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1. Find free cluster in FAT directory (if entry == 0 it's free) (I know that clusters 0 and 1 are restricted and cannot be used)
2. Finde free entry in Root directory (if entry.name == "\0\0\0\0..." it's free)
3. Fill root directory entry with: name, size, first cluster. Fill FAT cluster entry with 0xff8
4. Write content of the file to [free_cluster + 31] to floppy disk (31 is because Data Directory starts from there)
5. Save changes to root and FAT

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Filesystem     Type     1K-blocks      Used Available Use% Mounted on
/dev/loop0     vfat          1424        34      1390   3% /mnt/disk

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 FAT_ENTRY_No   VALUE
0 -> 0x0
1 -> 0x0
2 -> 0x0
3 -> 0x4
4 -> 0xFFFFFF00
5 ->0xFF0
6 -> 0xFFFFF800
...

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int fat12_find_if_free_cluster(int free_cluster)
{
  unsigned char* sector;

  /* Calculate FAT entry offset, etc. */
  unsigned int FAT_Offset = free_cluster + (free_cluster / 2); //multiply by 1.5 (o3mujemy adres info o nast. klastrze)
  unsigned int FAT_Sector = 1 + (FAT_Offset / ROZMIAR_SEKTORA);
  unsigned int entryOffset = FAT_Offset % ROZMIAR_SEKTORA;

  /* Read fat sectors */
  sector = (unsigned char*) floppy_read_sector( FAT_Sector );
  memcpy((unsigned char*)FAT, sector, 512);
  sector = (unsigned char*) floppy_read_sector ( FAT_Sector + 1 );
  memcpy((unsigned char*)FAT + ROZMIAR_SEKTORA, sector, 512);

  /* Read where is next cluster of file (if any) */
  short nextCluster = *(unsigned short*)&FAT[entryOffset];
  short actual_cluster;

  if( nextCluster & 0x0001 )
    actual_cluster = nextCluster >> 4;        //grab high 12 bits
  else
    actual_cluster = nextCluster & 0x0FFF;    //grab low 12 bits

  if(actual_cluster == 0) {
      actual_cluster = 0xff8;

      /* Zapisujemy w FAT */
      if(nextCluster & 0x0001) {
          nextCluster &= 0x000F;
          nextCluster |= (actual_cluster << 4);
      } else {
          nextCluster &= 0xF000;
          nextCluster |= actual_cluster;
      }
      short* tmp = (short*)&FAT[entryOffset];
      *tmp = nextCluster;
      floppy_write_sector(FAT_Sector, FAT);
      floppy_write_sector(FAT_Sector+1, FAT+ROZMIAR_SEKTORA);

      return free_cluster;
  }

  return -1;
}

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[FDD ]
Message: io: norm r/w parms out of range: sec#01h cyl#6fh eot#01h head#01