DOCUMENT:Q62571  17-DEC-2000  [msdos]
TITLE   :Converting Drive Letters to MS-DOS INT 13H Disk Drive Numbers
PRODUCT :Microsoft Disk Operating System
PROD/VER:MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22
OPER/SYS:
KEYWORDS:

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The information in this article applies to:

 - Microsoft MS-DOS operating system versions 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
=======

The BIOS Interrupt 13H uses a zero-based number for floppy disk drives ("A"= 0,
"B"= 1, and so on), and a zero-based number with the high bit (bit 7) set for
hard disks ("C"= 80H, "D"= 81H, and so on).

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

You can use the following algorithm to convert any logical drive letter (ASCII)
to the drive numbers that Interrupt 13H uses:

1. Identify the block device with Interrupt 21H IOCTL function 44H, subfunction
   0dH, minor code 60H (Get Device Parameters), as either a floppy disk drive or
   a hard disk drive by checking the returned parameter block "device type"
   field byte at offset +1. A value of 0-4 or 7 indicates that it is a floppy
   disk drive. A value of 5 indicates that it is a hard disk drive.

2. If the device is a floppy disk drive, subtract 1 from the BL drive number
   input to the function Get Device Parameters to make it zero based, or take
   the uppercase ASCII logical drive letter and subtract the value of "A". For
   example:

      Drive letter "A" - "A" = 0,  "B" - "A" = 1, and so on

3. If the device is a hard disk drive, you need to interpret the hard disk
   partition table to differentiate between a primary partition and a logical
   partition because the ordering of logical drive numbers does not necessarily
   reflect the physical order and corresponding BIOS physical drive number.

   For more information, query on the following keywords:

   prod(msdos) and driver.sys and order

   Once you know how MS-DOS assigns logical drive letters on system
   initialization, you need to address the correct physical hard disk and
   compute the offset (starting sector number) of any logical drive by using the
   starting sector values supplied in the hard disk's partition table. These
   starting sector numbers would be supplied to the BIOS Int 13H function to
   access those sectors representing a logical drive. "Advanced MS-DOS
   Programming" by Ray Duncan has more information on the boot record and
   partition tables.

   Example
   -------

   HD 1: Bios# 80H      HD 2: Bios# 81H      HD X: Bios# X...

   C: (primary)         E: (logical)
   -----------          -----------
   D: (logical)         F: (logical)

   To access logical drive D, address HD 1: Bios# 80H, but sectors starting at
   logical partition D:.

Additional query words: 6.22 3.30 4.01 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
Version           : MS-DOS:3.x,4.x,5.x,6.0,6.2,6.21,6.22

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