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USING A BOOT MONITOR IN EMBEDDED SYSTEMS

Part 1ýConstructing the Platform
by Ed Sutter

Start ý The Typical Package ý What are the Alternatives? ý Key Ingredients ý MicroMonitor Run Time Startup ý Just After Reset ý Establish Exception Handlers ý I/O Initialization ý Flash Memory Drivers ý Sources and PDF

ESTABLISH EXCEPTION HANDLERS

The next step is to initialize the CPU vector table, which is a block of memory (either in a fixed location or pointed to by a system register) that contains either code or pointers to code. The code is asynchronously branched to from running application code as a result of some system condition occurring. This could be an interrupt, invalid instruction, breakpoint trap, address alignment error, or so on. There are a lot of different exceptions, and for each CPU, the list is different, as well as the implementation of the vector table itself. This is CPU-specific, and it is beyond the scope of this article to go much further regarding specific CPU implementations.

From a generic point of view, the monitor has two jobsýit must create the table and be prepared to handle the exception. Afterward, it must cache the state of the registers at the time of the exception. The table could be part of the boot flash if the boot flash is in memory space that allows this. However, I find it more practical to create the table in RAM space whenever possible. This means that at startup there is a window of time when there is no table, but it provides the flexibility of allowing the content of the table to be changed. This is handy if you are trying to share the table between an application and monitor because the monitor can write default handlers for all exceptions, then the application can overwrite the handlers that it needs to deal with, leaving the others to default to monitor-based exception handling.

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