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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[]

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 End tags (before 1.10, 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 End tags (before 1.10, 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[]

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[]

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

  • Entity data

Chunk format/Entity

Mobs[]

Mob Entities
Entity ID Name
bat Bat
blaze Blaze
cave_spider Cave Spider
chicken Chicken
cow Cow
creeper Creeper
donkey Donkey
elder_guardian Elder Guardian
ender_dragon Ender Dragon
enderman Enderman
endermite Endermite
evocation_illager Evoker
ghast Ghast
giant Giant
guardian Guardian
horse Horse
husk Husk
llama Llama
magma_cube Magma Cube
mooshroom Mooshroom
mule Mule
ocelot Ocelot
pig Pig
polar_bear Polar Bear
rabbit Rabbit
sheep Sheep
shulker Shulker
silverfish Silverfish
skeleton Skeleton
skeleton_horse Skeleton Horse
slime Slime
snowman Snow Golem
spider Spider
squid Squid
stray Stray
vex Vex
villager Villager
villager_golem Iron Golem
vindication_illager Vindicator
witch Witch
wither Wither
wither_skeleton Wither Skeleton
wolf Wolf
zombie Zombie
zombie_horse Zombie Horse
zombie_pigman Zombie Pigman
zombie_villager Zombie Villager

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.

Projectiles[]

Projectile Entities
Entity ID Name
arrow Arrow
dragon_fireball Ender Dragon Fireball
egg Egg
ender_pearl Ender Pearl
fireball Ghast Fireball
potion Splash Potion
small_fireball Blaze Fireball/Fire Charge
snowball Snowball
spectral_arrow Spectral Arrow
wither_skull Wither Skull
xp_bottle Bottle o' Enchanting

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. While all projectiles share tags, they are all independently implemented through Throwable and ArrowBase.

Items and XPOrbs[]

Item Entities
Entity ID Name
item Dropped Item
xp_orb Experience Orb

Items and XPOrbs are a subclass of Entity.

Vehicles[]

Vehicle Entities
Entity ID Name
boat Boat
Minecart (deprecated) Minecart
Minecart with Chest
Minecart with Furnace
minecart Minecart
chest_minecart Minecart with Chest
commandblock_minecart Minecart with Command Block
furnace_minecart Minecart with Furnace
hopper_minecart Minecart with Hopper
spawner_minecart Minecart with Spawner
tnt_minecart Minecart with TNT

Vehicles are subclasses of Entity.

Dynamic tiles[]

Dynamic Block Entities
Entity ID Name
falling_block Dynamic Tile
tnt TNT

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

Other[]

Other Entities
Entity ID Name
area_effect_cloud Area Effect Cloud
armor_stand Armor Stand
ender_crystal End Crystal
evocation_fangs Evocation Fangs
eye_of_ender_signal Eye of Ender
fireworks_rocket Firework Rocket
item_frame Item Frame
leash_knot Lead Knot
llama_spit Llama spit
painting Painting
shulker_bullet 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[]

Block Entities
Block Entity ID Associated Block
banner Banner
beacon Beacon
brewing_stand Brewing Stand
cauldron (PE) Cauldron
chest Chest
Trapped Chest
comparator Redstone Comparator
command_block Command Block
daylight_detector Daylight Sensor
dispenser Dispenser
dropper Dropper
enchanting_table Enchantment Table
ender_chest Ender Chest
end_gateway End Gateway (block)
end_portal End Portal (block)
flower_pot Flower Pot
furnace Furnace
hopper Hopper
jukebox Jukebox
mob_spawner Monster Spawner
noteblock Note Block
piston Piston Moving
sign Sign
skull Mob head
structure_block Structure Block

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[]

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[]

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.
1.4.212w34aAdded entity WitherBoss.
1.513w02aAdded entity MinecartTNT.
Minecart is now deprecated.
13w03aAdded entity MinecartHopper.
13w06aAdded entity MinecartSpawner.
1.6.113w16aAdded entity EntityHorse.
13w21aRemoved ArmorType from EntityHorse.
Removed Saddle from EntityHorse.
1.6.1-preReadded Saddle to EntityHorse.
1.7.213w39aAdded entity MinecartCommandBlock.
1.814w02aAdded Lock to containers.
Item IDs are no longer used when specifying NBT data.
Added Block to FallingSand, using the alphabetical ID format.
14w06aAdded ShowParticles to all mobs.
Added PickupDelay to item entities.
Setting Age to -32768 makes items which never expire.
Removed AttackTime from mobs.
14w10aAdded 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.
14w11aAdded entity Endermite.
Added EndermiteCount to Enderman.
14w21aCustomName and CustomNameVisible now work for all entities.
14w25aAdded 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.
14w27aAdded entity Rabbit. Added CommandStats to command blocks and signs.
14w28aRemoved EndermiteCount from Enderman.
14w30aAdded Silent for all entities.
14w32aAdded NoAI for all mobs.
Added entity ArmorStand.
14w32cAdded NoBasePlate to ArmorStand.
1.915w31aAdded 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 Entity.
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.
15w32aTags and DataVersion tag can now be applied on entities.
Changed the Fuse tag's type for the PrimedTnt entity from "Byte" to "Short".
15w32cIntroduced 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."
15w33aAdded 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.
15w33cAdded BeamTarget to EnderCrystal.
15w34aAdded 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.
15w35aAdded VillagerProfession to Zombie.
15w37aAdded Enabled to MinecartHopper.
15w38aAdded SkeletonTrap and SkeletonTrapTime to EntityHorse.
15w41aReplaced Riding with Passengers for all entities.
Added RootVehicle for all passengers.
Added Type to Boat.
15w42aAdded Fuel to Cauldron (brewing stands).
15w43aAdded LootTable and LootTableSeed to Chest, Minecart and MinecartHopper.
Added DeathLootTable and DeathLootTableSeed to all mobs.
Added life and power to all fireballs (FireballBase).
15w44aAdded ShowBottom to EnderCrystal.
15w44bAdded Potion and CustomPotionEffects to Arrow.
Added Potion to AreaEffectCloud.
15w45aRemoved 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.
15w46aMoreCarrotTicks from Rabbit is now set to 40 when rabbits eat a carrot crop, but is not used anyway.
15w47aAdded PaymentItem to Beacon.
15w49aRemoved PaymentItem from Beacon.
In order for a sign to accept text, all 4 tags ("Text1", "Text2", "Text3", and "Text4") must exist.
15w51bThe original values of DisabledSlots in ArmorStand have changed in nature.
1.1016w20aAdded entity PolarBear.
Added ZombieType to Zombie, replacing VillagerProfession and IsVillager. Value of 6 indicates the "husk" zombie.
A value of 2 on SkeletonType indicates the "stray" skeleton.
NoGravity extended to all entities.
Added powered, showboundingbox and showair to Structure.
16w21aAdded FallFlying to mobs and armor stands.
Added integrity and seed to Structure.
1.10-pre1Added ParticleParam1 and ParticleParam2 to AreaEffectCloud.
1.1116w32aHorse was split into entity IDs horse, donkey, mule, skeleton_horse and zombie_horse for their respective types. Type and HasReproduced removed from horse, ChestedHorse and Items tags now apply only to mule and donkey, and SkeletonTrap and SkeletonTrapTime tags now apply only to skeleton_horse.
Skeleton was split into entity IDs skeleton, stray and wither_skeleton. SkeletonType tag is removed from all skeleton types.
Zombie was split into entity IDs zombie, zombie_villager and husk. ZombieType tag is removed from all zombie types, ConversionTime and Profession tags now apply only to zombie_villager.
Guardian was split into entity IDs guardian and elder_guardian. Elder tag is removed from guardian.
Unused savegame IDs Mob and Monster were removed.
Pumpkin byte tag is added to snowman.
16w35aAdded CustomName to banner.
16w39aAdded llama, llama_spit, vindication_illager, vex, evocation_fangs and evocation_illager.
Added Color to shulker.
Added LocName, CustomPotionColor and ColorMap to items.
16w40aAdded Johnny to vindication_illager.
Removed xTile, yTile, zTile, inTile, inGround from the FireballBase class (in large fireballs, small fireballs, dragon fireballs, and wither skulls).
16w41allama_spit is now available as a save game entity.
16w42aAdded crit to arrow.
16w43aAdded OwnerUUIDLeast and OwnerUUIDMost to evocation_fangs.
16w44aRemoved xTile, yTile, zTile, inTile, inGround from the fishing bobber entity.
1.11-pre1Changed ench to require at least one compound.


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