1. IMG File Format (part 3)

1.3 FAT Block

/c%H5} X^S1615After the block sequence comes what can be labeled a File Allocation Table, or FAT. The data section of the IMG file contains a number of “sub-files”, and the location and type for each is defined in a series of FAT blocks. Collectively, these blocks make up the FAT itself. Each FAT block is 512 bytes in length and corresponds to a single sub-file.
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+uq9M;p;t2s"|.B1615As with the header, a block sequence counter takes up the final 480 bytes of each FAT block, mapping the sub-file to particular file blocks inside the IMG file, itself. The numbering of these sequence numbers continues from the header sequence, and from the previous FAT block sequences. This definition implies that an IMG file can never have more than 65,535 blocks, but the use of large block sizes (2048, 4096, etc…) allow the IMG file to hold several gigabytes of data. The only practical limit on IMG file size seems to be the use of 32-bit integers for file offsets, which imposes a hard limit of 4 GB.

Large sub-files, those with more than 240 blocks of data, are spanned across multiple FAT blocks with each sub-file part numbered in a 32-bit integer at offset 0x10. File size is only present in the first of these (part 0x0000).