How to Handle Exception in ARM: Simple Guide

In ARM architecture, exceptions are handled by setting up an exception vector table that points to specific handler routines. When an exception occurs, the processor jumps to the corresponding handler code defined in vector table to manage the error safely.

Why This Happens

Exceptions in ARM occur when the processor encounters unexpected events like undefined instructions, memory access errors, or interrupts. Without proper handling, the system can crash or behave unpredictably.

Here is an example of broken ARM assembly code that does not set up an exception handler, causing the system to fail on an undefined instruction exception.

armasm

    AREA Reset, CODE, READONLY
    ENTRY

    MOV R0, #0
    UDF #0      ; Undefined instruction triggers exception

    END

Output

Processor enters undefined instruction exception but no handler is defined, causing system crash or lockup.

The Fix

To fix this, you must define an exception vector table at a fixed memory location. Each vector points to a handler routine that safely manages the exception. For example, the undefined instruction vector points to a handler that can log the error or reset the system.

armasm

    AREA Reset, CODE, READONLY
    ENTRY

    ; Exception Vector Table at address 0x00000000
    VECTOR_TABLE
        B Reset_Handler           ; Reset
        B Undefined_Handler       ; Undefined Instruction
        B SWI_Handler             ; Software Interrupt
        B Prefetch_Handler        ; Prefetch Abort
        B Data_Handler            ; Data Abort
        B Reserved_Handler        ; Reserved
        B IRQ_Handler             ; IRQ
        B FIQ_Handler             ; FIQ

    Reset_Handler
        MOV R0, #0
        B End

    Undefined_Handler
        ; Handle undefined instruction here
        B End

    SWI_Handler
        B End

    Prefetch_Handler
        B End

    Data_Handler
        B End

    Reserved_Handler
        B End

    IRQ_Handler
        B End

    FIQ_Handler
        B End

    End
        B End

    END

Output

Processor jumps to the correct handler on exception, preventing crashes and allowing controlled recovery.

Prevention

To avoid exceptions causing system failure, always set up a complete exception vector table before running your main code. Test each handler to ensure it manages errors properly. Use debugging tools to catch exceptions early and write safe code to minimize exceptions.

Best practices include:

• Place vector table at address 0x00000000 or remap memory accordingly.
• Write clear, minimal handlers that log or reset safely.
• Use hardware debugging features to trace exceptions.
• Keep exception handlers fast and avoid complex logic.

Related Errors

Other common ARM exceptions include:

• Prefetch Abort: Occurs when instruction fetch fails; fix by checking memory mapping.
• Data Abort: Happens on invalid data access; fix by validating pointers and memory permissions.
• IRQ/FIQ: Interrupts that require proper interrupt service routines.

Each requires its own handler in the vector table to avoid system crashes.

Key Takeaways

Set up an exception vector table at a fixed address to handle ARM exceptions.

Write simple, safe handler routines for each exception type to prevent crashes.

Test exception handlers thoroughly to ensure reliable error management.

Use hardware debugging tools to detect and analyze exceptions early.

Keep exception handlers minimal and fast to maintain system stability.