Note
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An FMap page provides a mechanism to quickly locate contiguous free space. Each byte in the FMap corresponds to one AMap page. The value of each byte indicates the longest number of free bits found in the corresponding AMap page. Because each bit in the AMap maps to 64 bytes, the FMap contains the maximum amount of contiguous free space in that AMap, up to about 16 kilobytes. Generally, because each AMap covers about 250 kilobytes of data, each FMap page (496 bytes) covers around 125 megabytes of data.
However, a special case exists for the initial FMap. As shown in the diagram in section 1.3.2, the HEADER contains an initial FMap, which is only 128 bytes, and which covers the first 32 megabytes of data.