ARM Exception Types: Overview and Usage

In ARM architecture, exception types are categories of events that interrupt normal program flow, such as Reset, Undefined Instruction, Software Interrupt (SWI), Prefetch Abort, Data Abort, and IRQ/FIQ. Each exception type triggers a specific handler to manage the event and maintain system stability.

How It Works

ARM exceptions are like special signals that tell the processor something unusual happened. Imagine driving a car and suddenly seeing a warning light; you need to stop and check. Similarly, when an exception occurs, the ARM processor pauses the current task and jumps to a special piece of code called an exception handler.

Each exception type corresponds to a different cause, such as a reset, an illegal instruction, or a hardware interrupt. The processor saves its current state and switches to a mode designed to handle that exception safely. After the handler finishes, the processor can return to the original task or take other actions.

Example

This example shows how an ARM processor handles a Software Interrupt (SWI) exception by jumping to the SWI handler address.

armasm

AREA Reset, CODE, READONLY
ENTRY

    MOV R0, #0          ; Prepare a value
    SWI 0x01            ; Trigger Software Interrupt

SWI_Handler
    ; This is where SWI exception is handled
    MOV R1, #1          ; Example action in handler
    BX LR               ; Return from exception

    END

Output

The processor executes the SWI instruction, then jumps to SWI_Handler to process the interrupt before returning.

When to Use

Exception types in ARM are used whenever the processor needs to respond to unexpected or important events. For example, Reset exceptions start the system safely after power-up. Undefined Instruction exceptions catch invalid code that could cause errors. Software Interrupts (SWI) allow programs to request operating system services.

Hardware interrupts like IRQ and FIQ let external devices signal the processor to handle input/output or time-critical tasks. Using these exceptions properly helps build reliable and responsive systems.

Key Points

• ARM exceptions interrupt normal execution to handle special events.
• Each exception type has a dedicated handler and processor mode.
• Common types include Reset, Undefined Instruction, SWI, Prefetch Abort, Data Abort, IRQ, and FIQ.
• Proper use of exceptions improves system stability and responsiveness.

Key Takeaways

ARM exceptions are special events that interrupt normal program flow to handle errors or signals.

Each exception type triggers a specific handler designed to manage that event safely.

Common ARM exception types include Reset, Undefined Instruction, SWI, Prefetch Abort, Data Abort, IRQ, and FIQ.

Using exceptions correctly is essential for building stable and responsive ARM-based systems.