DOCUMENT:Q39531  17-DEC-2000  [msdos]
TITLE   :How ROM BIOS Disk Limitation of 1024 Cylinders Affects MS-DOS
PRODUCT :Microsoft Disk Operating System
PROD/VER:MS-DOS:2.x,3.x,4.x,5.x,6.0,6.2,6.21,6.22
OPER/SYS:
KEYWORDS:

======================================================================
-------------------------------------------------------------------------------
The information in this article applies to:

 - Microsoft MS-DOS operating system versions 2.11, 3.1, 3.2, 3.21, 3.3, 3.3a, 4.0, 4.01, 5.0, 5.0a, 6.0, 6.2, 6.21, 6.22 
-------------------------------------------------------------------------------

SUMMARY
=======

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 " (without the quotation marks)

MORE INFORMATION
================

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.

REFERENCES
==========

"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          :  
Technology        : kbMSDOSSearch kbMSDOS321 kbMSDOS400 kbMSDOS320 kbMSDOS330a kbMSDOS621 kbMSDOS622 kbMSDOS620 kbMSDOS600 kbMSDOS310 kbMSDOS500 kbMSDOS330 kbMSDOS401 kbMSDOS500a kbMSDOS211
Version           : MS-DOS:2.x,3.x,4.x,5.x,6.0,6.2,6.21,6.22

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS
PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND.  MICROSOFT DISCLAIMS
ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES
OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE.  IN NO
EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR
ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL,
CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF
MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE
POSSIBILITY OF SUCH DAMAGES.  SOME STATES DO NOT ALLOW THE EXCLUSION
OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES
SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 2000.