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The Primary FAT starts at Sector 1, byte #0x0200.
Suppose a file has a starting cluster of 0x49 (73).
The file starts on the media at cluster #73
The FAT entry pointing to the next cluster is in the FAT at byte #(73*1.5 + 1) = 110 (0x6e)
Since the FAT starts at 0x0200, the FAT entry for this file is at 0x026e
Example:
1st cluster is 0x49 (73). FAT entry starts at high-order nibble of 0x026e (110) = 0x04a (74)
2nd cluster is 0x4a (74). FAT entry starts at low-order nibble of 0x0270 (112) = 0x04b (75)
3rd cluster is 0x04b (75). FAT entry starts at high-order nibble of 0x0271 (113) = 0x04c (76)
4th cluster is 0x04c (76). FAT entry starts at low-order nibble of 0x0273 (115) = 0x04d (77)
5th cluster is 0x04d (77). FAT entry starts at high-order nibble of 0x0274 (116) = 0xfff (end of list)
The physical size of this file is five clusters (2560 bytes), and occupies clusters 73, 74, 75, 76, and 77 on the medium. (It is merely a coincidence that the clusters are contiguous.)
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