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Older Size Barriers

The first widely-publicized hard disk barrier was the infamous 504 MiB / 528 MB barrier that showed up in the mid 1990s. Though not widely known, there were a number of older capacity barriers that affected hard drives before the 504/528 limit made so many PC headlines. Most of these got little attention, most likely because there were fewer PC users then, but also because upgrading was less common. (Upgrades are often the cause of barrier troubles.)

At any rate, these have no relevance at all to modern computing, but might be of interest to those who have much older machines. I mention them here for completeness, if for no other reason, but I will describe them only briefly since again, they have no impact on modern PCs:

• PC/XT Parameter (10.4 MiB / 10.9 MB) Barrier: The very first PC to use a hard disk was IBM's PC/XT. This machine was specifically designed to use a particular type of disk with 312 cylinders, 4 heads and 17 sectors per track. As such, it was hard-wired to the approximately 10 MB capacity of those very early drives.
• FAT12 Partition Size (16 MiB / 16.7 MB) Barrier: The first FAT format used for hard disks was the 12-bit FAT12 partition type (still used for floppy disks). This allowed a maximum of 4,086 clusters of 4,096 bytes, for a total of 16,736,256 bytes per disk.
• 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.
• DOS 4 (128 MiB / 134 MB) Barrier: DOS 4.x improved over DOS 3.x by allowing 65,526 clusters instead of 16,384, quadrupling maximum partition size to about 128 MiB. Cluster size was still fixed at 2,048 bytes.

As you can see, most of these early limits were not due to BIOS issues, but rather some very short-sighted thinking on the part of the MS-DOS design team, which was apparently only trying to stay a year or two ahead of the hard disk technology curve! In addition to the above, there was a 512 MiB barrier as well, caused by the change made with DOS 5. That operating system changed DOS 4 by allowing cluster size in a single partition to increase to 8,192 bytes, allowing a theoretical maximum partition size of about 512 MiB or 537 MB. However, most systems that had drives large enough for this to be an issue were not able to use that full size due to the slightly smaller 504 MiB / 528 MB barrier, caused by BIOS issues.

Next: The 1,024 Cylinder (504 MiB / 528 MB) Barrier

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