What is an illegal address (ILAD) reset?

* Question

What is an illegal address (ILAD) reset?

* Answer

An Illegal Address (ILAD) Reset refers to a system reset that is triggered when a processor or controller attempts to access an invalid or restricted memory address—an address outside the defined or allowed range of the system’s memory map.

1. Definition of Illegal Address (ILAD)

An illegal address is any memory location that:

Does not exist within the system’s physical or logical memory space.

Is not mapped to any peripheral or valid memory region.

Is protected or restricted due to access control (e.g., privileged-only regions).

Violates alignment rules in architectures with strict memory alignment.

2. What Triggers an ILAD Reset?

An ILAD reset is typically triggered when:

A program attempts to read from or write to a non-existent memory location.

A peripheral or DMA controller tries to access an invalid memory region.

A corrupted instruction or faulty pointer results in undefined memory access.

Stack overflows or incorrect function return addresses lead to illegal jumps.

In many microcontrollers and SoCs, hardware trap mechanisms detect such access violations and initiate a system reset or exception handling procedure.

3. Common Contexts Where ILAD Occurs

Embedded systems: Accessing an unimplemented peripheral register or invalid flash/ROM area.

RTOS or bare-metal firmware: Dereferencing a null or misaligned pointer.

Bootloaders: Trying to jump to an application address that has not been programmed.

4. System Behavior on ILAD Reset

Depending on the processor architecture and system configuration:

A reset signal is issued, bringing the system back to its initial state.

An exception handler may log the cause and try recovery (if supported).

Debug registers may log the offending address and instruction for analysis.

5. Prevention and Debugging Techniques

Use memory protection units (MPUs) or MMUs to define valid address regions.

Enable watchdog timers and software assertions to detect faults early.

Utilize debug interfaces (e.g., JTAG, SWD) to trace instruction flow.

Implement bounds checking in code, especially in pointer arithmetic.

Summary

Conclusion:
An ILAD reset is a protective mechanism that prevents the system from operating in an undefined or dangerous state due to invalid memory access. Understanding its triggers and implementing safeguards is critical in embedded and real-time systems to ensure system integrity and reliability.