what is Kernel Control Path?

jevinlin

参考www.linfo.org文章：http://www.linfo.org/kernel_control_path.html的定义：
Kernel Control Path Definition
A kernel control path is the sequence of instructions executed by a
kernel
to handle a system call, an interrupt or an exception.
The kernel is the core of an
operating system
, and it controls virtually everything that occurs on a computer.
An interrupt is a signal to the kernel that an event has occurred. Hardware interrupts
are initiated by hardware devices, including the keyboard, the mouse, a
printer or a disk drive. Interrupt signals initiated by
programs
are called software interrupts or exceptions.
A system call is a request in
Linux
or other
Unix-like
operating systems made via a software interrupt by an active process for a service performed by the kernel, such as input/output (I/O) or
process
creation (i.e., creation of a new process). A process is an
instance
of a program in execution. An active process is a process that is
currently progressing in the CPU (central processing unit), in contrast
to processes that are currently awaiting their turns for the CPU.
In the most simple situation, the CPU executes a kernel control path
sequentially, that is, beginning with the first instruction and ending
with the last instruction. However, the CPU interleaves
(i.e., alternates running parts of the two separate sequences) the
kernel control path in several types of situations: (1) when the CPU
detects an exception while running a kernel control path (e.g.,
attempting to access data that is currently not in
memory
), (2) when a process in
user mode
invokes a system call and the corresponding kernel control path
confirms that the request cannot be immediately granted and (2) when a
hardware interrupt occurs while the CPU is running a kernel control
path with the interrupts enabled.
User mode is one of two distinct execution modes for the CPU in Linux. It is a non-privileged
mode in which each process starts out. It is non-privileged in that it
is forbidden for processes in this mode to access those portions of
memory that have been allocated to the kernel or to other programs. The
kernel is not a process, but rather a controller of processes, and it
alone has access to all resources on the system.
Context switching
is not permitted when the CPU is executing a kernel control path
associated with an interrupt. A context switch is the switching of the
CPU from one process to another.
               
               
               

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