Maximum Hard Drive Partitions with MS OS/2 1.3 for Server (86863)

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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.