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HLW Chapter 1

Links: 701 How Linux Works Index

Chapter 1

  • Processes - the running programs that the kernel manages—collectively make up the system’s upper level, called user space.

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  • The kernel runs in kernel mode, and the user processes run in user mode.
    • Code running in kernel mode has unrestricted access to the processor and main memory.
  • User space refers to the parts of main memory that the user processes can access.
    • If a process makes a mistake and crashes, the consequences are limited and can be cleaned up by the kernel.
  • Kernel does 4 things:
    • Manage processes: context switching
    • Manage main memory: RAM
    • Device drivers
    • System calls: system calls (or syscalls) perform specific tasks that a user process alone cannot do well or at all. For example, the acts of opening, reading, and writing files all involve system calls.
  • Users exist primarily to support permissions and boundaries.
  • Every user-space process has a user owner, and processes are said to run as the owner.
    • A user may terminate or modify the behaviour of its own processes (within certain limits), but it cannot interfere with other users’ processes.
  • In addition, users may own files and choose whether to share them with other users.
  • The root user is an exception to the preceding rules because root may terminate and alter another user’s processes and access any file on the local system.
Operating as root can be dangerous. It can be difficult to identify and correct mistakes because the system will let you do anything, even if it is harmful to the system

As powerful as the root user is, it still runs in the operating system’s user mode, NOT kernel mode.

Summary

  • User processes make up the environment that we directly interact with.
  • The kernel manages processes and hardware.
  • Both the kernel and processes reside in memory (RAM).

Last updated: 2023-01-27