Interrupts are critical components in computer systems, allowing efficient and responsive interaction between the CPU and various hardware and software elements. They are signals that cause the processor to temporarily halt its current execution and switch to executing a predefined routine, known as the Interrupt Service Routine (ISR), to handle the interrupt event.
Hardware Interrupts
A hardware interrupt is triggered by an external hardware device to signal that it requires the CPU’s attention. Examples include pressing a key on a keyboard or moving a mouse, which generates an interrupt that tells the processor to read the input. Hardware interrupts are often asynchronous, meaning they can occur at any time, regardless of the processor’s current activities.
Key Features:
- Asynchronous Nature: Hardware interrupts can occur at any time, independent of the CPU clock.
- IRQ Lines: Many systems use Interrupt Request (IRQ) lines, where each device is associated with a specific IRQ signal, simplifying the identification and servicing of interrupts.
- Interrupt Vector Tables: Modern systems typically have a distinct interrupt routine for each type of interrupt, often implemented via interrupt vector tables.
Software Interrupts
A software interrupt is generated by executing specific instructions or when certain conditions within the CPU are met. These interrupts are used to request services from the operating system or to handle exceptional conditions within the program, such as errors.
Key Features:
- Intentional Generation: Software interrupts can be intentionally triggered using special instructions.
- Handling by OS: Typically managed by the operating system kernel, which executes an appropriate handler or performs a default action like terminating a faulty program.
Types of Software Interrupts:
- Synchronous Interrupts: These occur as a direct result of executing an instruction, such as a division by zero or invalid memory access.
- Asynchronous Interrupts: Triggered by events that are not directly related to the execution of the current instruction set.
Masking Interrupts:
- Maskable Interrupts: These can be enabled or disabled by the processor using an interrupt mask register. The associated interrupt signal can be ignored or deferred when masked.
- Non-Maskable Interrupts (NMIs): These are high-priority interrupts that cannot be ignored or disabled, used for critical events like hardware failures.
Common Issues:
- Missing Interrupts: When the expected interrupt is not generated, potentially causing the system to wait indefinitely.
- Spurious Interrupts: Interrupts with no identifiable source, often due to electrical anomalies or faulty circuit design.
Terminology and Classification
The terminology associated with interrupts can vary across different architectures:
- Trap: Often used interchangeably with interrupt but can specifically refer to synchronous interrupts caused by exceptional conditions or instructions designed to invoke an interrupt.
- Fault: A type of synchronous interrupt where the return address points to the faulting instruction, allowing for a potential restart of the operation.
- Abort: Used for severe errors, often non-restartable, indicating critical hardware issues or illegal values in system tables.
Different architectures have specific classifications:
- x86: Divides interrupts into hardware interrupts and software exceptions, with further subcategories like faults, traps, and aborts.
- ARM: Uses “exception” to refer to all interrupts and further divides them into hardware interrupts, aborts, resets, and exception-generating instructions.
- RISC-V: Uses “interrupt” for external signals and “exception” for internal conditions.
Example of Interrupt Usage:
- Keyboard Input: When a key is pressed, the keyboard sends an interrupt signal to the CPU. The CPU stops its current tasks, saves its state, and executes the ISR to process the keypress, such as updating the display or storing the character in a buffer. Once the ISR is finished, the CPU restores its previous state and continues with its prior activities.
Key Points about Interrupts:
- Immediate Attention: Interrupts prompt the CPU to stop its current activities and address an urgent task, which ensures that high-priority events are handled promptly.
- Interrupt Service Routine (ISR): When an interrupt occurs, the CPU executes a specific piece of code known as the Interrupt Service Routine (ISR) or interrupt handler, which is designed to deal with the event causing the interrupt.
- Types of Interrupts:
- Hardware Interrupts: These are generated by hardware devices, such as a keyboard, mouse, or network card, to signal events like key presses or incoming data.
- Software Interrupts: These are generated by software instructions, often to request a system service from the operating system.
- Maskable Interrupts: These can be temporarily ignored or “masked” by setting a particular bit in the CPU’s status register.
- Non-Maskable Interrupts (NMI): These cannot be ignored and must be processed immediately, often used for critical hardware errors.
- Interrupt Vector: The interrupt vector is a specific memory location that holds the address of the ISR. The CPU uses this address to jump to the appropriate ISR when an interrupt occurs.
- Priority: Interrupts can have different priorities. Higher-priority interrupts can preempt lower-priority ones, ensuring that the most critical tasks are handled first.
- Context Saving and Restoring: When an interrupt occurs, the CPU saves the current context (the state of the CPU registers and program counter) so that it can resume the interrupted task once the ISR is completed.
Frequently Asked Questions on Interrupt – FAQs
What happens to the CPU’s current state when an interrupt occurs?
When an interrupt occurs, the CPU saves its current state (the state of the CPU registers and program counter) so that it can resume the interrupted task once the ISR is completed.
Which component typically handles the signaling and management of interrupts?
The Interrupt Controller is responsible for signaling and managing interrupts. It helps the CPU prioritize and handle multiple interrupt requests efficiently.
Which register typically controls the enabling and disabling of hardware interrupts in a processor?
The Interrupt Mask Register (IMR) in a processor controls the enabling and disabling of hardware interrupts. It allows selective masking of interrupts, meaning certain interrupts can be ignored or deferred by the CPU.
What is the primary difference between a hardware interrupt and a software interrupt?
Hardware interrupts are generated by external devices like keyboards or disk drives to signal the CPU, while the processor itself generates software interrupts upon executing specific instructions to request services from the OS.
What is the main advantage of using vectored interrupts over non-vectored interrupts?
Vectored interrupts provide faster interrupt processing because each interrupt has a predefined ISR address in the interrupt vector table, allowing the CPU to quickly jump to the appropriate routine.