How to use ARM's data-abort exception

By Roger Lynx
Aug 22 2006 (18:30 PM), Courtesy of Embedded Systems Design

Processors giveth and processors taketh away. They can fetch and store data or they can refuse to do either. When your processor aborts a data access, what can you do? This in-depth article explains the hows and whys of data aborts on the ARM7 family of processors, including working code for a useful data-abort exception handler.

The late Joseph Campbell, well-known scholar of comparative religion and mythology, once expressed his sentiments about computers from his perspective: "Computers are like Old Testament gods; lots of rules and no mercy." Add to his observations the familiar wisdom "where there are rules, there are also exceptions," and your Old Testament machine becomes more forgiving.

The data-abort exception (with the help of an exception handler) may be God's gift to ARM programmers. A data-abort exception is a response by a memory system to an invalid data access. The data-abort exception handler is a program that can inform the programmer where in his or her code this exception has occurred (after the application has crashed). The exception handler ought to handle the consequences of the aborted instruction gracefully, rather than forcing the processor to hang in an infinite loop. If you understand the fundamental rules of the ARM architecture and data-abort exception handling, you'll spend less time begging for mercy.

Not all ARM processors, however, come with these data-abort exception handlers. The exception handlers are usually not provided by the compiler, RTOS, or silicon vendors since it would necessitate quite a high level of integration. Ideally, a perfectly designed system doesn't need an exception handler. However, in a process of striving for perfection, engineers can come across moments when the processor showers them with a slew of undesired abort exceptions. These exceptions may originate in software, such as improper C-structures, or appear in code ported from a different processor architecture. Alternatively, exceptions may become signs of improper memory system design or manifestations of environmental effects on a marginal hardware design or on a specific component.

This article is an introduction to programming data-abort exceptions handlers on the ARM architecture. I'll demonstrate many of the concepts related to exceptions using the LPC 2148 from Philips, which necessitates a side trip through the underworld of the LPC 2000 family's undocumented features. I'll also explore how the discrete implementations of ARM core in a general-purpose MCU may provide a disparate saga about the causes of the data-abort exception. I'll also dispel some myths associated with exceptions and data alignment and show you how to create a data-abort exception handler for the LPC 2148 (and other ARM processors). The material presented here may help you develop your own exception handler for a similar ARM7TDMI processor and reduce your debugging time.

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