An operating system has a number of the device controller and each device controller's role for a specific device has been fixed, such as disk drives, audio devices etc. These device controllers and CPU are connected through a common bus so that they can easily access to the shared memory. In this case, a memory controller plays the role of referee to access orderly to the shared memory.
A bootstrap program or firmware stored within the hardware Read-Only-Memory(ROM) or Electrically-Erasable-Programmable-Read-Only-Memory (EEPROM) initializes all the aspects of the operating system when the computer starts to run. For this initial task, firmware at first locates the operating-system kernel and load it into memory.
There are some services like the kernel that are loaded into memory at the beginning as system processes or system daemons. After complete the beginning/booting phase, the system is ready to get some events to happen.
An event can be happening by an interrupt from either the hardware or the software. An interrupt from the hardware may be happening by sending a signal to the CPU. But the software triggers an interrupt by executing an operation, this operation is also known as a system call or a monitor call.