Embedded Systems Design - July 2008 - (Page 35)

20 years ago exception primer BY THOMAS BESEMER An The following article ﬁrst appeared in the July 1989 issue of Embedded Systems Programming magazine. ost developers are somewhat familiar with the notion of exception handling. Software is rarely implemented with perfect hardware in a perfect environment, and the possibility of an anomalous event or error condition has to be considered. Exception handling in embedded applications is often vitally important; a confused processor can generate expensive and even life-threatening results. Having a program give up and post an Abort, Retry, Fail? message is rarely an acceptable option in avionics or medical applications. The way exceptional conditions are handled depends on the supporting software. In general, the exception handler is an operating-system resource. In a UNIX environment, for example, an error during a program’s execution (such as an invalid memory access) will cause the operating system to generate a core ﬁle that’s written to the disk, then halt program execution. That’s simple exception handling: a fault is recognized, a ﬁle is generated to indicate the system state at the time of the fault, the process is killed, and the operating system returns to the user prompt. The operator is then expected to take appropriate action, such as determining the cause of the error and re-executing the program. In contrast to the UNIX scenario, embedded systems call for sophisticated exception handling techniques. Embedded usually implies that the system performs a known, deﬁned function in the real world. The operating personnel will likely be untrained in computer engineering, or perhaps not even present. As a result, the system must be able to resolve the anomalous condition and continue operation. AN EXCEPTIONAL METHODOLOGY The ﬁrst step in any exception-handling scheme is to deﬁne what will constitute an exception. The deﬁnition depends largely on whether the underlying operating system is a simple monitor, a kernel such as VRTX or pSOS, or a full-blown operating system such as UNIX. *Editor’s note: Remember, this article was written 20 years ago. M There’s nothing new about the need for exception handling, which is a key feature in embedded applications. This article, which ﬁrst appeared 20 years ago, describes exception handling in detail. www.embedded.com | embedded systems design | JULY 2008 35 http://www.embedded.com

Table of Contents Feed for the Digital Edition of Embedded Systems Design - July 2008

Embedded Systems Design - July 2008 Contents #Include Parity Bit Programming Pointers Interactive C-code Cleaning Tool Supports Multiprocessor SoC Design Building a Power Supply for Discontinuous Transmission Wireless Networks An Exception Primer Advertising Index Break Points Marketplace

Embedded Systems Design - July 2008

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