Inside the CPU Interrupt Code 2025: How Processors Handle the Unexpected

In the world of modern computing, a CPU doesn’t just run programs—it responds, reacts, and adapts in real-time. Interrupts make this responsiveness possible. Every time hardware signals the processor—like a keyboard press, a network packet arrival, or a timer tick—the CPU sets aside its current task to assess and manage the incoming request.

Think of an interrupt as a high-priority alarm at a busy help desk. Rather than waiting in line, urgent matters instantly draw attention, ensuring critical tasks don't get delayed. Without this mechanism, CPUs would need to constantly poll devices for updates, wasting cycles and draining efficiency.

There are three main categories of interrupts shaping this dynamic: hardware interrupts generated by external devices, software interrupts triggered by program instructions, and exceptions caused by events like invalid memory access or division by zero.

This blog delves into the CPU interrupt code: the low-level logic that processes these signals. You'll discover how interrupt vectors, priorities, flags, and handlers work together beneath the surface to maintain control, speed, and precision—all in the space of microseconds.

How the CPU Orchestrates Interrupt Management

CPU Architecture and Its Interrupt Handling Capabilities

Modern CPUs integrate a dedicated pathway for interrupt handling directly into their microarchitecture. At its core, the interrupt mechanism connects the processor’s control logic with input/output devices and internal error traps. Whether it's a simple x86 processor or an ARM-based multi-core architecture, the CPU contains key hardware components—including the interrupt request lines (IRQs), interrupt descriptor tables (IDT or IVT), and control registers—that manage the lifecycle of an interrupt.

Multiplexing plays a critical role. CPUs can manage multiple interrupt sources through prioritized vectors and mask registers. Coordination between the control unit and the arithmetic logic unit (ALU) ensures that interrupt execution neither corrupts register context nor drops execution threads. This fine-grained governance becomes especially relevant in real-time systems, where timing precision and deterministic response dictate architectural design choices down to the microinstruction level.

Detection, Flagging, and Processing an Interrupt

As soon as a hardware device or software routine sends an interrupt signal, the CPU’s interrupt controller identifies the type of interrupt. It uses IRQ lines and status signals to flag the request. The control unit then temporarily suspends the current execution state, storing the program counter and relevant CPU flags onto the stack.

Next, the processor checks the interrupt mask registers. If the signal isn’t masked or blocked by a higher-priority interrupt, the processor triggers an interrupt acknowledge cycle. This involves resolving the appropriate interrupt vector, jumping to the address of the associated Interrupt Service Routine (ISR), and processing the event that prompted the interrupt.

The entire operation takes place within a few CPU clock cycles. For example, on Intel's x86 processors, a typical interrupt latency ranges from 20 to 100 clock cycles depending on system state and active threads. On tightly optimized ARM Cortex-M microcontrollers using Tail-Chaining, this transition can shrink to six cycles, minimizing response time for high-priority events.

CPU Registers and Flags in Interrupt Execution

Register preservation defines the stability of interrupt handling. The processor saves key general-purpose and segment registers—EAX, EBX, ECX, EDX, as well as ESP and EIP on the x86 architecture—before branching to the ISR. Restoring these values after the routine ensures seamless continuation of the previously interrupted task.

The FLAGS register holds the interrupt flag (IF), which controls whether the CPU will respond to maskable interrupts. Clearing the IF bit using the CLI instruction disallows new interrupts until execution of a critical section is complete. Restoring the IF bit via the STI instruction resumes normal interrupt acceptance. These bit-level manipulations guarantee atomicity when context integrity is at risk.

Internal CPU Functions for Interrupt Processing

Instruction-level mechanisms within the CPU drive robust interrupt handling. Key functions include:

• INT – triggers a software interrupt using a specified interrupt vector.
• IRET/IRETD – returns from an interrupt by restoring the instruction pointer, code segment, and flags.
• CLI/STI – clears or sets the interrupt enable flag respectively, halting or allowing new interrupt handling.
• PUSHAD/POPAD – saves and restores all general-purpose registers to avoid data corruption during ISR execution.

These functions, implemented in microcode or hardware-specific logic blocks, directly interface with architectural features like the Instruction Decoder and Execution Unit. They enable high-speed transition between normal execution and interrupt response without breaking memory coherency or processing order.

Dissecting Hardware and Software Interrupts: Origins and Execution

How Hardware Interrupts Originate

Hardware interrupts originate outside the CPU and signal the need for immediate attention from an external device. These signals arrive asynchronously, meaning they occur independent of the processor's current tasks. The moment a hardware device, such as a network card, disk drive, or I/O controller, requires service, it sends an electrical signal along a dedicated interrupt line to the CPU.

Consider a keystroke on a keyboard. This action doesn't wait for the CPU to check the keyboard at intervals. Instead, it fires a hardware interrupt that tells the system exactly when input data is available. Another tangible example centers around programmable timers. When a countdown hits zero, the timer asserts its interrupt line, prompting the CPU to act—perhaps to switch tasks, manage scheduling, or update internal state.

How Software Interrupts Arise

Software interrupts are deliberately triggered by the running program. A specific machine instruction—typically INT on x86 architectures—causes the CPU to divert control flow to a predefined interrupt vector. These interrupts are synchronous and carefully placed within code to signal transitions between privilege levels or request kernel services.

For example, in Linux systems, a user program initiates a system call by issuing interrupt 0x80. This leads the CPU to a handler responsible for interpreting the request and executing privileged operations on the application's behalf. Software-generated traps also fall into this category, often used to catch division by zero errors or invalid memory access.

Key Differences: Origin, Timing, and Purpose

• Origin: Hardware interrupts stem from external peripherals; software interrupts originate from deliberate instructions within the executing program.
• Timing: Hardware interrupts occur asynchronously with the CPU’s instruction cycle, while software interrupts are synchronous—they arise at specific points in execution.
• Interrupt Handling: Hardware signals reach the CPU via physical interrupt lines managed by a controller, whereas software interrupts are decoded directly from machine-level instructions and mapped to the appropriate handler via standardized trap tables.

Each type of interrupt serves distinct roles in system architecture. Hardware interrupts provide responsiveness to real-time events, making them indispensable in device communication. In contrast, software interrupts facilitate structured, controlled access to lower-level system resources or serve as instrumentation points in application logic.

Interrupt Handling: How Interrupts Are Processed

High-Level Steps in Interrupt Processing

Interrupt handling starts with a disruption: a device or process signals the CPU to pause its current execution. The following actions occur in a precise sequence:

• Interrupt signal arrives: A hardware or software interrupt is triggered.
• CPU acknowledges the interrupt: It completes the current instruction cycle and checks the interrupt flag.
• Interrupt is prioritized: The CPU consults the interrupt controller or a priority scheme to determine which interrupt to handle if multiple are pending.
• Context is saved: The current execution context—including register states and program counter—is pushed to the stack.
• ISR is executed: The CPU loads the corresponding interrupt handler address from the Interrupt Vector Table and executes the associated Interrupt Service Routine (ISR).
• Context is restored: After the ISR completes, the saved state is popped from the stack.
• Execution continues: The CPU resumes the interrupted program as though no interruption had occurred.

This entire flow can execute in microseconds, and in well-optimized systems, even faster.

Role of Interrupt Flags

Interrupt flags control whether the CPU will respond to incoming interrupts. The interrupt enable (IE) flag determines if the processor will recognize future interrupts. When the CPU enters an ISR, this flag is typically cleared—sometimes automatically—to prevent nested interrupts (unless explicitly enabled).

Some architectures include additional flags like the priority level register or machine status register, which offer more granular control over interrupt reception. For instance, in x86 systems, the IF (Interrupt Flag) in the EFLAGS register enables maskable interrupts when set to 1.

CPU Actions: From Interruption to Program Resumption

The processor performs several low-level operations between recognizing an interrupt and returning to the original program:

• Pushes the instruction pointer and processor state onto the system stack.
• Loads the interrupt vector from a fixed memory location or table.
• Jumps to the interrupt handler address.
• Executes the ISR—which may involve I/O handling, signaling other threads, or clearing hardware buffers.
• Executes the IRET or equivalent instruction to restore state and return to the previous execution.

Some processors automatically save and restore context; others rely on the ISR code to perform these steps manually. This affects ISR complexity and system determinism.

Problems If Interrupts Are Not Handled Correctly

Mishandling interrupts causes unpredictable behavior at both the hardware and application level. When an ISR fails to clear an interrupt or acknowledge it properly, the IRQ line may remain active, resulting in repeated triggers—also known as an unhandled interrupt storm.

If multiple interrupts arrive and the system lacks priorities or nesting support, some may be lost or indefinitely delayed. This leads to data loss in devices that operate with time-sensitive buffers—network cards and audio devices being prime examples.

Incorrect context saving or restoring can corrupt stack frames, compromise register values, and lead to errant behavior that is difficult to trace. In real-time systems, unbounded interrupt latency damages timing guarantees, breaking hard assurance conditions and crashing control loops.

What happens if an interrupt triggers while interrupts are disabled? If the interrupt is maskable, the CPU ignores it. If it's non-maskable, the result is undefined unless the architecture includes dedicated handling for such conditions.

Inside the Interrupt Vector Table (IVT): Mapping the Flow of Execution

What the IVT Is and Where It Lives in Memory

The Interrupt Vector Table (IVT) is a predefined region of system memory that holds the addresses of all interrupt service routines (ISRs). Every time an interrupt request (IRQ) occurs, the processor consults this table to locate the corresponding ISR and transfers control accordingly.

On x86-based systems operating in real mode, the IVT begins at memory address 0x0000:0000 and occupies the first 1024 bytes (0x0000 to 0x03FF). Each entry in the IVT is 4 bytes long—2 bytes for the segment and 2 bytes for the offset—allowing for a maximum of 256 interrupt vectors.

In protected mode or on architectures like x86_64, the mechanism varies. Instead of the traditional IVT, these platforms use an Interrupt Descriptor Table (IDT) which is more versatile and capable of supporting advanced privilege-level protection and segment selectors.

Translating IRQ to Action: How IVT Maps to ISRs

The processor uses the interrupt request number as an index into the table. For instance, interrupt number 0 corresponds to IVT entry at memory address 0x0000, number 1 maps to 0x0004, and so on—simple arithmetic, as each entry spans 4 bytes.

An IRQ from a keyboard (commonly IRQ1) triggers interrupt number 0x09 (decimal 9). The processor multiplies this number by 4 to get the IVT offset, retrieves the ISR address stored there, and jumps directly to that memory location to start executing the interrupt handler.

This design keeps the transfer of control deterministic and fast, with no branching logic required during interrupt lookup. ISR developers place the handler's address directly in the vector, ensuring hardware coherence and system responsiveness.

IVT in Hardware and Software Interrupt Scenarios

For hardware-generated interrupts—like those from timers, keyboards, or network cards—the IVT bridges the gap between physical signal and software execution. The device signals the interrupt controller, which passes the IRQ to the CPU. The CPU consults the IVT without hesitation.

Software interrupts, such as those triggered by the INT instruction (e.g., INT 0x10 for BIOS video services), follow the identical path. The opcode references an interrupt number, the CPU looks it up in the IVT, and then loads the pointer to launch the corresponding ISR.

This dual-purpose application of the IVT—covering both hardware interactions and system-level software calls—demonstrates its foundational role in CPU architecture. Without needing a separate mechanism for different interrupt types, the IVT streamlines execution flow and enhances predictability.

• The IVT provides direct, numeric indexing for fast ISR lookup.
• In real mode, it's fixed at the lowest 1 KB of memory.
• Every interrupt, regardless of source, maps to a 4-byte vector containing the ISR location.

Interrupt Service Routine (ISR): The Core of Interrupt Response

Definition and Purpose

When a CPU detects an interrupt, it temporarily suspends execution of the current instruction flow and begins executing an Interrupt Service Routine (ISR). The ISR contains the specific code that handles the event associated with the interrupt—whether it's a hardware signal from a disk controller, a keystroke, or a software request. Each interrupt has a corresponding ISR that carries out appropriate actions based on the type and source of the signal.

The ISR performs targeted operations, often low-level and time-sensitive. It ensures that input/output tasks proceed without delay. This mechanism enables real-time or near-real-time responsiveness in operating systems and embedded systems. Once the ISR completes, the CPU resumes the previously paused process, preserving computational continuity.

How ISRs Are Registered and Associated with Devices or Traps

Interrupt Service Routines must be explicitly registered with the operating system or the firmware. This process links a specific interrupt request line (IRQ) or trap number to a memory address pointing to the relevant ISR. During system initialization, device drivers register these routines using kernel-level APIs or system calls depending on the platform.

• In x86 architecture, the ISR's address is stored in the Interrupt Descriptor Table (IDT), which replaces the older Interrupt Vector Table (IVT).
• On Linux, the request_irq() function ties a device’s IRQ line to a handler function defined in the driver code.
• For ARM Cortex-M systems, the vector table entries are set during boot and can be modified using NVIC (Nested Vector Interrupt Controller) configurations.

This mapping ensures that when a device triggers a specific IRQ, the CPU immediately executes the correct ISR without ambiguity or delay.

Common Functions Performed Inside an ISR

ISRs are designed for speed and determinism. Their code typically performs a narrow set of tasks that are completed quickly, often deferring non-essential processing to the main program or a background thread. Inside an ISR, developers usually:

• Acknowledge the interrupt—clear or reset the interrupt status flag in the hardware to prevent retriggering.
• Read or write data—interact with device buffers such as reading data from a UART register or writing to a DAC.
• Queue events—mark completion flags, increment counters, or invoke task notifications for deferred processing.
• Perform minimal error checks—detect framing or parity errors using status registers if applicable.

Operations within the ISR must be deterministic, taking a predictable number of CPU cycles to maintain system reliability, particularly in real-time environments.

Challenges: Responding Quickly and Minimizing Latency

Two critical performance metrics define ISR effectiveness: interrupt response time and ISR latency. The response time covers the delay from interrupt occurrence to ISR execution, while latency denotes the total time the ISR holds up other operations. System architects and embedded developers reduce these through:

• Priority masking—assigning appropriate interrupt priorities to ensure higher-priority devices preempt lower-priority ones.
• ISR minimization—keeping ISRs short and offloading long tasks to deferred service routines (DSRs) or kernel threads.
• Efficient vector table management—ensuring fast lookup and minimal CPU cycles from interrupt trigger to ISR entry.

Latency issues often surface when ISRs disable lower-priority interrupts or when shared interrupt lines cause contention. Identifying these bottlenecks requires profiling tools that track ISR entry/exit times and system-wide interrupt statistics.

Managing Interrupt Priorities with the Programmable Interrupt Controller (PIC)

Introduction to PIC and Its Communication with the CPU

The Programmable Interrupt Controller (PIC) acts as a mediator between hardware devices and the CPU. When multiple devices issue interrupt requests at the same time, the CPU relies on the PIC to determine which interrupt to handle first. PIC hardware monitors all incoming signals and relays only the most pertinent to the processor’s interrupt line.

In x86 architecture, the most widely known PIC is the Intel 8259A. It connects to the CPU via the Interrupt Request (IRQ) lines. The PIC receives interrupt signals from I/O peripherals through these IRQ lines. It then sends an interrupt number to the CPU over the data bus, which the CPU uses to locate the corresponding entry in the Interrupt Vector Table (IVT).

Role in Organizing and Prioritizing Interrupts

The PIC sorts interrupt requests based on priority levels it has been configured to follow. It uses a priority resolver to decide which IRQ to send to the CPU when more than one is active. On the 8259A, IRQ0 has the highest default priority, while IRQ7 holds the lowest. However, this hierarchy can be dynamically modified by reprogramming the PIC.

• Fixed priority mode: Interrupts are handled based on a predefined hierarchy.
• Rotating priority mode: The priority changes dynamically, giving each device equal chance over time.
• Special fully nested mode: Optimized for systems with cascaded PICs, allowing more granular control over multiple interrupts.

To support additional IRQ lines, systems often cascade two 8259A PICs—configured as a master and a slave—expanding the number of distinct interrupts from 8 to 15 (IRQ2 is used to link the slave to the master).

Defining Maskable and Non-Maskable Interrupts through Programmability

A key feature of the PIC is its ability to mask certain interrupts, blocking them from reaching the CPU unless explicitly allowed. Each IRQ line has a corresponding bit in the Interrupt Mask Register (IMR). Setting a bit to 1 masks that interrupt; clearing it to 0 unblocks it.

Maskable interrupts can be selectively disabled to ensure that high-priority operations complete without disruption. Non-maskable interrupts (NMIs), on the other hand, bypass the PIC entirely. They connect directly to a dedicated CPU pin and always execute when triggered. NMIs are reserved for critical events such as memory parity errors or hardware failures.

Modern systems have largely replaced the 8259A with the more sophisticated Advanced Programmable Interrupt Controller (APIC), which supports more cores, interrupt vectors, and greater granularity. However, understanding the architecture and functions of the traditional PIC remains foundational for low-level systems programming and legacy hardware support.

Understanding Maskable and Non-Maskable Interrupts

Two Fundamental Categories of CPU Interrupts

Interrupts fall into two broad categories: maskable and non-maskable. This distinction determines whether the processor can defer or ignore the interrupt request during critical operations. Grasping the difference between them is key to writing effective and predictable interrupt code.

Maskable Interrupts (IRQ)

Maskable interrupts are the most common and are also known as IRQ (Interrupt Requests). These can be temporarily disabled by the CPU using processor flags—more specifically, the Interrupt Flag (IF) in x86 architecture. When this flag is cleared, incoming maskable interrupts are ignored until it is set again.

Developers call CLI (Clear Interrupt Flag) and STI (Set Interrupt Flag) assembly instructions to control this behavior. During execution of time-sensitive or atomic instructions—critical sections where data consistency is essential—maskable interrupts must stay disabled to avoid race conditions.

• Programmable Interrupt Controller (PIC) or Advanced Programmable Interrupt Controller (APIC) can be configured to mask specific interrupt lines (IRQs) at the hardware level.
• Operating Systems often disable maskable interrupts when context switching or manipulating the scheduler's data structures.

Non-Maskable Interrupts (NMI)

Unlike their maskable counterparts, NMIs cannot be disabled or deferred. These interrupts are reserved for high-priority tasks that must be addressed immediately, regardless of the current CPU state or interrupt flags.

Use cases for NMIs include:

• Hardware failure detection: Memory corruption detectors, parity errors, or bus lockups invoke NMIs to signal unrecoverable errors.
• System watchdog timers: If the system fails to reset within a given time, an NMI can trigger to prevent infinite hangs.
• Manual interrupt buttons: Diagnostic tools may include physical NMI switches to halt operation for debugging regardless of running code.

On x86 systems, NMIs are typically routed through IRQ 2 by legacy PC architecture conventions, and handled using the NMI_Handler label in the interrupt descriptor table.

Making the Right Choice

Why not just make every interrupt non-maskable? Because NMIs bypass CPU-controlled protection mechanisms, they can easily disrupt system stability if misused. Maskable interrupts strike a balance: they allow prioritization and controlled dispatching without risking the same level of disruption.

Understanding when to assert control with maskable interrupts versus when to force CPU attention using NMIs is key. Are you wrapping a memory buffer in a real-time data acquisition application? Disable maskable interrupts. Are you detecting a hardware fault about to corrupt memory? Trigger an NMI and take over the system’s state immediately.

Understanding Context Switching During Interrupt Handling

What Happens When a CPU Interrupt Occurs?

When an interrupt occurs, the CPU pauses the current execution thread to process the interrupt. This abrupt shift doesn’t erase the current process—it preserves it. To continue later, the CPU needs to save the current execution state meticulously.

Saving the Execution State

Before the CPU executes the interrupt service routine (ISR), it captures the context of the interrupted task. This context includes:

• Program Counter (PC): Indicates the next instruction that would have executed.
• Stack Pointer (SP): Points to the top of the call stack.
• CPU Registers: General-purpose and special-purpose registers, such as AX, BX, CX, DX in x86 architecture.
• Status Flags: Bits in the CPU flag register that influence conditional operations (e.g., Zero Flag, Carry Flag).

Depending on the architecture (e.g., x86, ARM, RISC-V), this information is pushed onto the stack either by hardware mechanisms, software routines, or a combination of both.

Resuming Execution After the Interrupt

Once the ISR completes its task, the CPU must restore the saved context to continue the previously paused thread seamlessly. This involves:

• Popping saved values—including the program counter, flags, and CPU registers—off the stack.
• Resetting the stack pointer and instruction pointer to their previous states.
• Clearing or re-enabling interrupts, depending on the system design and interrupt nesting behavior.

When done correctly, the interrupted process resumes as if nothing happened. The user never sees this switch, but the efficiency of this operation has a direct effect on system responsiveness.

Timing Issues and Race Conditions

Ever imagined two interrupts colliding in real time? This happens. Timing issues arise when context switches occur faster than proper synchronization mechanisms can handle. Without atomic save-restore operations or interrupt masking, race conditions may corrupt system state—especially when multiple threads share resources.

To mitigate this, interrupt handlers should be concise, non-blocking, and should offload work to lower-priority execution contexts like deferred procedure calls (DPCs) or threaded ISRs. Multicore systems also amplify these risks, requiring careful design of lock mechanisms or use of atomic instructions for state preservation.

Nested Interrupts and Priority Levels: How CPUs Handle Complex Interrupt Scenarios

What Are Nested Interrupts?

Nested interrupts occur when a second interrupt is triggered while the CPU is in the process of handling a first. Instead of ignoring the second interrupt or queuing it until later, the processor temporarily suspends the current interrupt service routine (ISR) and jumps to address the new, higher-priority ISR.

Not all architectures support nested interrupts equally. ARM Cortex-M processors, for example, implement hardware support via the Nested Vectored Interrupt Controller (NVIC), which allows automatic prioritization and interrupt stacking with minimal software overhead. On x86 architectures, handling nested interrupts requires careful manipulation of CPU flags and interrupt masks to maintain system stability and control flow.

CPU Flags and Interrupt Mask Levels

To allow interruption of an ISR, the CPU must clear the interrupt disable flag (commonly referred to as IF in x86 architecture). If IF remains unset, the CPU will not acknowledge new interrupts. Operating systems and interrupt controllers manipulate this flag, alongside interrupt mask registers, to enable or block specific interrupt lines at defined execution points.

Mask levels operate like filters. For example, advanced PICs and modern APICs use Interrupt Request Level (IRQL) assignments. A current IRQL of 5 means any incoming interrupt must have a priority higher than 5 to nest the current ISR. This mapping lets the operating system and hardware controller coordinate which ISRs can preempt others.

Managing Interrupt Priority: PIC and OS Cooperation

The Programmable Interrupt Controller (PIC) plays a central role in enforcing interrupt priorities. In systems built on the Intel 8259 PIC or its advanced programmable interrupt controller (APIC) successors, priority levels can be statically or dynamically set. The OS must configure the PIC correctly to ensure time-critical interrupts—like those for system clocks or real-time I/O—preempt lower-priority ones.

• The OS kernel tracks ISR nesting depth and ensures stack space is preserved with each new interrupt call.
• Interrupt masking through the IMR (Interrupt Mask Register) prevents specific lines from being serviced temporarily.
• When combined, staking registers, IF bit, and the IMR construct a system that filters and directs interrupt flows without losing control of execution flow.

Coordination between software and hardware must be precise. The OS scheduler must be aware of interrupt priorities and nesting states to avoid deadlocks, race conditions, or unpredictable latency impacts. This synchronization enables responsive scheduling in soft and hard real-time systems.

Problem Scenario: Low-Priority Interrupt Blocking High-Priority Events

Consider a misconfigured IMR where a low-priority interrupt for a serial port remains unmasked while a time-critical audio buffer underrun interrupt is delayed. Since the CPU is servicing the unmasked low-priority ISR and not accepting further interrupts due to IF being cleared, the higher-priority request doesn't preempt as expected. The result: audible dropout in audio playback or missed frames in streaming output.

Developers must map interrupts thoughtfully, set fitting priorities, and ensure that ISRs either re-enable interrupts quickly or delegate non-critical computation to kernel threads. Writing ISRs that are short, atomic, and quick to clear interrupt masks enhances system reactivity and throughput under high interrupt load conditions.

Reinforcing Command over CPU Interrupt Code

Through this content series, each layer of the CPU interrupt ecosystem has been dissected—from hardware triggers to software traps, from the interrupt vector table to the nuances of context switching. Understanding these elements in isolation is helpful, but mastery comes from recognizing their interdependencies.

Every low-level CPU signal, every ISR, every line in the interrupt vector contributes to a larger architecture of system responsiveness. Interrupt handling doesn't exist in a vacuum—it drives determinism in real-time operating systems, mediates hardware-device interaction, and ensures stable execution under high I/O workloads. Poorly handled interrupts introduce unpredictable behavior. Precise, efficient interrupt code guarantees performance consistency and hardware stability.

Static theories and system diagrams are only half of the journey. Moving forward, dive into real-world codebases and challenge assumptions. Use emulators or hardware simulators to monitor ISR behavior under load. Benchmark your interrupt latency with tools like perf or ftrace. Experiment with platform-specific registers or open-source kernels to observe nested interrupts in action.

Begin with the following:

• Write a minimal ISR that toggles an output pin on a microcontroller—see how often it fires and if latencies vary.
• Inspect the IVT of an x86 machine with a memory debugger. Where do different vectors point?
• Use virtualization tools to trigger and trace system calls routed through software interrupts.

Engaging directly with interrupt code makes all abstractions tangible. Complexity fades when you watch a breakpoint hit inside your handler and see the registers change. So what will you test first?