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.

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

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

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

  // unknown (0x0600)
  UINT16 Unknown1;

  // description
  CHAR   Descr[FDFFILE_LEN_DESCR];

  // all zero
  BYTE   Reserved1[8];

  // unknown (0x0100)
  UINT16 Unknown2;

  // unknown (0x0034)
  UINT16 Unknown3;

  // all zero
  BYTE   Reserved2[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 */

// default block size is 9216 bytes
#define FDFFILE_DEFAULT_BLKSIZE   (9*SIZE_KB)


// 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
//------------------------------------------------------------------------------
// - decompressed blocks do not need to be the same size as stored blocks
//   (stored blocks are FDFFILE_DEFAULT_BLKSIZE in size)
//------------------------------------------------------------------------------

Software

• Software download and Emulation

Sample File

• DOS Mouse Driver Image file

Links

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