Exception Vector Table in ARM: Definition and Usage

The exception vector table in ARM is a special memory area that holds addresses of code to run when different exceptions occur, like interrupts or errors. It acts like a map that the processor uses to quickly find the right response code for each exception type.

How It Works

The exception vector table in ARM is like a directory or a list of emergency phone numbers. When the processor encounters an unexpected event, such as an interrupt or fault, it needs to quickly find the correct code to handle that event. The vector table stores the starting addresses of these handler routines in a fixed order.

Each entry in the table corresponds to a specific exception type, such as reset, undefined instruction, software interrupt, prefetch abort, data abort, IRQ, and FIQ. When an exception happens, the ARM processor uses the exception type as an index to jump to the matching address in the vector table and start executing the handler code.

This mechanism ensures fast and organized handling of exceptions, allowing the system to respond correctly without confusion or delay.

Example

This example shows a simplified ARM assembly snippet defining an exception vector table with handlers for reset and IRQ exceptions.

armasm

    .section .vectors, "a"
    .global _start
_start:
    LDR PC, =reset_handler    @ Reset vector
    LDR PC, =undef_handler    @ Undefined instruction vector
    LDR PC, =swi_handler      @ Software interrupt vector
    LDR PC, =prefetch_abort_handler @ Prefetch abort vector
    LDR PC, =data_abort_handler @ Data abort vector
    LDR PC, =reserved_handler  @ Reserved vector
    LDR PC, =irq_handler       @ IRQ vector
    LDR PC, =fiq_handler       @ FIQ vector

reset_handler:
    B reset_handler           @ Loop here for demo

irq_handler:
    B irq_handler             @ Loop here for demo

When to Use

The exception vector table is essential in any ARM-based system to manage unexpected events safely and efficiently. It is used in embedded systems, operating systems, and firmware to handle hardware interrupts, system faults, and software-triggered exceptions.

For example, when a button is pressed on a device, an interrupt triggers the IRQ handler via the vector table to run code that responds to the button press. Similarly, if the processor encounters an illegal instruction, it uses the vector table to jump to the undefined instruction handler to manage the error.

Setting up the vector table correctly is critical for system stability and responsiveness.

Key Points

• The exception vector table holds addresses of exception handlers in a fixed order.
• It allows the ARM processor to quickly jump to the right code when exceptions occur.
• Common exceptions include reset, IRQ, FIQ, and aborts.
• Proper setup of the vector table is vital for system reliability.
• It is used in embedded systems and operating system kernels.

Key Takeaways

The exception vector table maps exception types to handler addresses for quick response.

Each exception type has a fixed position in the vector table used by the ARM processor.

It is crucial for handling interrupts and faults in ARM-based systems.

Correctly setting up the vector table ensures system stability and proper exception management.

Used widely in embedded systems and operating system kernels for managing exceptions.