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Since nonce is a 32 bit number, the maximum value of nonce can only be 4,294,967,296. So, if a miner doesn't find the target hash within this number, what happens?
Maximum # of hashes for finding a block?
Answers
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The miner can change the block header hash several different ways:
- Change the timestamp. This is the least intrusive, but works just fine. If you can do more than 4 Th/s, this starts to become a problem.
- Add transactions. Adding any new transaction changes the Merkle root, so this gives you entirely new nonce spaces.
- Change the coinbase transaction. This is the most common method. 32 bits are typically used to act as an additional nonce.
1How can a miner change the coinbase transaction? More specifically, does this changeable field mean anything? – galahad – 2016-07-20T21:25:39.173
The scriptSig field of the coinbase transaction's txin can contrain 96 bytes of freeform data. – Pieter Wuille – 2016-07-20T23:00:07.070
Note: Although a signed 32-bit integer's max value is 2^32 - 1 = 2,147,483,647, that's not really relevant: The miners will iterate through all 32-bit values, so they have 4,294,967,296 tries really. Put another way, think of that 32-bit number as an unsigned integer. – Alin Tomescu – 2016-07-21T17:34:07.500