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The subject matter of the "SuperFloppy", Here to Learn
post Sep 10 2008, 06:27 PM
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Whatever you say. (IMG:../forums/style_emoticons/default/smile.gif)

In my book, and in the context I was talking about, a partition type existed that used a filesystem (hex code 04) using 16 bit addressing that was limited to 32 Mb in size.

What I was trying to convey was that there was no way at the time to let MS-DOS access anything bigger than 32 Mb, hence the need to partition the drive, dividing it in parts.

Initial FAT16
In 1984 IBM released the PC AT, which featured a 20 MB hard disk. Microsoft introduced MS-DOS 3.0 in parallel. Cluster addresses were increased to 16-bit, allowing for up to 65,517 clusters per volume, and consequently much greater file system sizes. However, the maximum possible number of sectors and the maximum (partition, rather than disk) size of 32 MB did not change. Therefore, although technically already "FAT16", this format was not what today is commonly understood as FAT16. A 20 MB hard disk formatted under MS-DOS 3.0 was not accessible by the older MS-DOS 2.0. Of course, MS-DOS 3.0 could still access MS-DOS 2.0 style 8 KB cluster partitions.

MS-DOS 3.0 also introduced support for high-density 1.2 MB 5.25" diskettes, which notably had 15 sectors per track, hence more space for FAT. This probably prompted a dubious optimization of the cluster size, which went down from 2 sectors to just 1. The net effect was that high density diskettes were significantly slower than older double density ones.[dubious - discuss]

As well, according to my sources, the limit is attributed to the way the "initial" FAT16 was implemented:
DOS 3 (32 MiB / 33.6 MB) Barrier: To get around the 16 MiB barrier, DOS 3.x was altered when the IBM PC/AT was introduced with larger drives. The first support for the FAT16 file system was added. However, a new barrier was introduced by the rather limited way in which FAT16 was originally implemented: cluster size was set to 2,048 bytes, and only 16,384 FAT entries were allowed, fixing maximum capacity at around 32 MiB. The ability to have multiple partitions was introduced at around the same time, but each partition could only be 32 MiB or less.

You are right in saying that the theoretical addressing limit of FAT16 is around 65536, actually 65517 but the limit was the above.

In other words, the "initial" FAT16 was implemented in a different way from the "normal" FAT16 (type 06) we are familiar with.

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