Chunk format

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Chunks store the terrain and entities within a 16×256×16 area. They also store precomputed lighting, heightmap data for Minecraft's performance, and other meta information.

NBT structure[edit | edit source]

See also: Information from the Anvil format page

Chunks are stored in NBT format, with this structure (see Block Format below for the ordering of the blocks within each array):

  • The root tag.

    •  DataVersion: Version of the chunk NBT structure.

    •  Level: Chunk data.

      •  xPos: X position of the chunk.

      •  zPos: Z position of the chunk.

      •  LastUpdate: Tick when the chunk was last saved.

      •  LightPopulated: 1 or 0 (true/false) - If true, the server/client has already calculated lighting values for this chunk after generation.

      •  TerrainPopulated: 1 or not present (true/false) indicate whether the terrain in this chunk was populated with special things. (Ores, special blocks, trees, dungeons, flowers, waterfalls, etc.) If set to zero then Minecraft will regenerate these features in the blocks that already exist.

      •  V: Currently always saved as 1 and not actually loaded by the game. Likely a chunk version tag.

      •  InhabitedTime: The cumulative number of ticks players have been in this chunk. Note that this value increases faster when more players are in the chunk. Used for regional difficulty: increases the chances of mobs spawning with equipment, the chances of that equipment having enchantments, the chances of spiders having potion effects, the chances of mobs having the ability to pick up dropped items, and the chances of zombies having the ability to spawn other zombies when attacked. Note that at values 3600000 and above, regional difficulty is effectively maxed for this chunk. At values 0 and below, the difficulty is capped to a minimum (thus, if this is set to a negative number, it will behave identically to being set to 0, apart from taking time to build back up to the positives). See Regional Difficulty for more specifics.

      •  Biomes: May not exist. 256 bytes of biome data, one byte for each vertical column in the chunk. See Data Values for biome IDs. If this tag does not exist it will be created and filled by Minecraft when the chunk is loaded and saved. If any values in this array are -1, Minecraft will also set them to the correct biome.

      •  HeightMap: 1024 bytes(256 TAG_Int) of heightmap data. 16 × 16. Each byte records the lowest level in each column where the light from the sky is at full strength. Speeds computing of the SkyLight.

      •  Sections: List of Compound tags, each tag is a sub-chunk of sorts.

        • An individual Section.

          •  Y: The Y index (not coordinate) of this section. Range 0 to 15 (bottom to top), with no duplicates but some sections may be missing if empty.

          •  Blocks: 4096 bytes of block IDs defining the terrain. 8 bits per block, plus the bits from the below Add tag.

          •  Add: May not exist. 2048 bytes of additional block ID data. The value to add to (combine with) the above block ID to form the true block ID in the range 0 to 4095. 4 bits per block. Combining is done by shifting this value to the left 8 bits and then adding it to the block ID from above.

          •  Data: 2048 bytes of block data additionally defining parts of the terrain. 4 bits per block.

          •  BlockLight: 2048 bytes recording the amount of block-emitted light in each block. Makes load times faster compared to recomputing at load time. 4 bits per block.

          •  SkyLight: 2048 bytes recording the amount of sunlight or moonlight hitting each block. 4 bits per block.

      •  Entities: Each TAG_Compound in this list defines an entity in the chunk. See Entity Format below. If this list is empty it will be a list of Byte tags.

      •  TileEntities: Each TAG_Compound in this list defines a block entity in the chunk. See Block Entity Format below. If this list is empty it will be a list of Byte tags.

      •  TileTicks: May not exist. Each TAG_Compound in this list is an "active" block in this chunk waiting to be updated. These are used to save the state of redstone machines, falling sand or water, and other activity. See Tile Tick Format below. This tag may not exist.

Block format[edit | edit source]

In the Anvil format, block positions are ordered YZX for compression purposes.

The coordinate system is as follows:

  • X increases East, decreases West
  • Y increases upwards, decreases downwards
  • Z increases South, decreases North

This also happens to yield the most natural scan direction, because all indices in the least significant dimension (i.e. X in this case) appear for each index in the next most significant dimension; so one reads an array ordered YZX as one would a book lying with its top northward, all letters (or X-indices) on a single line (or Z-index) at a time, and all lines on a single page (or Y-index) at a time. For the 2D arrays (i.e. "Biomes" and "HeightMap") the inapplicable Y dimension is simply omitted, as though the book is only one page thick.

Each section in a chunk is a 16x16x16-block area, with up to 16 sections in a chunk. Section 0 is the bottom section of the chunk, and section 15 is the top section of the chunk. To save space, completely empty sections are not saved. Within each section is a byte tag "Y" for the Y index of the section, 0 to 15, and then byte arrays for the blocks. The "Block" byte array has 4096 partial block IDs at 8 bits per block. Another byte array "Add" is used for block with IDs over 255, and is 2048 bytes of the other part of the 4096 block IDs at 4 bits per block. When both the "Block" and "Add" byte arrays exist, the partial ID from the "Add" array is shifted left 8 bits and added to the partial ID from the "Blocks" array to form the true Block ID. The "Data" byte array is also 2048 bytes for 4096 block data values at 4 bits per block. The "BlockLight" and "SkyLight" byte arrays are the same as the "Data" byte array but they are used for block light levels and sky light levels respectively. The "SkyLight" values represent how much sunlight or moonlight can potentially reach the block, independent of the current light level of the sky.

The endianness of the 2048-byte arrays (i.e. "Add," "Data," "BlockLight," & "SkyLight"), which give only 4 bits per block, seems to stand as the one anomalous exception to the otherwise consistent, format-wide standard of big-endian data storage. It also runs counter to the presumably natural human-readable printing direction. If the blocks begin at 0, they are grouped with even numbers preceding odd numbers (i.e. 0 & 1 share a byte, 2 & 3 share the next, etc.); under these designations Minecraft stores even-numbered blocks in the least significant half-byte, and odd-numbered blocks in the most significant half-byte. Thus block[0] is byte[0] at 0x0F, block[1] is byte[0] at 0xF0, block[2] is byte[1] at 0x0F, block[3] is byte[1] at 0xF0, etc. ...

The pseudo-code below shows how to access individual block information from a single section. Hover over text to see additional information or comments.

byte Nibble4(byte[] arr, int index){ return index%2 == 0 ? arr[index/2]&0x0F : (arr[index/2]>>4)&0x0F; }
int BlockPos = y*16*16 + z*16 + x;
byte BlockID_a = Blocks[BlockPos];
byte BlockID_b = Nibble4(Add, BlockPos);
short BlockID = BlockID_a + (BlockID_b << 8);
byte BlockData = Nibble4(Data, BlockPos);
byte Blocklight = Nibble4(BlockLight, BlockPos);
byte Skylight = Nibble4(SkyLight, BlockPos);

Entity format[edit | edit source]

Every entity is an unnamed
 TAG_Compound contained in the Entities list of a chunk file. The sole exception is the Player entity, stored in level.dat, or in <player>.dat files on servers. All entities share this base:

id Pos Motion Rotation FallDistance Fire Air OnGround Dimension Invulnerable PortalCooldown UUIDMost UUIDLeast UUID CustomName CustomNameVisible Silent Riding CommandStats SuccessCountObjective SuccessCountName AffectedBlocksObjective AffectedBlocksName AffectedEntitiesObjective AffectedEntitiesName AffectedItemsObjective AffectedItemsName QueryResultObjective QueryResultName

Mobs[edit | edit source]

Mob Entities
Entity ID Name
Bat Bat
Blaze Blaze
CaveSpider Cave Spider
Chicken[note 1] Chicken
Cow[note 1] Cow
Creeper Creeper
EnderDragon Ender Dragon
Enderman Enderman
Endermite Endermite
Ghast Ghast
Giant Giant
Guardian Guardian
Elder Guardian
EntityHorse[note 1] Horse
LavaSlime Magma Cube
MushroomCow[note 1] Mooshroom
Ozelot[note 1][note 2] Ocelot
Pig[note 1] Pig
PigZombie Zombie Pigman
Rabbit[note 1] Rabbit
Sheep[note 1] Sheep
Shulker[upcoming] Shulker
Silverfish Silverfish
Skeleton Skeleton
Wither Skeleton
Slime Slime
SnowMan Snow Golem
Spider Spider
Squid Squid
Villager[note 1] Villager
VillagerGolem Iron Golem
Witch Witch
WitherBoss Wither
Wolf[note 1][note 2] Wolf
Zombie Zombie
Zombie Villager
  1. a b c d e f g h i j
    Can breed
  2. a b
    Can be tamed

Mobs are a subclass of Entity with additional tags to store their health, attacking/damaged state, potion effects, and more depending on the mob. Players are a subclass of Mob.

HealF Health AbsorptionAmount AttackTime HurtTime HurtByTimestamp DeathTime Attributes Name Base Modifiers Name Amount Operation UUIDMost UUIDLeast ActiveEffects Id Amplifier Duration Ambient ShowParticles Equipment DropChances CanPickUpLoot NoAI PersistenceRequired Leashed Leash UUIDMost UUIDLeast X Y Z GoldenAppleOverflow

Projectiles[edit | edit source]

Projectile Entities
Entity ID Name
Arrow Arrow
DragonFireball Ender Dragon Fireball
Fireball Ghast Fireball
SmallFireball Blaze Fireball/Fire Charge
Snowball Snowball
SpectralArrow Spectral Arrow
ThrownEgg Egg
ThrownEnderpearl Ender Pearl
ThrownExpBottle Bottle o' Enchanting
ThrownPotion Splash Potion
WitherSkull Wither Skull

Projectiles are a subclass of Entity and have very obscure tags such as X,Y,Z coordinate tags despite Entity Pos tag, inTile despite inGround, and shake despite most projectiles not being arrows.

Items and XPOrbs[edit | edit source]

Item Entities
Entity ID Name
Item Dropped Item
XPOrb Experience Orb

Items and XPOrbs are a subclass of Entity.

Vehicles[edit | edit source]

Vehicle Entities
Entity ID Name
Boat Boat
Minecart (deprecated) Minecart
Minecart with Chest
Minecart with Furnace
MinecartRideable Minecart
MinecartChest Minecart with Chest
MinecartFurnace Minecart with Furnace
MinecartSpawner Minecart with Spawner
MinecartTNT Minecart with TNT
MinecartHopper Minecart with Hopper
MinecartCommandBlock Minecart with Command Block

Vehicles are subclasses of Entity.

Dynamic tiles[edit | edit source]

Dynamic Block Entities
Entity ID Name
PrimedTnt TNT
FallingSand Dynamic Tile

Dynamic tiles are a subclass of Entity and are used to simulate realistically moving blocks.

Other[edit | edit source]

Other Entities
Entity ID Name
AreaEffectCloud Area Effect Cloud
ArmorStand Armor Stand
EnderCrystal Ender Crystal
EyeOfEnderSignal Eye of Ender
FireworksRocketEntity Firework Rocket
ItemFrame Item Frame
LeashKnot Lead Knot
Painting Painting
ShulkerBullet[upcoming] Shulker Bullet
Fishing Rod Bobber

Other entity types that are a subclass of Entity but do not fit into any of the above categories.

Block entity format[edit | edit source]

Block Entities
Block Entity ID Associated Block
Airportal End Portal (block)
Banner Banner
Beacon Beacon
Cauldron Brewing Stand
Chest Chest
Trapped Chest
Comparator Redstone Comparator
Control Command Block
DLDetector Daylight Sensor
Dropper Dropper
EnchantTable Enchantment Table
EnderChest Ender Chest
EndGateway[upcoming] End Gateway (block)
FlowerPot Flower Pot
Furnace Furnace
Hopper Hopper
MobSpawner Monster Spawner
Music Note Block
Piston Piston Moving
RecordPlayer Jukebox
Sign Sign
Skull Mob head
Trap Dispenser

A block entity (not related to entity) is used by Minecraft to store information about a block that can't be stored in the four bits of block data the block has. Block entities were called "tile entities" until the 1.8 snapshots and that term is still used in some command usage.

Tile tick format[edit | edit source]

Tile Ticks represent block updates that need to happen because they could not happen before the chunk was saved. Examples reasons for tile ticks include redstone circuits needing to continue updating, water and lava that should continue flowing, recently placed sand or gravel that should fall, etc. Tile ticks are not used for purposes such as leaf decay, where the decay information is stored in the leaf block data values and handled by Minecraft when the chunk loads. For map makers, tile ticks can be used to update blocks after a period of time has passed with the chunk loaded into memory.

  • A Tile Tick

    •  i: The ID of the block; used to activate the correct block update procedure.

    •  t: The number of ticks until processing should occur. May be negative when processing is overdue.

    •  p: If multiple tile ticks are scheduled for the same tick, tile ticks with lower p will be processed first. If they also have the same p, the order is unknown.

    •  x: X position

    •  y: Y position

    •  z: Z position

History[edit | edit source]

Chunks were first introduced in Minecraft Infdev. Before the addition of the MCRegion format in Beta 1.3, chunks were stored as individual chunk files ".dat" where the file names contained the chunk's position encoded in Base36 - this is known as the Alpha level format. MCRegion changed this by storing groups of 32×32 chunks in individual ".mcr" files with the coordinates in Base10, with the goal being to reduce disk usage by cutting down on the number of file handles Minecraft had open at once. MCRegion's successor is the current format, Anvil, which only made changes to the chunk format. The region file technique is still used, but the region file extensions are ".mca" instead.

The major change from MCRegion to Anvil was the division of Chunks into Sections; each chunk has up to 16 individual 16×16×16 block Sections so that completely empty sections will not be saved at all. Preparation has also been made to support blocks with IDs in the range 0 to 4095, as compared to the previous 0 to 255 limitation. However, Minecraft is not fully prepared for such blocks to exist as items; many item IDs are already taken in the range 256 to 4095.

The Blocks, Data, BlockLight, and SkyLight arrays are now housed in individual chunk Sections. The Data, SkyLight, and BlockLight are arrays of 4-bit values, and the BlockLight and SkyLight arrays no longer house part of the block ID. The Blocks array is 8 bits per block, and the 4096-blocks support exists in the form of an optional Add byte array of 4-bits per block for additional block ID information. With the Anvil format, the NBT Format was changed from Notch's original specification to include an integer array tag similar to the existing byte array tag. It is currently only used for HeightMap information in chunks.

Official release
? Removed MaxExperience, RemainingExperience, ExperienceRegenTick, ExperienceRegenRate and ExperienceRegenAmount from MobSpawner.
1.4.2 12w34a Added entity WitherBoss.
1.5 13w02a Added entity MinecartTNT.
Minecart is now deprecated.
13w03a Added entity MinecartHopper.
13w06a Added entity MinecartSpawner.
1.6.1 13w16a Added entity EntityHorse.
13w21a Removed ArmorType from EntityHorse.
Removed Saddle from EntityHorse.
1.6.1-pre Readded Saddle to EntityHorse.
1.7.2 13w39a Added entity MinecartCommandBlock.
1.8 14w02a Added Lock to containers.
Item IDs are no longer used when specifying NBT data.
Added Block to FallingSand, using the alphabetical ID format.
14w06a Added ShowParticles to all mobs.
Added PickupDelay to item entities.
Setting Age to -32768 makes items which never expire.
14w10a Added rewardExp to Villager.
Added OwnerUUID for mobs that can breed.
Added Owner to Skull.
Changes to item frames and paintings: added Facing, TileX, TileY and TileZ now represent the co-ordinates of the block the item is in rather than what it is placed on, deprecated Direction and Dir.
14w11a Added entity Endermite.
Added EndermiteCount to Enderman.
14w21a CustomName and CustomNameVisible now work for all entities.
14w25a Added entity Guardian.
Added Text1, Text2, Text3 and Text4 to signs. The limit no longer depends on the amount of characters (16), it now depends on the width of the characters.
14w27a Added entity Rabbit. Added CommandStats to command blocks and signs.
14w28a Removed EndermiteCount from Enderman.
14w30a Added Silent for all entities.
14w32a Added NoAI for all mobs.
Added entity ArmorStand.
14w32c Added NoBasePlate to ArmorStand.
1.9 15w31a Added tags HandItems, ArmorItems, HandDropChances, and ArmorDropChances to Living, which replace the DropChances and Equipment tags.
Added HandItems and ArmorItems to ArmorStand.
Added Glowing to Entity.
Added Team to LivingBase.
Added DragonPhase to EnderDragon
Added entity Shulker, child of SnowMan.
Added entity ShulkerBullet, child of Entity.
Added entity DragonFireball, which extends FireballBase and has no unique tags.
Added entities TippedArrow and SpectralArrow, children of Arrow.
Added block EndGateway, child of TileEntity.
15w32a Tags and DataVersion tag can now be applied on entities.
Changed the Fuse tag's type for the PrimedTnt entity from "Byte" to "Short".
15w32c Introduced a limit on the amount of tags an entity can have (1024 tags). When surpassed it displays an error saying: "Cannot add more than 1024 tags to an entity."
15w33a Added entity AreaEffectCloud, child of Entity.
Added ExactTeleport and renamed Life to Age for EndGateway.
Added Linger to ThrownPotion.
Removed DataVersion from Entity. It is now only applied to Player only, child of LivingBase.
Removed UUID from Entity.
HealF under LivingBase has become deprecated.
Health under LivingBase has changed from type "Short" to "Float".
Equipment removed from ArmorStand and Living entity, its usage replaced by HandItems and ArmorItems which were added earlier.
15w33c Added BeamTarget to EnderCrystal.
15w34a Added powered and conditional byte tags to Control tile entity for command blocks.
Added life and power to FireballBase.
Added id inside SpawnData to MobSpawner.
Added powered to the Music tile entity for note blocks.
15w35a Added VillagerProfession to Zombie.
15w37a Added Enabled to MinecartHopper.
15w38a Added SkeletonTrap and SkeletonTrapTime to EntityHorse.
15w41a Replaced Riding with Passengers for all entities.
Added RootVehicle for all passengers.
Added Type to Boat.
15w42a Added Fuel to Cauldron (brewing stands).
15w43a Added LootTable and LootTableSeed to Chest, Minecart and MinecartHopper.
Added DeathLootTable and DeathLootTableSeed to all mobs.
Added life and power to all fireballs (FireballBase).
15w44a Added ShowBottom to EnderCrystal.
15w44b Added Potion and CustomPotionEffects to Arrow.
Added Potion to AreaEffectCloud.
15w45a Removed Linger from ThrownPotion. Instead, the potion will linger if the stored item has an ID of minecraft:lingering_potion.
ThrownPotion will now render as its stored item even if the item is not a potion.
15w46a MoreCarrotTicks from Rabbit is now set to 40 when rabbits eat a carrot crop, but is not used anyway.