FDF Image

Revision as of 14:24, 22 September 2026

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 Files

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

Links

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