When you map a view of a file into your process's address space, you must specify two things. First, you must tell the system which byte in the data file should be mapped as the first byte in the view. You do this using the dwFileOffsetHigh and dwFileOffsetLow parameters. Because Windows supports files that can be up to 16 EB, you must specify this byte-offset using a 64-bit value of which the high 32 bits are passed in the dwFileOffsetHigh parameter and the low 32 bits are passed in the dwFileOffsetLow parameter. Note that the offset in the file must be a multiple of the system's allocation granularity. (To date, all implementations of Windows have an allocation granularity of 64 KB.)
Jeffrey Richter, Programming Applications for Microsoft Windows