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Canon® EOS Rebel XSi/450D Guide to Digital SLR Photography by David D. Busch

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Upgrading Your Firmware

If you’re computer savvy, you might wonder how your Rebel XSi is able to overwrite its own operating system—that is, how can the existing firmware be used to load the new version on top of itself? It’s a little like lifting yourself by reaching down and pulling up on your bootstraps. Not ironically, that’s almost exactly what happens: At your command (when you start the upgrade process), the XSi shifts into a special mode in which it is no longer operating from its firmware but, rather, from a small piece of software called a bootstrap loader—a separate, protected software program that functions only at startup or when upgrading firmware. The loader’s function is to look for firmware to launch or, when directed, to copy ...

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