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The first 4 KB of NVRAM space in a typical configuration may be reserved for use by operating systems other than Mac OS. The Macintosh firmware and system software does nothing with this space except to initialize the first 2 bytes to show that the available NVRAM size is 4 KB.
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Operating systems that use this space would need to provide their own protocols for allocating fields and for defining, updating, and checking data. In particular, they would need to follow rules for determining whether fields in the NVRAM operating-system partition use big-endian or little-endian addressing.