What parts of a block, other than the nonce, can a miner vary

Question

What are the list of all the variables whose values that the miner can cycle or modify to get a valid hash for this block (with very high probability) ? The only one I am aware of is:

1. Nonce - 4 Bytes.

But since Nonce is just 4 bytes it is simply not enough for generating many potential hash candidates that pass the difficuly condition of SHA256 (256 bits).

What are the other variables that can be tweaked for ensuring the existence of a passing hash candidate (with very high probability)?

P.S. We are assuming that all the hashes are calculated immediately so timestamp for each candidate is same. Thus, timestamp wont be able to help as far as I understand.

I am struggling with this for a while and can't find much detailed answer for this (other than nonce).

Answer 1

ExtraNonce: As well as the Nonce field, There is also an extranonce convention - a part of the unlocking script in the input of the coinbase transaction is used as an extra nonce by miners.

Timestamp: The miner can vary the timestamp. It only has to be accurate within about 2 hours. It doesn't have to be greater than the timestamp of the previous block.

Version: Miners can vary a part of the version number. There is a method known as ASICboost which helps miners who are using ASICs for mining.

Coinbase output: The miner can vary the coinbase output. Perhaps selecting a different receiving address or varying the type of script.

Transaction order: The miner can vary the order of transactions (other than for transactions that spend outputs of prior transactions in same block).

Transactions: The miner can make an arbitrary select of transactions from their mempool or create their own new arbitrary valid transactions to include. If inventing their own transactions they can vary the fee from zero to maximum and can vary the transaction type from legacy to latest. They can also vary the input amounts from near zero to whole UTXO. The transactions must be valid but that still allows a vast range of variations, assuming the miner owns some bitcoin to start with.

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From Bitcoin for Developers I

Table 2. The structure of a coinbase transaction input

Size	Field	Description
32 bytes	Transaction Hash	All bits are zero: Not a transaction hash reference
4 bytes	Output Index	All bits are ones: 0xFFFFFFFF
1–9 bytes (VarInt)	Coinbase Data Size	Length of the coinbase data, from 2 to 100 bytes
Variable	Coinbase Data	Arbitrary data used for extra nonce and mining tags. In v2 blocks; must begin with block height
4 bytes	Sequence Number	Set to 0xFFFFFFFF

See also BIP-34