dash-docs/_includes/ref_block_chain.md

Block Chain

The following subsections briefly document core block details.

Block Contents

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This section describes [version 2 blocks][v2 block]{:#term-v2-block}{:.term}, which are any blocks with a block height greater than 227,835. (Version 1 and version 2 blocks were intermingled for some time before that point.) Future block versions may break compatibility with the information in this section. You can determine the version of any block by checking its version field using bitcoind RPC calls.

As of version 2 blocks, each block consists of four root elements:

1. A [magic number][block header magic]{:#term-block-header-magic}{:.term} (0xd9b4bef9).

2. A 4-byte unsigned integer indicating how many bytes follow until the end of the block. Although this field would suggest maximum block sizes of 4 GiB, max block size is currently capped at 1 MiB and the default max block size (used by most miners) is 350 KiB (although this will likely increase over time).

3. An 80-byte block header described in the section below.

4. One or more transactions.

Every block must include a [coinbase transaction][]{:#term-coinbase-tx}{:.term} which should collect and spend any transaction fees paid by transactions included in this block. All blocks with a block height less than 6,930,000 are entitled to receive a [block reward][]{:#term-block-reward}{:.term} of newly created bitcoin value, which also should be spent in the coinbase transaction. (The block reward started at 50 bitcoins and is being halved approximately every four years. As of April 2014, it's 25 bitcoins.) A coinbase transaction is invalid if it tries to spend more value than is available from the transaction fees and block reward.

The coinbase transaction has the same basic format as any other transaction, but it references a single non-existent UTXO and a special [coinbase field][]{:#term-coinbase-field}{:.term} replaces the field that would normally hold a scriptSig and signature. In version 2 blocks, the coinbase parameter must begin with the current block's block height and may contain additional arbitrary data or a script up to a maximum total of 100 bytes.

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Block Header

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The 80-byte block header contains the following six fields:

Field	Bytes	Format
1. Version	4	Unsigned Int
2. hashPrevBlock	32	Unsigned Int (SHA256 Hash)
3. hashMerkleRoot	32	Unsigned Int (SHA256 Hash)
4. Time	4	Unsigned Int (Epoch Time)
5. Bits	4	Internal Bitcoin Target Format
6. Nonce	4	(Arbitrary Data)

1. The [block version][]{:#term-block-version}{:.term} number indicates which set of block validation rules to follow so Bitcoin Core developers can add features or fix bugs. As of block height 227,836, all blocks use version number 2.

2. The hash of the previous block header puts this block on the block chain and ensures no previous block can be changed without also changing this block's header.

3. The Merkle root is a hash derived from hashes of all the transactions included in this block. It ensures no transactions can be modified in this block without changing the block header hash.

4. The [block time][]{:#term-block-time}{:.term} is the approximate time when this block was created in Unix Epoch time format (number of seconds elapsed since 1970-01-01T00:00 UTC). The time value must be greater than the median time of the previous 11 blocks. No peer will accept a block with a time currently more than two hours in the future according to the peer's clock.

5. Bits translates into the target threshold value---the maximum allowed value for this block's hash. The bits value must match the network difficulty at the time the block was mined.

6. The [header nonce][]{:#term-header-nonce}{:.term} is an arbitrary input that miners can change to test different hash values for the header until they find a hash value less than or equal to the target threshold. If all values within the nonce's four bytes are tested, the time can be updated or the coinbase transaction can be changed and the Merkle root updated.

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