherit
  info from parent.
• The appropriate linkages are set for scheduling,
  state queues..
• Create and initialize other data structures

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Process interruption

• Two kinds of process interruptions interrupt and
  trap.
• Interrupt Caused by some event external to and
  asynchronous to the currently running process,
  such as completion of IO.
• Trap Error or exception condition generated
  within the currently running process. Ex illegal
  access to a file, arithmetic exception.
• Supervisor call explicit interruption

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Unix System V

• All user processes in the system have as root
  ancestor a process called init. When a new
  interactive user logs onto the system,
  initcreates a user process, subsequently this
  user process can create child processes and so
  on. init is created at the boot-time.
• Process states User running , kernel running,
  Ready in memory, sleeping in memory (blocked),
  Ready swapped (ready-suspended), sleeping swapped
  (blocked-suspended), created (new), zombie ,
  preempted (used in real-time scheduling).

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Cont.

• Similar to our 7 state model
• 2 running states User and Kernel
• ??transitions to other states (blocked,
  ready) must come from kernel running
• Sleeping states (in memory, or swapped)
  correspond to our blocking states
• A preempted state is distinguished from the ready
  state (but they form 1 queue)
• Preemption can occur only when a process is about
  to move from kernel to user mode

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Process States and Transitions
exit
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Data Structures for Process
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Data Structure for Process (contd.)
per process region table
Kernel region table
u area
main memory
Kernel process table
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State of a Process

• Process table entry
• Contains general fields of processes that must be
  always be accessible to the kernel
• U area
• further characteristics of the process only need
  to be accessible to the running process itself

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Process table entry

• State field user running, kernel running etc.
• Fields that allow the kernel to locate the
  process and u area. Requires while context switch
• Process size kernel know how much space to
  allocate for the process.
• User ID
• Process ID

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Process table entry (contd.)

• Event descriptor.
• Used when the process is in the "sleep" state.
• Scheduling parameters.
• Allow the kernel to determine the order in which
  processes move to the states "kernel running" and
  "user running
• A signal field.
• keeps the signals sent to a process but not yet
  handled.
• Various timers process execution time, resource
  utilization etc.

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U Area

• A pointer to the process table entry
• User IDs
• various Timer
• Execution time in user mode
• Execution Time in kernel mode
• An error field keeps error during system call
• Return value field result of system call

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U Area (contd.)

• I/O parameters
• Amount of data transfer
• Address of source and target etc.
• The current directory and current root
• User file descriptor table
• Limit fields
• Restrict process size
• Restrict size of the file it can write
• The control terminal field
• login terminal associated with the process, if
  one exists
• An array indicates how the process wishes to
  react to signal

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Per Process Region Table (Pregion)

• Each pregion entry points to the kernel region
  table
• Starting virtual (absolute) address of the region
• Permission filed
• read-only, read-write, read-execute

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Kernel Region table

• Kernel region table contains the pointer to the
  page table which keeps the physical memory address

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Regions
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Pages and Page Tables
Logical Page Number Physical Page
Number 0 177 1 54 2 209 3 17
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Pages and Page Tables (contd.)
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Process and Context Switching

• Clock interrupt The OS determines if the time
  slice of the currently running process is over,
  then switches it to Ready state, and dispatches
  another from Ready queue. Process switch
• Memory fault (Page fault) A page fault occurs
  when the requested program page is not in the
  main memory. OS (page fault handler) brings in
  the page requested, resumes faulted process.
• IO Interrupt OS determines what IO action
  occurred and takes appropriate action.

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Cont

• Process switch A transition between two
  memory-resident processes in a multiprogramming
  environment.
• Context switch Changing context from a executing
  program to an Interrupt Service Routine (ISR).
  Part of the context that will be modified by the
  ISR needs to be saved. This required context is
  saved and restored by hardware as specified by
  the ISR.

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The context of a process

• The context of the process consists of the
  contents of its (user) address space and the
  contents of hardware register and kernel data
  structure that relate to the process.
• User-level context
• ??Process Text (ie code read-only)
• ??Process Data
• ??User Stack (calls/returns in user mode)
• ??Shared memory (for IPC)
• Register level context
• Program counter,Processor status register,Stack
  pointer, General purpose register

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• System-level context
• ??Process table entry
• ??the actual entry concerning this process in the
  Process Table maintained by OS
• ??Process state, UID, PID, priority, event
  awaiting, signals sent, pointers to memory
  holding text, data...
• ??U (user) area
• ??additional process info needed by the kernel
  when executing in the context of this process
• ??effective UID, timers, limit fields, files in
  use .??Kernel stack (calls/returns in kernel
  mode)
• ??Per Process Region Table (used by memory
  manager).