Terminology (TODO: Remove it)#

  • Kernel
    The kernel is the program that manages the system, including (depending on the kernel model) hardware devices, memory, and CPU scheduling. It runs in a privileged CPU mode that allows direct access to hardware, called kernel mode.

  • Process
    An OS abstraction and environment for executing a program. The program runs in user mode, with access to kernel mode (e.g., for performing device I/O) via system calls or traps into the kernel.

  • Thread
    An executable context that can be scheduled to run on a CPU. The kernel has multiple threads, and a process contains one or more.

  • Task
    A Linux runnable entity, which can refer to a process (with a single thread), a thread from a multithreaded process, or kernel threads.

  • Virtual memory
    An abstraction of main memory that supports multitasking and oversubscription. It is, practically, an infinite resource.

  • Kernel space
    The virtual memory address space for the kernel.

  • User space
    The virtual memory address space for processes.

  • User land
    User-level programs and libraries (/usr/bin, /usr/lib…).

  • Context switch
    A switch from running one thread or process to another.

  • Mode switch
    A switch between kernel and user modes.

  • Trap
    A signal sent to the kernel to request a system routine (privileged action). Trap types include system calls, processor exceptions, and interrupts.

  • Hardware interrupt
    A signal sent by physical devices to the kernel, usually to request servicing of I/O. An interrupt is a type of trap.

Kernel and User Modes#

Kernel mode allows full access to devices and the execution of privileges instructions. User mode request privileges operations to kernel via system calls.

Switching between user and kernel modes is a mode switch.

Protection Ring#

mechanisms to protect data and functionality from faults (by improving fault tolerance) and malicious behavior (by providing computer security)

Context Switch#

Process of storing the system state for one task, so that task can be paused and another task resumed.

There are three potential triggers for a context switch:

  1. Multitasking

  2. Interrupt handling

  3. User and Kernel mode switching

System Calls#

Programmatic way in which a computer program requests a service from the kernel. System calls provide an essential interface between a process and the operating system.

System calls are generally not invoked directly, but rather via wrapper functions in glibc.