# Begin block

The base fee is calculated at the beginning of each block.

### Base Fee <a href="#base-fee" id="base-fee"></a>

#### Disabling base fee <a href="#disabling-base-fee" id="disabling-base-fee"></a>

We introduce two parameters : `NoBaseFee`and `EnableHeight`

`NoBaseFee` controls the feemarket base fee value. If set to true, no calculation is done and the base fee returned by the keeper is zero.

`EnableHeight` controls the height we start the calculation.

* If `NoBaseFee = false` and `height < EnableHeight`, the base fee value will be equal to `base_fee` defined in the genesis and the `BeginBlock` will return without further computation.
* If `NoBaseFee = false` and `height >= EnableHeight`, the base fee is dynamically calculated upon each block at `BeginBlock`.

Those parameters allow us to introduce a static base fee or activate the base fee at a later stage.

#### Enabling base fee <a href="#enabling-base-fee" id="enabling-base-fee"></a>

To enable EIP1559 with the EVM, the following parameters should be set :

* NoBaseFee should be false
* EnableHeight should be set to a positive integer >= upgrade height. It defines at which height the chain starts the base fee adjustment
* LondonBlock evm's param should be set to a positive integer >= upgrade height. It defines at which height the chain start to accept EIP1559 transactions

#### Calculation <a href="#calculation" id="calculation"></a>

The base fee is initialized at `EnableHeight` to the `InitialBaseFee` value defined in the genesis file.

The base fee is after adjusted according to the total gas used in the previous block.

```go
parent_gas_target = parent_gas_limit / ELASTICITY_MULTIPLIER

if EnableHeight == block.number
    base_fee = INITIAL_BASE_FEE
else if parent_gas_used == parent_gas_target:
    base_fee = parent_base_fee
else if parent_gas_used > parent_gas_target:
    gas_used_delta = parent_gas_used - parent_gas_target
    base_fee_delta = max(parent_base_fee * gas_used_delta / parent_gas_target / BASE_FEE_MAX_CHANGE_DENOMINATOR, 1)
    base_fee = parent_base_fee + base_fee_delta
else:
    gas_used_delta = parent_gas_target - parent_gas_used
    base_fee_delta = parent_base_fee * gas_used_delta / parent_gas_target / BASE_FEE_MAX_CHANGE_DENOMINATOR
    base_fee = parent_base_fee - base_fee_delta
```


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