Process

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Process

• Processes are executing or executable instances
  of programs.
• Processes in a modern OS have
• management information
• resources
• unique process identifier or PID
• Management information covers
• allocated resources
• event handling
• permissions
• scheduling information
• utilisation of resources
• Resources include
• memory allocated - address space
• open files, I/O channels, devices
• secondary storage allocations - swap space,
  memory mapping
• Address space is protected so only process may
  read/write to it.
• Memory protection stops process
• writing to address space of other processes or
  kernel
• crashing OS or other processes if it goes wrong

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Unix Process (I)

• Address space of Unix process is divided into 3
  memory segments
•  text read only program code 
• Data dynamic values of program data structures 
• Stack execution stack of program
• Unix processes operate either in application or
  kernel mode.
• Unix processes in kernel mode
• execute system calls synchronously
• run in separate address space
• use copy of calling arguments as input
• copy return value into process address space
• Unix kernel keeps process information in fixed
  size process table.

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Unix Process (II)

• Unix processes are in one of 5 states
•   State Status Description 
• nascent  created but without resources 
• runnable R running or in a run queue 
• sleeping S awaiting an event 
• stopped T stopped by a signal or by parent
  process 
• zombie Z deceased but termination not notified
  to parent

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Process Creation (I)
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Process Creation (II)
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