Microsoft KB Archive/39531

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How ROM BIOS Disk Limitation of 1024 Cylinders Affects MS-DOS

Article ID: 39531

Article Last Modified on 11/16/2006


  • Microsoft MS-DOS 2.11 Standard Edition
  • Microsoft MS-DOS 3.1
  • Microsoft MS-DOS 3.2 Standard Edition
  • Microsoft MS-DOS 3.21 Standard Edition
  • Microsoft MS-DOS 3.3 Standard Edition
  • Microsoft MS-DOS 3.3a
  • Microsoft MS-DOS 4.0 Standard Edition
  • Microsoft MS-DOS 4.01 Standard Edition
  • Microsoft MS-DOS 5.0 Standard Edition
  • Microsoft MS-DOS 5.0a
  • Microsoft MS-DOS 6.0 Standard Edition
  • Microsoft MS-DOS 6.2 Standard Edition
  • Microsoft MS-DOS 6.21 Standard Edition
  • Microsoft MS-DOS 6.22 Standard Edition

This article was previously published under Q39531


Systems that have hard disks with more than 1024 cylinders cannot use the entire drive with a standard ROM BIOS or with a generic MS-DOS packaged product or the Microsoft MS-DOS 5 Upgrade and later MS-DOS versions. This is because the default IBM-PC ROM BIOS can only comprehend disks that have 1024 or fewer cylinders. Microsoft MS-DOS relies on an IBM-compatible ROM BIOS to communicate with the system hardware, and thus cannot understand disks that the ROM BIOS cannot understand.

For information on common workarounds for the 1024-cylinder limitation, query on the following words in the Microsoft Knowledge Base:

1024 and cylinder and workarounds


The design of the IBM ROM BIOS disk interface did not anticipate disks with more than 1024 cylinders. The ROM BIOS Interrupt 13H disk interface has a standard register interface in which the cylinder number of a disk is partly contained in two different registers: the 8-bit CH register contains the low-order 8 bits of the cylinder number, and the 8-bit CL register contains the high-order 2 bits of the cylinder number. When combined, this creates a 10-bit cylinder number, zero based, thus giving the upper limit of 1024 (0 to 1023).

For OEM ROM BIOS extensions, refer to your OEM hardware technical reference manuals.


"IBM PS/2 and PC BIOS Interface Technical Reference," part number 68X2260, available from IBMB.

"Programmer's Quick Reference Series: IBM ROM BIOS" by Ray Duncan, ISBN 1-55615-135-7, Microsoft Press

Additional query words: 6.22 3.0 3.00 3.1 3.10 3.2 3.20 3.21 3.22 3.3 3.30 3.3a 3.30a 4.0 4.00 4.0a 4.00a 4.01 4.01a 5.00 5.00a 6.00 6.20

Keywords: KB39531