Computer System Organisation

The computer system is a combination of many parts such as peripheral devices, secondary memory, CPU, device controllers, and a shared memory bus. These components work together to execute programs and handle input/output operations.

Computer System Organisation

The following diagram shows how the CPU, memory, and I/O device controllers are connected through a common system bus −

The salient points about the above figure displaying Computer System Organisation are −

• The I/O devices and the CPU both execute concurrently. Some processes are scheduled for the CPU and at the same time, some are undergoing input/output operations.

• There are multiple device controllers, each in charge of a particular device such as keyboard, mouse, printer, or disk drive.

• There is a buffer available for each of the devices. The input and output data can be stored in these buffers temporarily.

• The data is moved from memory to the respective device buffers by the CPU for I/O operations, and then this data is moved back from the buffers to memory.

• The device controllers use an interrupt to inform the CPU that the I/O operation is completed.

Interrupt Handling

An interrupt is a necessary part of Computer System Organisation as it is triggered by hardware and software parts when they need immediate attention. An interrupt can be generated by a device or a program to inform the operating system to halt its current activities and focus on something else.

Hardware interrupts and software interrupts are the two types of interrupts. Hardware interrupts are triggered by hardware peripherals (such as a keyboard press or disk read completion), while software interrupts are triggered by software function calls (such as system calls).

Hardware interrupts are further divided into two types −

• Maskable interrupts − These can be ignored or disabled by the CPU when it is handling a higher-priority task. For example, a timer interrupt can be temporarily masked.

• Non-maskable interrupts (NMI) − These cannot be ignored by the CPU. They signal critical events such as hardware failures or memory parity errors that require immediate attention.

Interrupt Handling Process

When an interrupt occurs, the CPU follows these steps −

• The CPU stops executing the current process and saves its state (program counter, registers).

• The CPU identifies the source of the interrupt using the interrupt vector table.

• Control is transferred to the appropriate Interrupt Service Routine (ISR).

• After the ISR completes, the CPU restores the saved state and resumes the original process.

Conclusion

Computer System Organisation describes how the CPU, memory, device controllers, and system bus work together to execute processes and handle I/O operations concurrently. Interrupts are a fundamental mechanism that allows hardware and software to signal the CPU for immediate attention, ensuring efficient multitasking and device management.