Services provided by the kernel

• Always resides in the main memory
• Controls the execution of processes by allowing their creation, termination or suspension, and communication
• Schedules processes fairly for execution on the CPU.
  • Processes share the CPU in a time-shared manner
    • CPU executes a process
    • Kernel suspends it when its time quantum elapses
    • Kernel schedules another process to execute
    • Kernel later reschedules the suspended process
• Allocates main memory for an executing process
  • The kernel allows processes to share portions of their address space under certain conditions, but protects the private address space of a process from outside tampering
  • If the system runs low on free memory, the kernel frees memory by writing a process temporarily to secondary memory, or a {\sl swap} device
  • If the kernel writes entire processes to a swap device, the implementation of the Unix system is called a {\sl swapping} system; if it writes pages of memory to a swap device, it is called a {\sl paging} system.
  • Coordinates with the machine hardware to set up a virtual to physical address that maps the compiler-generated addresses to their physical addresses.
• File system maintenance
  • Allocates secondary memory for efficient storage and retrieval of user data
  • Allocates secondary storage for user files
  • Reclaims unused storage
  • Structures the file system in a well understood manner
  • Protects user files from illegal access
• Allows processes controlled access to peripheral devices such as terminals, tape drives, disk drives, and network devices.

Services provided by the kernel transparently

• Kernel recognizes that a given file is a regular file or a device but hides the distinction from user processes
• Kernel formats data in a file for internal storage but hides the internal format from user processes, returning an unformatted byte stream
• Kernal allows shell to read terminal input, to spawn processes dynamically, to synchronize process execution, to create pipes, and to redirect I/O

From Maurice J. Bach. The Design of the Unix Operating System. Prentice-Hall, Englewood Cliffs, NJ. 1986.