This page hosts a formal specification of vfat using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.
digraph {
rankdir=LR;
node [shape=plaintext];
subgraph cluster__vfat {
label="Vfat";
graph[style=dotted];
vfat__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="boot_sector_pos">0</TD><TD PORT="boot_sector_size">116</TD><TD>BootSector</TD><TD PORT="boot_sector_type">boot_sector</TD></TR>
</TABLE>>];
vfat__inst__fats [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="fats_pos">boot_sector.pos_fats</TD><TD PORT="fats_size">boot_sector.size_fat</TD><TD></TD><TD PORT="fats_type">fats</TD></TR>
<TR><TD COLSPAN="4" PORT="fats__repeat">repeat boot_sector.bpb.num_fats times</TD></TR>
</TABLE>>];
vfat__inst__root_dir [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="root_dir_pos">boot_sector.pos_root_dir</TD><TD PORT="root_dir_size">boot_sector.size_root_dir</TD><TD>RootDirectory</TD><TD PORT="root_dir_type">root_dir</TD></TR>
</TABLE>>];
subgraph cluster__ext_bios_param_block_fat32 {
label="Vfat::ExtBiosParamBlockFat32";
graph[style=dotted];
ext_bios_param_block_fat32__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="ls_per_fat_pos">0</TD><TD PORT="ls_per_fat_size">4</TD><TD>u4le</TD><TD PORT="ls_per_fat_type">ls_per_fat</TD></TR>
<TR><TD PORT="has_active_fat_pos">4</TD><TD PORT="has_active_fat_size">1b</TD><TD>BitsType1(LittleBitEndian)</TD><TD PORT="has_active_fat_type">has_active_fat</TD></TR>
<TR><TD PORT="reserved1_pos">4:1</TD><TD PORT="reserved1_size">3b</TD><TD>b3</TD><TD PORT="reserved1_type">reserved1</TD></TR>
<TR><TD PORT="active_fat_id_pos">4:4</TD><TD PORT="active_fat_id_size">4b</TD><TD>b4</TD><TD PORT="active_fat_id_type">active_fat_id</TD></TR>
<TR><TD PORT="reserved2_pos">5</TD><TD PORT="reserved2_size">1</TD><TD></TD><TD PORT="reserved2_type">reserved2</TD></TR>
<TR><TD PORT="fat_version_pos">6</TD><TD PORT="fat_version_size">2</TD><TD>u2le</TD><TD PORT="fat_version_type">fat_version</TD></TR>
<TR><TD PORT="root_dir_start_clus_pos">8</TD><TD PORT="root_dir_start_clus_size">4</TD><TD>u4le</TD><TD PORT="root_dir_start_clus_type">root_dir_start_clus</TD></TR>
<TR><TD PORT="ls_fs_info_pos">12</TD><TD PORT="ls_fs_info_size">2</TD><TD>u2le</TD><TD PORT="ls_fs_info_type">ls_fs_info</TD></TR>
<TR><TD PORT="boot_sectors_copy_start_ls_pos">14</TD><TD PORT="boot_sectors_copy_start_ls_size">2</TD><TD>u2le</TD><TD PORT="boot_sectors_copy_start_ls_type">boot_sectors_copy_start_ls</TD></TR>
<TR><TD PORT="reserved3_pos">16</TD><TD PORT="reserved3_size">12</TD><TD></TD><TD PORT="reserved3_type">reserved3</TD></TR>
<TR><TD PORT="phys_drive_num_pos">28</TD><TD PORT="phys_drive_num_size">1</TD><TD>u1</TD><TD PORT="phys_drive_num_type">phys_drive_num</TD></TR>
<TR><TD PORT="reserved4_pos">29</TD><TD PORT="reserved4_size">1</TD><TD>u1</TD><TD PORT="reserved4_type">reserved4</TD></TR>
<TR><TD PORT="ext_boot_sign_pos">30</TD><TD PORT="ext_boot_sign_size">1</TD><TD>u1</TD><TD PORT="ext_boot_sign_type">ext_boot_sign</TD></TR>
<TR><TD PORT="volume_id_pos">31</TD><TD PORT="volume_id_size">4</TD><TD></TD><TD PORT="volume_id_type">volume_id</TD></TR>
<TR><TD PORT="partition_volume_label_pos">35</TD><TD PORT="partition_volume_label_size">11</TD><TD>str(ASCII)</TD><TD PORT="partition_volume_label_type">partition_volume_label</TD></TR>
<TR><TD PORT="fs_type_str_pos">46</TD><TD PORT="fs_type_str_size">8</TD><TD>str(ASCII)</TD><TD PORT="fs_type_str_type">fs_type_str</TD></TR>
</TABLE>>];
}
subgraph cluster__boot_sector {
label="Vfat::BootSector";
graph[style=dotted];
boot_sector__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="jmp_instruction_pos">0</TD><TD PORT="jmp_instruction_size">3</TD><TD></TD><TD PORT="jmp_instruction_type">jmp_instruction</TD></TR>
<TR><TD PORT="oem_name_pos">3</TD><TD PORT="oem_name_size">8</TD><TD>str(ASCII)</TD><TD PORT="oem_name_type">oem_name</TD></TR>
<TR><TD PORT="bpb_pos">11</TD><TD PORT="bpb_size">25</TD><TD>BiosParamBlock</TD><TD PORT="bpb_type">bpb</TD></TR>
<TR><TD PORT="ebpb_fat16_pos">36</TD><TD PORT="ebpb_fat16_size">26</TD><TD>ExtBiosParamBlockFat16</TD><TD PORT="ebpb_fat16_type">ebpb_fat16</TD></TR>
<TR><TD PORT="ebpb_fat32_pos">62</TD><TD PORT="ebpb_fat32_size">54</TD><TD>ExtBiosParamBlockFat32</TD><TD PORT="ebpb_fat32_type">ebpb_fat32</TD></TR>
</TABLE>>];
boot_sector__inst__pos_fats [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>pos_fats</TD><TD>(bpb.bytes_per_ls * bpb.num_reserved_ls)</TD></TR>
</TABLE>>];
boot_sector__inst__ls_per_fat [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>ls_per_fat</TD><TD>(is_fat32 ? ebpb_fat32.ls_per_fat : bpb.ls_per_fat)</TD></TR>
</TABLE>>];
boot_sector__inst__ls_per_root_dir [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>ls_per_root_dir</TD><TD>((((bpb.max_root_dir_rec * 32) + bpb.bytes_per_ls) - 1) / bpb.bytes_per_ls)</TD></TR>
</TABLE>>];
boot_sector__inst__is_fat32 [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>is_fat32</TD><TD>bpb.max_root_dir_rec == 0</TD></TR>
</TABLE>>];
boot_sector__inst__size_fat [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>size_fat</TD><TD>(bpb.bytes_per_ls * ls_per_fat)</TD></TR>
</TABLE>>];
boot_sector__inst__pos_root_dir [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>pos_root_dir</TD><TD>(bpb.bytes_per_ls * (bpb.num_reserved_ls + (ls_per_fat * bpb.num_fats)))</TD></TR>
</TABLE>>];
boot_sector__inst__size_root_dir [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>size_root_dir</TD><TD>(ls_per_root_dir * bpb.bytes_per_ls)</TD></TR>
</TABLE>>];
}
subgraph cluster__bios_param_block {
label="Vfat::BiosParamBlock";
graph[style=dotted];
bios_param_block__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="bytes_per_ls_pos">0</TD><TD PORT="bytes_per_ls_size">2</TD><TD>u2le</TD><TD PORT="bytes_per_ls_type">bytes_per_ls</TD></TR>
<TR><TD PORT="ls_per_clus_pos">2</TD><TD PORT="ls_per_clus_size">1</TD><TD>u1</TD><TD PORT="ls_per_clus_type">ls_per_clus</TD></TR>
<TR><TD PORT="num_reserved_ls_pos">3</TD><TD PORT="num_reserved_ls_size">2</TD><TD>u2le</TD><TD PORT="num_reserved_ls_type">num_reserved_ls</TD></TR>
<TR><TD PORT="num_fats_pos">5</TD><TD PORT="num_fats_size">1</TD><TD>u1</TD><TD PORT="num_fats_type">num_fats</TD></TR>
<TR><TD PORT="max_root_dir_rec_pos">6</TD><TD PORT="max_root_dir_rec_size">2</TD><TD>u2le</TD><TD PORT="max_root_dir_rec_type">max_root_dir_rec</TD></TR>
<TR><TD PORT="total_ls_2_pos">8</TD><TD PORT="total_ls_2_size">2</TD><TD>u2le</TD><TD PORT="total_ls_2_type">total_ls_2</TD></TR>
<TR><TD PORT="media_code_pos">10</TD><TD PORT="media_code_size">1</TD><TD>u1</TD><TD PORT="media_code_type">media_code</TD></TR>
<TR><TD PORT="ls_per_fat_pos">11</TD><TD PORT="ls_per_fat_size">2</TD><TD>u2le</TD><TD PORT="ls_per_fat_type">ls_per_fat</TD></TR>
<TR><TD PORT="ps_per_track_pos">13</TD><TD PORT="ps_per_track_size">2</TD><TD>u2le</TD><TD PORT="ps_per_track_type">ps_per_track</TD></TR>
<TR><TD PORT="num_heads_pos">15</TD><TD PORT="num_heads_size">2</TD><TD>u2le</TD><TD PORT="num_heads_type">num_heads</TD></TR>
<TR><TD PORT="num_hidden_sectors_pos">17</TD><TD PORT="num_hidden_sectors_size">4</TD><TD>u4le</TD><TD PORT="num_hidden_sectors_type">num_hidden_sectors</TD></TR>
<TR><TD PORT="total_ls_4_pos">21</TD><TD PORT="total_ls_4_size">4</TD><TD>u4le</TD><TD PORT="total_ls_4_type">total_ls_4</TD></TR>
</TABLE>>];
}
subgraph cluster__root_directory_rec {
label="Vfat::RootDirectoryRec";
graph[style=dotted];
root_directory_rec__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="file_name_pos">0</TD><TD PORT="file_name_size">11</TD><TD></TD><TD PORT="file_name_type">file_name</TD></TR>
<TR><TD PORT="attrs_pos">11</TD><TD PORT="attrs_size">1</TD><TD>AttrFlags</TD><TD PORT="attrs_type">attrs</TD></TR>
<TR><TD PORT="reserved_pos">12</TD><TD PORT="reserved_size">10</TD><TD></TD><TD PORT="reserved_type">reserved</TD></TR>
<TR><TD PORT="last_write_time_pos">22</TD><TD PORT="last_write_time_size">4</TD><TD>DosDatetime</TD><TD PORT="last_write_time_type">last_write_time</TD></TR>
<TR><TD PORT="start_clus_pos">26</TD><TD PORT="start_clus_size">2</TD><TD>u2le</TD><TD PORT="start_clus_type">start_clus</TD></TR>
<TR><TD PORT="file_size_pos">28</TD><TD PORT="file_size_size">4</TD><TD>u4le</TD><TD PORT="file_size_type">file_size</TD></TR>
</TABLE>>];
subgraph cluster__attr_flags {
label="Vfat::RootDirectoryRec::AttrFlags";
graph[style=dotted];
attr_flags__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="read_only_pos">0</TD><TD PORT="read_only_size">1b</TD><TD>BitsType1(LittleBitEndian)</TD><TD PORT="read_only_type">read_only</TD></TR>
<TR><TD PORT="hidden_pos">0:1</TD><TD PORT="hidden_size">1b</TD><TD>BitsType1(LittleBitEndian)</TD><TD PORT="hidden_type">hidden</TD></TR>
<TR><TD PORT="system_pos">0:2</TD><TD PORT="system_size">1b</TD><TD>BitsType1(LittleBitEndian)</TD><TD PORT="system_type">system</TD></TR>
<TR><TD PORT="volume_id_pos">0:3</TD><TD PORT="volume_id_size">1b</TD><TD>BitsType1(LittleBitEndian)</TD><TD PORT="volume_id_type">volume_id</TD></TR>
<TR><TD PORT="is_directory_pos">0:4</TD><TD PORT="is_directory_size">1b</TD><TD>BitsType1(LittleBitEndian)</TD><TD PORT="is_directory_type">is_directory</TD></TR>
<TR><TD PORT="archive_pos">0:5</TD><TD PORT="archive_size">1b</TD><TD>BitsType1(LittleBitEndian)</TD><TD PORT="archive_type">archive</TD></TR>
<TR><TD PORT="reserved_pos">0:6</TD><TD PORT="reserved_size">2b</TD><TD>b2</TD><TD PORT="reserved_type">reserved</TD></TR>
</TABLE>>];
attr_flags__inst__long_name [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">id</TD><TD BGCOLOR="#E0FFE0">value</TD></TR>
<TR><TD>long_name</TD><TD> ((read_only) && (hidden) && (system) && (volume_id)) </TD></TR>
</TABLE>>];
}
}
subgraph cluster__root_directory {
label="Vfat::RootDirectory";
graph[style=dotted];
root_directory__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="records_pos">0</TD><TD PORT="records_size">32</TD><TD>RootDirectoryRec</TD><TD PORT="records_type">records</TD></TR>
<TR><TD COLSPAN="4" PORT="records__repeat">repeat _root.boot_sector.bpb.max_root_dir_rec times</TD></TR>
</TABLE>>];
}
subgraph cluster__ext_bios_param_block_fat16 {
label="Vfat::ExtBiosParamBlockFat16";
graph[style=dotted];
ext_bios_param_block_fat16__seq [label=<<TABLE BORDER="0" CELLBORDER="1" CELLSPACING="0">
<TR><TD BGCOLOR="#E0FFE0">pos</TD><TD BGCOLOR="#E0FFE0">size</TD><TD BGCOLOR="#E0FFE0">type</TD><TD BGCOLOR="#E0FFE0">id</TD></TR>
<TR><TD PORT="phys_drive_num_pos">0</TD><TD PORT="phys_drive_num_size">1</TD><TD>u1</TD><TD PORT="phys_drive_num_type">phys_drive_num</TD></TR>
<TR><TD PORT="reserved1_pos">1</TD><TD PORT="reserved1_size">1</TD><TD>u1</TD><TD PORT="reserved1_type">reserved1</TD></TR>
<TR><TD PORT="ext_boot_sign_pos">2</TD><TD PORT="ext_boot_sign_size">1</TD><TD>u1</TD><TD PORT="ext_boot_sign_type">ext_boot_sign</TD></TR>
<TR><TD PORT="volume_id_pos">3</TD><TD PORT="volume_id_size">4</TD><TD></TD><TD PORT="volume_id_type">volume_id</TD></TR>
<TR><TD PORT="partition_volume_label_pos">7</TD><TD PORT="partition_volume_label_size">11</TD><TD>str(ASCII)</TD><TD PORT="partition_volume_label_type">partition_volume_label</TD></TR>
<TR><TD PORT="fs_type_str_pos">18</TD><TD PORT="fs_type_str_size">8</TD><TD>str(ASCII)</TD><TD PORT="fs_type_str_type">fs_type_str</TD></TR>
</TABLE>>];
}
}
vfat__seq:boot_sector_type -> boot_sector__seq [style=bold];
boot_sector__inst__pos_fats:pos_fats_type -> vfat__inst__fats:fats_pos [color="#404040"];
boot_sector__inst__size_fat:size_fat_type -> vfat__inst__fats:fats_size [color="#404040"];
bios_param_block__seq:num_fats_type -> vfat__inst__fats:fats__repeat [color="#404040"];
boot_sector__inst__pos_root_dir:pos_root_dir_type -> vfat__inst__root_dir:root_dir_pos [color="#404040"];
boot_sector__inst__size_root_dir:size_root_dir_type -> vfat__inst__root_dir:root_dir_size [color="#404040"];
vfat__inst__root_dir:root_dir_type -> root_directory__seq [style=bold];
boot_sector__seq:bpb_type -> bios_param_block__seq [style=bold];
boot_sector__seq:ebpb_fat16_type -> ext_bios_param_block_fat16__seq [style=bold];
boot_sector__seq:ebpb_fat32_type -> ext_bios_param_block_fat32__seq [style=bold];
bios_param_block__seq:bytes_per_ls_type -> boot_sector__inst__pos_fats [color="#404040"];
bios_param_block__seq:num_reserved_ls_type -> boot_sector__inst__pos_fats [color="#404040"];
boot_sector__inst__is_fat32:is_fat32_type -> boot_sector__inst__ls_per_fat [color="#404040"];
ext_bios_param_block_fat32__seq:ls_per_fat_type -> boot_sector__inst__ls_per_fat [color="#404040"];
bios_param_block__seq:ls_per_fat_type -> boot_sector__inst__ls_per_fat [color="#404040"];
bios_param_block__seq:max_root_dir_rec_type -> boot_sector__inst__ls_per_root_dir [color="#404040"];
bios_param_block__seq:bytes_per_ls_type -> boot_sector__inst__ls_per_root_dir [color="#404040"];
bios_param_block__seq:bytes_per_ls_type -> boot_sector__inst__ls_per_root_dir [color="#404040"];
bios_param_block__seq:max_root_dir_rec_type -> boot_sector__inst__is_fat32 [color="#404040"];
bios_param_block__seq:bytes_per_ls_type -> boot_sector__inst__size_fat [color="#404040"];
boot_sector__inst__ls_per_fat:ls_per_fat_type -> boot_sector__inst__size_fat [color="#404040"];
bios_param_block__seq:bytes_per_ls_type -> boot_sector__inst__pos_root_dir [color="#404040"];
bios_param_block__seq:num_reserved_ls_type -> boot_sector__inst__pos_root_dir [color="#404040"];
boot_sector__inst__ls_per_fat:ls_per_fat_type -> boot_sector__inst__pos_root_dir [color="#404040"];
bios_param_block__seq:num_fats_type -> boot_sector__inst__pos_root_dir [color="#404040"];
boot_sector__inst__ls_per_root_dir:ls_per_root_dir_type -> boot_sector__inst__size_root_dir [color="#404040"];
bios_param_block__seq:bytes_per_ls_type -> boot_sector__inst__size_root_dir [color="#404040"];
root_directory_rec__seq:attrs_type -> attr_flags__seq [style=bold];
root_directory_rec__seq:last_write_time_type -> dos_datetime__seq [style=bold];
attr_flags__seq:read_only_type -> attr_flags__inst__long_name [color="#404040"];
attr_flags__seq:hidden_type -> attr_flags__inst__long_name [color="#404040"];
attr_flags__seq:system_type -> attr_flags__inst__long_name [color="#404040"];
attr_flags__seq:volume_id_type -> attr_flags__inst__long_name [color="#404040"];
root_directory__seq:records_type -> root_directory_rec__seq [style=bold];
bios_param_block__seq:max_root_dir_rec_type -> root_directory__seq:records__repeat [color="#404040"];
}