![]() ![]() A multitasking operating system is one in which multiple processes execute on a single CPU seemingly simultaneously and without interfering with each other. This is referred to as a mode switch rather than a context switch, because it does not change the current process.Ĭontext switching is an essential feature of multitasking operating systems. ![]() The existence of these two modes in Unix-like operating systems means that a similar, but simpler, operation is necessary when a system call causes the CPU to shift to kernel mode. RAM), that is, communication between this combination and the computer's users (e.g., via the keyboard or mouse), its storage devices (e.g., disk or tape drives), or other computers. I/O can be defined as any movement of information to or from the combination of the CPU and main memory (i.e. A system call is a request in a Unix-like operating system by an active process (i.e., a process currently progressing in the CPU) for a service performed by the kernel, such as input/output (I/O) or process creation (i.e., creation of a new process). KERNEL RETRIEVE TIMESLICE USED CODEOther programs, including applications, initially operate in user mode, but they can run portions of the kernel code via system calls. Kernel mode is a privileged mode of the CPU in which only the kernel runs and which provides access to all memory locations and all other system resources. Thus the wording suspending progression of a process might be preferable.Ĭontext switches can occur only in kernel mode. Although this wording can help clarify the concept, it can be confusing in itself because a process is, by definition, an executing instance of a program. A program counter is a specialized register that indicates the position of the CPU in its instruction sequence and which holds either the address of the instruction being executed or the address of the next instruction to be executed, depending on the specific system.Ĭontext switching can be described in slightly more detail as the kernel (i.e., the core of the operating system) performing the following activities with regard to processes (including threads) on the CPU: (1) suspending the progression of one process and storing the CPU's state (i.e., the context) for that process somewhere in memory, (2) retrieving the context of the next process from memory and restoring it in the CPU's registers and (3) returning to the location indicated by the program counter (i.e., returning to the line of code at which the process was interrupted) in order to resume the process.Ī context switch is sometimes described as the kernel suspending execution of one process on the CPU and resuming execution of some other process that had previously been suspended. A register is a small amount of very fast memory inside of a CPU (as opposed to the slower RAM main memory outside of the CPU) that is used to speed the execution of computer programs by providing quick access to commonly used values, generally those in the midst of a calculation. In Linux, threads are lightweight processes that can run in parallel and share an address space (i.e., a range of memory locations) and other resources with their parent processes (i.e., the processes that created them).Ī context is the contents of a CPU's registers and program counter at any point in time. A context switch (also sometimes referred to as a process switch or a task switch) is the switching of the CPU (central processing unit) from one process or thread to another.Ī process (also sometimes referred to as a task) is an executing (i.e., running) instance of a program. ![]()
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