OS Operations

• This is 1.4
• Quick OS load
  • Turn the computer on.
  • It starts (if it is an intel like chip) executing the BIOS (basic input/output system) or UEFI (unified Extensible Firmware Interface) or something else hardware dependent
    • In the old days this was called bootstrap program.
      • Believe it or not, at one point, I keyed one of these into a vax 750.
    • This is firmware or software written to EEPROM
    • It it is responsible for items like
      • Initializing things such as memory and CPU
      • Initializing devices
      • Discovering the boot mode/devices
      • Loading the kernel and any temporary services needed.
      • Starting the kernel
• Once the kernel starts and is configured, it starts the first program.
  • On linux/unix this is accomplished through systemd.
    • This replaced init and is not the only way.
    • The system service daemon.
    • Responsible for starting and restarting services.
      • Daemons.
      • Starts the service on boot.
      • If the service dies, it will restart it.
    • Also a set of control functions
      • systemctl status httpd
      • systemctl restart httpd
      • Configured in //usr/lib/systemd/system
      • peek at httpd.service
  • So systemd will start the other daemons.
  • Things start to change as interaction happens
    • For example, someone may connect via ssh.
    • They contact the sshd, which starts another sshd
    • Which handles login and might start a shell
    • And the shell will start processes
    • ps -aux --forest
  • Just in case someone asks, kthreadd manages the kernel threads (or processes that are running for the kernel)
  • We might look at the lightdm (this is sort of the parent of the graphics display)
• As we said before, the user will need to get back to the kernel eventually
  • This is done through a system call. (intel centric)
    • This is done by generating an system interrupt 80
    • With RAX containing the service you want.
    • a table.
      • This allows the kernel to operate in secure mode.
      • More on this in a second.
• Some terms
  • A ruining program in called a process.
  • A thread is a lightweight process
    • A process can have multiple cooperating threads
    • That can share memory
    • Either at the process or kernel level.
  • A system that allows more than one process at a time is called multiprogramming.
    • This allows more efficient use of the cpu. (especially when there was only one, no cores)
    • Remember back in the bad old days, there were very few of these, so we wanted to keep them busy.
    • Remember, to do I/O we need to make a system call
      • This allows the kernel to take over.
      • And I/O is very slow.
      • So there is time to let another program run while we are getting the data the current program needs.
      • We will see this in the future.
    • This lead to multitasking
    • At least a primitive scheduler is needed here.
    • At least primitive memory management is needed.
  • Multitasking
    • The OS switches frequently between tasks.
    • Each task runs for a quantum, or time slice, or until it makes a system call and is blocked for a resource
    • The OS really needs a scheduler to do this.
    • The attempt is to provide the illusion that all programs are running at the same time.
  • Multitasking requires more sophisticated memory management systems.
    • Physical memory is the reality.
    • But logical memory is a user's view of memory.
    • And virtual memory allows the OS to manage the two.
      • A chunk of memory can be written to the hard drive when a process is "swapped out"
      • This can be loaded again when the process is swapped in.