MS-DOS 6.22 Extended Partition Size Limits

In MS-DOS 6.22, the maximum capacity of a secondary extended DOS partition is primarily governed by the 8.4 GB (8,422 MB) physical limit imposed by the standard BIOS INT 13h interface, while any individual logical drive created inside that extended partition is limited to 2 GB (2,048 MB) due to the constraints of the 16-bit File Allocation Table (FAT16) file system. Consequently, while the extended partition container itself can occupy the remaining space of an 8.4 GB disk—up to approximately 8.4 GB if the primary partition is negligible—it must be divided into smaller logical drives of no more than 2 GB each.

BIOS and Interface Constraints (The 8.4 GB Limit)

MS-DOS 6.22 does not natively support Logical Block Addressing (LBA) or the extended INT 13h BIOS functions introduced in later operating systems like Windows 95 OSR2 and MS-DOS 7.10. Instead, it relies on standard BIOS Cylinder-Head-Sector (CHS) translation.

Under the INT 13h standard:

  • Maximum Cylinders: 1,024
  • Maximum Heads: 255 (or 256 in theoretical hardware limits, typically 255 in BIOS)
  • Maximum Sectors per Track: 63
  • Sector Size: 512 bytes

Multiplying these parameters (\(1024 \times 255 \times 63 \times 512\)) yields approximately 8,422,686,720 bytes, or roughly 7.84 GiB / 8.4 GB. Because MS-DOS 6.22 cannot address sectors beyond this threshold, the entire hard disk—including both primary and extended partitions combined—cannot exceed this boundary. If a primary partition uses the maximum allowed 2 GB, the extended partition can occupy the remaining space of roughly 6.4 GB.

FAT16 Logical Drive Limits (The 2 GB Limit)

Although the extended partition container can reach several gigabytes, MS-DOS 6.22 cannot format an extended partition as a single large drive. The extended partition simply serves as a container for logical DOS drives, each of which must use the FAT16B file system.

FAT16 is constrained by:

  • Cluster Addressing: A maximum of 65,526 clusters.
  • Maximum Cluster Size: 32 KB (64 sectors per cluster) under standard MS-DOS implementations.

Multiplying 65,526 clusters by 32 KB results in a maximum volume size of 2,096,832 KB, commonly referenced as 2 GB or 2,048 MB. While FAT16 theoretically supports 64 KB clusters (allowing 4 GB volumes), MS-DOS 6.22's native utilities, including FDISK and FORMAT, do not support 64 KB clusters due to compatibility and software stability constraints.

Structuring Storage in MS-DOS 6.22

To utilize the full capacity of an 8.4 GB hard disk in MS-DOS 6.22:

  1. Primary Partition (Drive C:): Allocated up to 2,048 MB.
  2. Extended Partition: Allocated the remainder of the disk (approximately 6.4 GB).
  3. Logical Drives: The extended partition is subdivided into multiple logical drives (D:, E:, F:, and G:), each assigned up to 2,048 MB.

MS-DOS supports drive letters up to Z:, providing more than enough letters to allocate multiple 2 GB volumes across the maximum addressable 8.4 GB space.