3.2 Applications
The kernel of the operating system is like an air traffic controller at an airport, and the applications are the airplanes under its control. The kernel decides which program gets which blocks of memory, it starts and kills applications, and it handles displaying text or graphics on a monitor.
Applications make requests to the kernel and in return receive resources, such as memory, CPU, and disk space. If two applications request the same resource, the kernel decides which one gets it, and in some cases, kills off another application to save the rest of the system and prevent a crash.
The kernel also abstracts some complicated details away from the application. For example, the application doesn’t know if a block of disk storage is on a solid-state drive, a spinning metal hard disk, or even a network file share. Applications need only follow the kernel’s Application Programming Interface (API) and therefore don’t have to worry about the implementation details. Each application behaves as if it has a large block of memory on the system; the kernel maintains this illusion by remapping smaller blocks of memory, sharing blocks of memory with other applications, or even swapping out untouched blocks to disk.
The kernel also handles the switching of applications, a process known as multitasking. A computer system has a small number of central processing units (CPUs) and a finite amount of memory. The kernel takes care of unloading one task and loading a new one if there is more demand than resources available. When one task has run for a specified amount of time, the CPU pauses it so that another may run. If the computer is doing several tasks at once, the kernel is deciding when to switch focus between tasks. With the tasks rapidly switching, it appears that the computer is doing many things at once.
When we, as users, think of applications, we tend to think of word processors, web browsers, and email clients, however, there are a large variety of application types. The kernel doesn’t differentiate between a user-facing application, a network service that talks to a remote computer, or an internal task. From this, we get an abstraction called a process. A process is just one task that is loaded and tracked by the kernel. An application may even need multiple processes to function, so the kernel takes ca