Tip 68: Undocumented FDISK

 Background

This page describes undocumented switches (or parameters) you can use with FDISK. Notably these switches can be used when running FDISK in non-interactive mode, e.g. in a batch file.

The switches work with the FDISK utility that is include with Windows 95 OSR2 (MS-DOS 7.1) and up.

 How To ...

Variables used in this document:

  • <size> Partition size in Mb - FDISK rounds this number up
  • <disk> Hard disk number, e.g. first HD is number 1
 Switches

/STATUS Show current partition layout
/MBR Recreate Master Boot Record on disk 1
/PRI Create primary partition
/PRIO Create primary partition with FAT16/FAT32 override
/EXT Create extended partition
/LOG Create logical drive
/LOGO Create logical drive with FAT16/FAT32 override
/FPRMT Prompt for FAT32/FAT16 in interactive mode
/Q Quit FDISK without rebooting. Used with input redirection (Piping keystrokes).
/X Do not use LBA partitions
/ACTOK Do not check drive integrity
/CMBR Recreate Master Boot Record on specified disk
Notes on /PRI, /PRIO and /LOG, /LOGO

 Switches explained
 
/STATUS - Show current partition layout

 FDISK /STATUS
 
If you have an extended partition with no logical drive defined, no information about the extended partition is shown.

 
/MBR - Recreate Master Boot Record on disk 1

This function is handy when an virus has infected the Master Boot Record. With /MBR you can wipe-out the virus.

 FDISK /MBR
 
/PRI - Create primary partition. Partition is set to active

Create a primary partition on disk number <disk> with the size of <size>. The partition is set to active.

 FDISK /PRI:<size> <disk>
 
If <size> is larger than the space on the HD all space is used for the primary partition.
/PRIO - Create primary partition with FAT16/FAT32 override. Partition is set to active

Works as /PRI.
  
/EXT - Create extended partition

Create an extended partition (to hold logical drives) on disk number <disk>with the size of <size>.

 FDISK /EXT:<size> <disk>
  
If <size> is larger than remaining free space, all free space is used. That is, you don’t have to know the exact remaining size in order to use this switch.
/LOG - Create logical drive

With /LOG you create a logical drive with the size of <size>. /LOG must be used together with /EXT.

 FDISK /EXT:<size> <disk> /LOG:<size>
  
/LOG must be used together with /EXT and <size> must be the same for both switches. Furthermore, <size> must be smaller or equal to free space.
/LOGO - Create logical drive with FAT16/FAT32 override

Works as /LOG.

/FPRMT - Prompt for FAT32/FAT16 in interactive mode

With /FPRMT you won’t get the FDISK start screen where you are asked for support for large disks. Instead, you will be prompted for FAT16/FAT32 each time you create a partition.

 FDISK /FPRMT

Note that the prompt for FAT16/FAT32 will be available for ALL partitions, so this option can be used to force FDISK to create FAT 32 partitions smaller than 540Mb. (By default, FDISK uses FAT32 for partitions greater than 540Mb only).
/X - Do not use LBA partitions

With /X you won’t get any LBA partitions.

 FDISK /X
/ACTOK - Do not check drive integrity

With /ACTOK the integrity of drive is not checked making FDISK faster.

 FDISK /ACTOK

/ACTOK may not work every time.
/CMBR - Recreate Master Boot Record on specified disk

Works as /MBR with the exception that you specify the disk to have it’s MBR recreated.

 FDISK /CMBR <disk>
Notes on /PRI, /PRIO and /LOG, /LOGO

PRI and LOG create FAT32 when partitions are larger than 512Mb and FAT16 when partitions are smaller than 512Mb. PRIO and LOGO create FAT16 even if partitions are larger than 512Mb (in effect, it’s like FDISK from DOS 5/6).
 Piping Keystrokes

You can pipe keystrokes into FDISK using a file which contains the keystrokes including <CR>. Most editors can’t insert the <CR> character without <LF> so you need a hex-editor to do the job. This might also be true for the <ESC> character, although <ESC> can be entered in DOS EDIT using P <ESC> Take a look at an example below.

Example:

 FDISK /Q < FDISK.BIN

Contents of FDISK.BIN:

N
 1
 1
 n
 2000
 <ESC>2
 1
 <ESC><ESC><ESC>

The above example will run FDISK, no user intervention, create a primary partition of 2000 MB, no large disk support, set the partition "active" and quit without rebooting.