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Sunday, 18 December 2011

Operating Systems FILE SYSTEM FAT 12

This initial version of FAT is now referred to as FAT12. As a filesystem for floppy disks, it
had a number of limitations: no support for hierarchical directories, cluster addresses were "only"
12-bits long (which made the code manipulating the FAT a bit tricky) and the disk size was stored
as a 16-bit count of sectors, which limited the size to 32MB.
An entry-level diskette at the time would be 5.25", single-sided, 40 tracks, with 8 sectors per
track, resulting in a capacity of slightly less than 160KB. The above limits exceeded this capacity
by one or more orders of magnitude and at the same time allowed all the control structures to fit
inside the first track, thus avoiding head movement during read and write operations. The limits
were successively lifted in the following years.
Since the sole root directory had to fit inside the first track as well, the maximum possible
number of files was limited to a few dozens.

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