Book Section: 1.7.
Operating system provides services to applications on base of associated hardware. The operating system works as a layer of software between the application program and the hardware as shown in below figure (layered view of computer system). All attempts by an application program to manipulate the
hardware must go through the operating system.
The operating system has two primary purposes.
- Protect the hardware from misuse by applications
- Provide applications with simple and uniform mechanisms for manipulating low-level hardware devices.
A process is the operating system’s abstraction for a running program.
Multiple processes can run concurrently on the same system, and each process appears to have exclusive use of the hardware. By concurrently, we mean that the instructions of one process are interleaved with the instructions of another process. In most systems, there are more processes to run than there are CPUs to run them.
Traditional systems could only execute one program at a time, while newer multicore processors can execute several programs simultaneously. A single CPU can execute multiple processes concurrently by having the processor switch among them. The operating system performs this interleaving with a mechanism known as context switching. The operating system keeps track of all the state information that the process needs in order to run. This state, which is known as the context, includes information such as the current values of the PC, the register file, and the contents of main memory.
At any point, a uniprocessor system can only execute the code for a single process. When the operating system decides to transfer control from the current process to some new process, it performs a context switch by saving the context of the current process, restoring the context of the new process, and then passing control to the new process. The new process picks up exactly where it left off. Below figure shows the basic idea for context switching.
Above figure indicates, the transition from one process to another is managed by the operating system kernel. The kernel is the portion of the operating system code that is always resident in memory. When an application program requires some action by the operating system, such as to read or write a file, it executes a special system call instruction, transferring control to the kernel. The kernel then performs the requested operation and returns back to the application program. Note that the kernel is not a separate process. Instead, it is a collection of code and data structures that the system uses to manage all the processes.
In modern systems, a process can actually consist of multiple execution units, called threads, each running in the context of the process and sharing the same code and global data.
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