Anex86 PC98 floppy image

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File Format
Name Anex86 PC98 floppy image
Extension(s) .fdi

The FDI disk image format for NEC PC-98 disk images seems to originate from the Anex86 PC98 emulator.

FDI File Header

The FDI header is 4096 bytes long, Intel little-endian, and starts with this 32-byte header.

   typedef struct {
       BYTE   dummy[4];
       BYTE   fddtype[4];
       BYTE   headersize[4];
       BYTE   fddsize[4];
       BYTE   sectorsize[4];
       BYTE   sectors[4];
       BYTE   surfaces[4];
       BYTE   cylinders[4];
   } FDIHDR;

The next 4,064 bytes of the header seem to always be pad bytes.

FDI file image contents

The remaining portion of the file is a raw dd-style image of the disk. In the case of an HDM image, you can prepend a 4096-byte header onto the image to turn it into an FDI (see example below).


For a 1261568-byte "2HD" disk image (like *.HDM format), the header is as follows:

Field Value Comment
dummy 0
fddtype 144 Unsure what this value actually means
header size 4096 Includes the 32-bytes in the header
fdd size 1261568
sector size 1024
sector count 8
surfaces 2 double-sided disk
cylinders 77
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