MS-DOS 6.22 Logical Sector Size Configuration

This article examines how MS-DOS 6.22 defines, reads, and configures logical sector sizes for storage devices. It identifies the primary system structure responsible for sector geometry—the BIOS Parameter Block (BPB)—explains the rigid 512-byte operating system standard for physical disks, and details the specific CONFIG.SYS device parameters available for defining sector sizes in custom and virtual drives.

In MS-DOS 6.22, the logical sector size of a formatted disk volume is defined by the Bytes Per Sector parameter located in the BIOS Parameter Block (BPB) at offset 0x0B of the volume boot record. MS-DOS does not feature a generic, global CONFIG.SYS command (such as a standalone SECTORSIZE= directive) to alter physical disk sector sizes. Instead, when a block device is mounted, the DOS kernel queries this specific BPB field from the partition's boot sector to establish how many bytes make up a single logical sector.

For physical fixed disks and standard floppy drives, MS-DOS 6.22 enforces a strict default of 512 bytes per sector. While the BPB structure mathematically supports larger sector sizes (such as 1024, 2048, or 4096 bytes), core MS-DOS 6.22 disk utilities—including FDISK.EXE and FORMAT.EXE—as well as standard ROM BIOS Interrupt 13h routines, hardcode and require 512-byte sector dimensions. Formatting a physical disk with a non-512-byte sector size typically leads to file system corruption or failure to boot under native DOS.

The only standard system configuration context where a user can explicitly define the sector size parameter in CONFIG.SYS is through virtual block device drivers, primarily RAMDRIVE.SYS. The configuration syntax for this driver is:

DEVICE=[drive:][path]RAMDRIVE.SYS [DiskSize] [SectorSize] [NumEntries] [/E | /A]

In this directive, SectorSize acts as the explicit configuration parameter, accepting values of 128, 256, or 512 bytes. When omitted, MS-DOS defaults the parameter to 512 bytes.

At the architecture level, custom block device drivers communicate logical sector sizes to the MS-DOS kernel during the driver initialization phase (INIT function, command code 0). The driver passes a pointer to its BPB array back to MS-DOS in the request header. MS-DOS 6.22 evaluates the largest sector size reported across all active block device drivers and uses that maximum value to size its internal input/output transfer buffers.