5.3 Processes

5.3 Processes

1. Processes are running programs and consist of the set or resources created and maintained by the OS to support the running of the program

   • Programs are basically sets of instructions stored in main memory
     • Computer runs many programs at a time and these are loaded into main memory
       • Computer will switch between programs in memory so they must be protected from each other

2. Process comprises several aspects

   • executable program
   • private address space both virtual and physical addresses
   • System resources that are allocated to the process by the OS
   • Unique identifier PID
   • Set of registers which store process info
   • A stack that is used to store local variables about the process and info about the flow of execution
     • Manipulating info on the stack has been exploited to create attacks

3. Process security issues

   • Process use resources like files and ports, process might try read a file or access a port or resource
   • Protection of data belonging to one process from corruption by another process is also vital
     • Memory management techniques can be used to limit the physical addresses that can be used by a process

4. Process states

   • Running
     • Using the CPU
   • Blocked
     • Waiting for some event to occur
   • Ready
     • Can use CPU when its next available
   • Inactive
     • Used for reducing system load
   • State transitions
     • Programs are executed and new process for that program is added to the ready list
       • When this process reaches the front of the list it is given to the CPU and enters the running state
     • Process scheduling controls ordering of processes
       • Certain processes have a higher priority and reach the front of the list faster

5. Dispatcher assignes CPU to a process

   • The amount of time assigned is called Quantum
     • Running processed may enter the blocked state during this quantum
       • Otherwise at the end of the Quantum the Dispatcher allocates a new process to the CPU.
         • Running process enters the ready state