Hardware:
The hardware is Centre of structure that provides the Operating System with basic services.
The hardware consists of all peripherals like memory (RAM, HDD, FDD etc) processor, mouse, and other input devices, terminals, printers etc.
The Kernel:
The kernel is the heart of the system - a collection of programs mostly written in ‘C’ which communicate with the hardware directly.
Kernel is an interface between hardware of the system and shell. It is loaded into the memory when the system is booted.
User programs that need to communicate with the hardware use the services of the kernel, which performs the job on the user’s behalf.
It manages the system’s memory, schedules processes, decides their priorities and performs other tasks.
Shell:
The shell is an interface between the user and the kernel that isolates the user from knowledge of kernel functions.
The shell accepts the commands keyed by the users and checks for their syntax and gives out error messages if something goes wrong.
It is a command interpreter of user requests.
Application programs:
The various compilers for languages like c, c++, pascal, fortran and other application programs written by programmers which are used by users for their operations falls in this layers.
Only those persons who maintain on “account” with the computer system can use the UNIX system.
User can directly access application programs through which they can interact with the system.
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