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Q86863: Maximum Hard Drive Partitions with MS OS/2 1.3 for Server

Article: Q86863
Product(s): Microsoft LAN Manager
Version(s): 
Operating System(s): 
Keyword(s): 
Last Modified: 30-JUL-2001

SUMMARY
=======

Microsoft OS/2 version 1.3 for Servers provides large drive translations, which
allows drives with a physical geometry over 1024 cylinders, 63 sectors/cylinder,
and 256 heads. For hard drives such as ESDI, IDE, MFM, and RLL, this is
accomplished by a logical translation via the disk-type-specific driver
(DISK.TSD).

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

To conform to IBM BIOS standards, all disk I/O requests are returned in CHS
(cylinder, head, sector) format, which is limited to 1024 cylinders, 63
sectors/cylinder, and 256 heads. Because cylinder addressing is limited to 10
bits (1024), heads to 8 bits (256), and sectors to 6 bits (64) minus 1 (because
sector addressing is 1-based), this gives a maximum addressable disk size of:

  1024*256*(64-1)*512 = 7.8 gigabytes

(Note: OS/2 sector size is 512 bytes.)

This is the maximum physical size of a hard disk that will be recognized. The
maximum partition size that can be addressed is limited to 2 gigabytes for HPFS
and for FAT partitions. HPFS386 partitions can address a 7.8 gigabyte
partition.

Under Microsoft OS/2 1.3 for Servers, with LADDR support installed, it is
possible to circumvent the restriction that a hard drive have no more than the
maximum allowed physical dimensions. This is accomplished by presenting OS/2 an
apparent drive geometry of acceptable limits. The apparent geometry is computed
by DISK.TSD, in the case of ESDI, IDE, RLL, and MFM drives. The translation
method basically takes any parameter that is outside of the allowed maximum,
divides it by 2, and doubles the number of one of the remaining parameters,
until the apparent geometry fits within the allowed maximum, or no further
translation can be accomplished. For example, if a drive has 1764 cylinders, 124
sectors, and 32 heads, it will be translated as follows:

              Original           Translation      Translation
              --------           -----------      -----------
  Cylinder     1764                882              882
  Sector        124                124               62
  Head           32                 64              128

The final geometry that DISK.TSD presents to OS/2 is that of a drive of 882
cylinders, 62 sectors, and 128 heads. DISK.TSD then performs the necessary
translation for reads and writes to the disk.

Additional query words: 2.10 1.30

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Keywords          :  

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