Interrupt vs. System Call

Attribute	Interrupt	System Call
Definition	An event that temporarily suspends the execution of a program and transfers control to a specific routine.	A request made by a program to the operating system for a specific service or resource.
Trigger	Hardware or software event, such as an external device signaling or an exception occurring.	User program explicitly makes a request to the operating system.
Execution Time	Interrupts are handled immediately and can occur asynchronously.	System calls are synchronous and require the operating system to switch to kernel mode to execute the requested service.
Privilege Level	Interrupts can occur at any privilege level, including kernel mode.	System calls can only be made from user mode to request services from the operating system running in kernel mode.
Context Switch	Interrupts may cause a context switch, where the current process is paused and another process is scheduled to run.	System calls do not necessarily cause a context switch, as the calling process continues execution after the system call is completed.
Handling	Interrupt handlers are predefined routines that handle specific types of interrupts.	System calls are handled by the operating system's kernel, which contains the necessary code to execute the requested service.
Examples	Timer interrupt, keyboard interrupt, hardware exception.	Opening a file, reading from or writing to a file, creating a new process.

Introduction

When it comes to computer systems and operating systems, there are various mechanisms in place to facilitate communication between different components. Two such mechanisms are interrupts and system calls. While both serve the purpose of allowing processes to interact with the underlying operating system, they differ in their attributes and usage. In this article, we will explore the attributes of interrupts and system calls, highlighting their differences and similarities.

Interrupts

Interrupts are a fundamental concept in computer systems. They are signals sent by hardware devices or software to the processor, indicating that an event has occurred that requires immediate attention. Interrupts can be classified into two types: hardware interrupts and software interrupts.

Hardware interrupts are generated by external devices such as keyboards, mice, or network cards. These interrupts are used to notify the processor about events that need to be handled promptly. For example, when a key is pressed on the keyboard, a hardware interrupt is triggered, allowing the operating system to respond and update the corresponding application.

On the other hand, software interrupts, also known as exceptions or traps, are generated by the processor itself. These interrupts are triggered by exceptional conditions, such as division by zero or accessing invalid memory addresses. Software interrupts allow the operating system to handle these exceptional situations and prevent the system from crashing.

Interrupts have several key attributes. Firstly, they are asynchronous, meaning they can occur at any time during the execution of a program. This makes interrupts suitable for handling time-sensitive events or responding to external stimuli. Secondly, interrupts have a higher priority than regular program execution. When an interrupt occurs, the processor suspends the current program and transfers control to the interrupt handler, which is a specific routine designed to handle the interrupt. Once the interrupt is handled, the processor resumes the execution of the interrupted program.

Furthermore, interrupts are typically associated with a specific interrupt vector, which is a unique identifier used to determine the type of interrupt and the corresponding interrupt handler. The interrupt vector allows the operating system to dispatch the interrupt to the appropriate handler, ensuring that the correct action is taken based on the interrupt's source.

System Calls

System calls, also known as kernel calls or system services, provide a mechanism for user-level processes to request services from the operating system. Unlike interrupts, system calls are synchronous, meaning they are initiated by the running program and require the program to wait for the operating system's response.

System calls allow processes to access privileged operations or resources that are not directly accessible from user space. For example, a process may need to read from or write to a file, allocate memory dynamically, or create a new process. These operations require the intervention of the operating system, which is achieved through system calls.

Similar to interrupts, system calls have specific attributes. Firstly, they are initiated by the running program using a software interrupt instruction or a dedicated system call instruction. This instruction transfers control from user space to the operating system, allowing the system call to be executed. Once the system call is completed, control is returned to the calling program.

System calls are typically identified by a unique number or identifier, known as a system call number. This number is used by the operating system to determine the requested service and invoke the corresponding system call handler. Each system call has its own set of parameters, which are passed from the calling program to the operating system. These parameters provide the necessary information for the operating system to perform the requested service.

Another important attribute of system calls is that they provide a boundary between user space and kernel space. User space is the memory space where user-level processes execute, while kernel space is the protected memory space where the operating system resides. System calls allow user-level processes to access the services provided by the operating system without compromising the security and stability of the system.

Comparison

While interrupts and system calls serve different purposes and have distinct attributes, they also share some similarities. Both interrupts and system calls involve a transfer of control from user space to the operating system. They allow processes to interact with the underlying operating system and access privileged operations or resources.

However, there are notable differences between interrupts and system calls. Interrupts are asynchronous and can occur at any time, while system calls are synchronous and initiated by the running program. Interrupts have a higher priority than regular program execution, whereas system calls require the program to wait for the operating system's response.

Interrupts are typically associated with specific interrupt vectors, allowing the operating system to dispatch the interrupt to the appropriate handler. On the other hand, system calls are identified by unique system call numbers, which are used by the operating system to determine the requested service.

Interrupts are commonly used for time-sensitive events or responding to external stimuli, while system calls are used for accessing privileged operations or resources that are not directly accessible from user space.

In summary, interrupts and system calls are essential mechanisms in computer systems and operating systems. While interrupts are asynchronous and have a higher priority, system calls are synchronous and provide a boundary between user space and kernel space. Understanding the attributes and differences between interrupts and system calls is crucial for developing efficient and reliable software systems.