FDF Image

File Format
Name	FDF Image
Ontology	Electronic File Formats Disk Image Formats FDF Image
Extension(s)	.fdf
Released	1991

Overview

Floppy Disk File (FDF) are disk images used by the DOS software EZ-DisKlone Plus. Introduced in 1991, the format is a compressed disk image validated by a 16bit CRC.

File Identification

FDF files have the header "1A 46 44 46".

Specifications

The FDF file format is reverse engineered in 86Box, the source code can be found at:

86Box-master\src\floppy\fdd_img.c

Basically FDF used RLE compression.

Integers are stored as little-endian.

EZ-DisKlone truncates the image, the unused sectors filled with 0xF6 at end of the disk are not stored.

• Each track/head is stored as a block.
• So number of bytes per block is equal to number of bytes per track.
• If two heads then the number of blocks is even.
• The number of stored blocks is equal/less then the number of track/heads.
• The number of stored tracks is in the file header.
• The CHS geometry is derived from the DiskType field.

//------------------------------------------------------------------------------
// Limits
//------------------------------------------------------------------------------

// string lengths
#define FDFFILE_LEN_DESCR     64

// sector size
#define FDFFILE_SECTOR_SIZE  512


//------------------------------------------------------------------------------
// File header
//------------------------------------------------------------------------------

// file signature
#define FDFFILE_SIGNATURE   MAKE_FOURCC(0x1A, 'F', 'D', 'F')

// disk (geometry) types
#define FDFFILE_DT_360K    0x03   /* 40/2/9  = 4608 bytes/track */
#define FDFFILE_DT_720K    0x04   /* 80/2/9  = 4608 bytes/track */
#define FDFFILE_DT_120M    0x05   /* 80/2/15 = 7680 bytes/track */
#define FDFFILE_DT_144M    0x06   /* 80/2/18 = 9216 bytes/track */


// file header
// - 128 bytes
typedef struct
{
  // equal to FDFFILE_SIGNATURE
  UINT32 Signature;

  // always zero
  UINT8  Reserved1;

  // one of FDFFILE_DT_xxx
  UINT8  DiskType;

  // description
  CHAR   Descr[FDFFILE_LEN_DESCR];

  // all zero
  UINT8  Reserved2[8];

  // unknown (0x0100)
  UINT16 Unknown1;

  // number of stored tracks
  // - must be multiplied by number of heads to get number of blocks
  UINT16 TrackCnt;

  // all zero
  UINT8  Reserved3[46];

  //
} TFdfFile_FileHdr;


//------------------------------------------------------------------------------
// Block header
//------------------------------------------------------------------------------

// block compression method
#define FDFFILE_METHOD_STORED   0x00
#define FDFFILE_METHOD_RLE      0x01
#define FDFFILE_METHOD_EOD      0xFF   /* end of data */


// block header
// - 5 bytes
// - followed by RLE compressed data
typedef struct
{
  // one of FDFFILE_METHOD_xxx
  UINT8  Method;

  // CRC16L checksum (LHA)
  UINT16 CRC;

  // number of compressed bytes
  UINT16 Size;

  //
} TFdfFile_BlkHdr;



//------------------------------------------------------------------------------
// RLE compression
//------------------------------------------------------------------------------
// - if FDFFILE_METHOD_EOD    then end of file
// - if FDFFILE_METHOD_STORED then block is stored uncompressed
// - if FDFFILE_METHOD_RLE    then:
//   * if b7 is set then the lower 7 bits are the repeat count and the next
//     byte is the byte to repeat
//   * If b7 is zero then its a store count, followed by the stored bytes
//------------------------------------------------------------------------------

Software

• Software download and Emulation

Sample Files

• DOS Mouse Driver Image file
• dexvert samples — archive/floppyDiskFile

Links

• EZX's EZ-DisKlone™ Plus for DOS
• SSW/EZX's Super DiskCopy