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Q39531: How ROM BIOS Disk Limitation of 1024 Cylinders Affects MS-DOS

Article: Q39531
Product(s): Microsoft Disk Operating System
Version(s): MS-DOS:2.x,3.x,4.x,5.x,6.0,6.2,6.21,6.22
Operating System(s): 
Keyword(s): 
Last Modified: 17-DEC-2000

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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 
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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

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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

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