Bootloader

In standard Ethereum clients, the process of executing blocks involves selecting and validating transactions one by one, executing them, and then applying the resulting state changes to the blockchain. This method is suitable for Ethereum's architecture but would be inefficient for ZKsync due to the need for running a complete proving workflow for each transaction.

Bootloader

Bootloader is the program that accepts an array of transactions and executes the entire ZKsync batch. This section will expand on its invariants and methods.

Playground bootloader vs proved bootloader

For convenience, we use the same implementation of the bootloader both in the mainnet batches and for emulating ethCalls or other testing activities. Only proved bootloader is ever used for batch-building and thus this document describes only it.

Why ZKsync Uses a Bootloader

To address this inefficiency, ZKsync employs a bootloader. This component allows for processing not just one transaction at a time but an entire batch of transactions as a single large operation. This approach is similar to how an EntryPoint works under EIP4337, which also manages transactions in arrays to support the Account Abstraction protocol.

You can learn more about Batches & L2 blocks on ZKsync.

Operational Mechanism of the Bootloader

The bootloader's code is not stored on Layer 2 (L2) but its hash is stored on Layer 1 (L1) and can only be modified through a system upgrade. This setup ensures that the bootloader functions as a kind of "formal" address that provides context and identity to this, msg.sender, and similar references during transaction processing. If someone interacts with this address, for instance, to handle transaction fees, it triggers the EmptyContract’s code.

Batch Start

It is enforced by the ZKPs, that the state of the bootloader is equivalent to the state of a contract transaction with empty calldata. The only difference is that it starts with all the possible memory pre-allocated (to avoid costs for memory expansion).

For additional efficiency (and our convenience), the bootloader receives its parameters inside its memory. This is the only point of non-determinism: the bootloader starts with its memory pre-filled with any data the operator wants. That’s why it is responsible for validating the correctness of it and it should never rely on the initial contents of the memory to be correct & valid.

For instance, for each transaction, we check that it is properly ABI-encoded and that the transactions go exactly one after another. We also ensure that transactions do not exceed the limits of the memory space allowed for transactions.

Transaction Types and Validation

While the main transaction format is the internal Transaction format, it is a struct that is used to represent various kinds of transactions types. It contains a lot of reserved fields that could be used depending in the future types of transactions without need for AA to change the interfaces of their contracts.

The exact type of the transaction is marked by the txType field of the transaction type. There are 6 types currently supported:

• txType: 0. It means that this transaction is of legacy transaction type. The following restrictions are enforced:
• maxFeePerErgs=getMaxPriorityFeePerErg since it is pre-EIP-1559 tx type.
• reserved1..reserved4 as well as paymaster are 0. paymasterInput is zero.
• Note, that unlike type 1 and type 2 transactions, reserved0 field can be set to a non-zero value, denoting that this legacy transaction is EIP-155-compatible and its RLP encoding (as well as signature) should contain the chainId of the system.
• txType: 1. It means that the transaction is of type 1, i.e. transactions access list. ZKsync does not support access lists in any way, so no benefits of fulfilling this list will be provided. The access list is assumed to be empty. The same restrictions as for type 0 are enforced, but also reserved0 must be 0.
• txType: 2. It is EIP1559 transactions. The same restrictions as for type 1 apply, but now maxFeePerErgs may not be equal to getMaxPriorityFeePerErg.
• txType: 113. It is ZKsync transaction type. This transaction type is intended for AA support. The only restriction that applies to this transaction type: fields reserved0..reserved4 must be equal to 0.
• txType: 254. It is a transaction type that is used for upgrading the L2 system. This is the only type of transaction is allowed to start a transaction out of the name of the contracts in kernel space.
• txType: 255. It is a transaction that comes from L1. There are almost no restrictions explicitly imposed upon this type of transaction, since the bootloader at the end of its execution sends the rolling hash of the executed priority transactions. The L1 contract ensures that the hash did indeed match the hashes of the priority transactions on L1.

You can also read more on L1->L2 transactions and upgrade transactions.

However, as already stated, the bootloader’s memory is not deterministic and the operator is free to put anything it wants there. For all of the transaction types above the restrictions are imposed in the following (method), which is called before starting processing the transaction.

Bootloader Memory Structure

The bootloader expects the following structure of the memory (here by word we denote 32-bytes, the same machine word as on EVM):

Batch Information

The first 8 words are reserved for the batch information provided by the operator.

• 0 — The address of the operator (the beneficiary of the transactions).
• 1 — The hash of the previous batch. Its validation will be explained later on.
• 2 — The timestamp of the current batch. Its validation will be explained later on.
• 3 — The number of the new batch.
• 4 — The L1 gas price provided by the operator.
• 5 — The “fair” price for L2 gas, i.e. the price below which the baseFee of the batch should not fall. For now, it is provided by the operator, but it in the future it may become hardcoded.
• 6 — The base fee for the batch that is expected by the operator. While the base fee is deterministic, it is still provided to the bootloader just to make sure that the data that the operator has coincides with the data provided by the bootloader.
• 7 — Reserved word. Unused on proved batch.

The batch information slots are used at the beginning of the batch. Once read, these slots can be used for temporary data.

Temporary Data Descriptions

(This temporary data are used for debug and transaction processing purposes.)

• [8..39] – reserved slots for debugging purposes
• [40..72] – slots for holding the paymaster context data for the current transaction. The role of the paymaster context is similar to the EIP4337’s one. You can read more about it in the account abstraction documentation.
• [73..74] – slots for signed and explorer transaction hash of the currently processed L2 transaction.
• [75..110] – 36 slots for the calldata for the KnownCodesContract call.
• [111..1134] – 1024 slots for the refunds for the transactions.
• [1135..2158] – 1024 slots for the overhead for batch for the transactions. This overhead is suggested by the operator, i.e. the bootloader will still double-check that the operator does not overcharge the user.
• [2159..3182] – slots for the “trusted” gas limits by the operator. The user’s transaction will have at its disposal min(MAX_TX_GAS(), trustedGasLimit), where MAX_TX_GAS is a constant guaranteed by the system. Currently, it is equal to 80 million gas. In the future, this feature will be removed.
• [3183..7282] – slots for storing L2 block info for each transaction. You can read more on the difference L2 blocks and batches.
• [7283..40050] – slots used for compressed bytecodes each in the following format:
  • 32 bytecode hash
  • 32 zeroes (but then it will be modified by the bootloader to contain 28 zeroes and then the 4-byte selector of the publishCompressedBytecode function of the BytecodeCompressor)
  • The calldata to the bytecode compressor (without the selector).
• [40051..40052] – slots where the hash and the number of current priority ops is stored. More on it in the priority operations section on Handling L1->L2 ops on ZKsync.

L1Messenger Pubdata

• [40053..248052] – slots where the final batch pubdata is supplied to be verified by the L1Messenger. More on how the L1Messenger system contracts handles the pubdata can be read on Handling Pubdata.

This [40053..248052] space is used for the calldata to the L1Messenger’s publishPubdataAndClearState function, which accepts:

• List of the user L2→L1 logs,
• Published L2→L1 messages
• Bytecodes
• List of full state diff entries, which describe how each storage slot has changed as well as compressed state diffs.

This method will then check the correctness of the provided data and publish the hash of the correct pubdata to L1.

Note, that while the realistic number of pubdata that can be published in a batch is 120kb, the size of the calldata to L1Messenger may be significantly larger due to the fact that this method also accepts the original, uncompressed state diff entries.

These will not be published to L1, but will be used to verify the correctness of the compression. In a worst-case scenario, the number of bytes that may be needed for this scratch space is if all the pubdata consists of repeated writes (i.e. we’ll need only 4 bytes to include key) that turn into 0 (i.e. they’ll need only 1 byte to describe it).

However, each of these writes in the uncompressed form will be represented as 272 byte state diff entry and so we get the number of diffs is 120k / 5 = 24k. This means that they will have accommodate 24k * 272 = 6528000 bytes of calldata for the uncompressed state diffs. Adding 120k on top leaves us with roughly 6650000 bytes needed for calldata. 207813 slots are needed to accommodate this amount of data. We round up to 208000 slots to give space for constant-size factors for ABI-encoding, like offsets, lengths, etc.

In theory, much more calldata could be used. For instance, if one byte is used for enum index. It is the responsibility of the operator to ensure that it can form the correct calldata for the L1Messenger.

Transaction's meta descriptions

• [586653..606652] words — 20000 slots for 10000 transaction’s meta descriptions (their structure is explained below).

For internal reasons related to possible future integrations of zero-knowledge proofs about some of the contents of the bootloader’s memory, the array of the transactions is not passed as the ABI-encoding of the array of transactions, but:

• We have a constant maximum number of transactions. At the time of this writing, this number is 10000.
• Then, we have 10000 transaction descriptions, each ABI encoded as the following struct:

struct BootloaderTxDescription {
    // The offset by which the ABI-encoded transaction's data is stored
    uint256 txDataOffset;
    // Auxilary data on the transaction's execution. In our internal versions
    // of the bootloader it may have some special meaning, but for the
    // bootloader used on the mainnet it has only one meaning: whether to execute
    // the transaction. If 0, no more transactions should be executed. If 1, then
    // we should execute this transaction and possibly try to execute the next one.
    uint256 txExecutionMeta;
}

Reserved slots for the calldata for the paymaster’s postOp operation

• [606653..606692] words — 40 slots which could be used for encoding the calls for postOp methods of the paymaster.

To avoid additional copying of transactions for calls for the account abstraction, we reserve some of the slots which could be then used to form the calldata for the postOp call for the account abstraction without having to copy the entire transaction’s data.

The actual transaction’s descriptions

• [606693..927496]

Starting from the 487312 word, the actual descriptions of the transactions start. (The struct can be found by this link). The bootloader enforces that:

• They are correctly ABI encoded representations of the struct above.
• They are located without any gaps in memory (the first transaction starts at word 653 and each transaction goes right after the next one).
• The contents of the currently processed transaction (and the ones that will be processed later on are untouched). Note, that we do allow overriding data from the already processed transactions as it helps to preserve efficiency by not having to copy the contents of the Transaction each time we need to encode a call to the account.

VM Hook Pointers

• [927497..927499]

These are memory slots that are used purely for debugging purposes (when the VM writes to these slots, the server side can catch these calls and give important insight information for debugging issues).

Result Pointer

• [927500..937499]

These are memory slots that are used to track the success status of a transaction. If the transaction with number i succeeded, the slot 937499 - 10000 + i will be marked as 1 and 0 otherwise.

Bootloader execution flow

1. At the start of the batch it reads the initial batch information and sends the information about the current batch to the SystemContext system contract.
2. It goes through each of transaction’s descriptions and checks whether the execute field is set. If not, it ends processing of the transactions and ends execution of the batch. If the execute field is non-zero, the transaction will be executed and it goes to step 3.
3. Based on the transaction’s type it decides whether the transaction is an L1 or L2 transaction and processes them accordingly. More on the processing of the L1 transactions can be read in the L1->L2 transactions section. More on L2 transactions can be read in the L2 transactions section.

L2 Transactions

On ZKsync, every address is a contract. Users can start transactions from their EOA accounts, because every address that does not have any contract deployed on it implicitly contains the code defined in the DefaultAccount.sol file. Whenever anyone calls a contract that is not in kernel space (i.e. the address is ≥ 2^16) and does not have any contract code deployed on it, the code for DefaultAccount will be used as the contract’s code.

Note, that if you call an account that is in kernel space and does not have any code deployed there, right now, the transaction will revert.

We process the L2 transactions according to our account abstraction protocol: https://code.zksync.io/tutorials/native-aa-multisig#prerequisites.

1. We deduct the transaction’s upfront payment for the overhead for the block’s processing. You can read more on how that works in the fee model description.
2. Then we calculate the gasPrice for these transactions according to the EIP1559 rules.
3. We conduct the validation step of the AA protocol:
   • We calculate the hash of the transaction.
   • If enough gas has been provided, we near_call the validation function in the bootloader. It sets the tx.origin to the address of the bootloader, sets the ergsPrice. It also marks the factory dependencies provided by the transaction as marked and then invokes the validation method of the account and verifies the returned magic.
   • Calls the accounts and, if needed, the paymaster to receive the payment for the transaction. Note, that accounts may not use block.baseFee context variable, so they have no way to know what exact sum to pay. That’s why the accounts typically firstly send tx.maxFeePerErg * tx.ergsLimit and the bootloader refunds for any excess funds sent.
4. We perform the execution of the transaction. Note, that if the sender is an EOA, tx.origin is set equal to the from the value of the transaction. During the execution of the transaction, the publishing of the compressed bytecodes happens: for each factory dependency if it has not been published yet and its hash is currently pointed to in the compressed bytecodes area of the bootloader, a call to the bytecode compressor is done. Also, at the end the call to the KnownCodeStorage is done to ensure all the bytecodes have indeed been published.
5. We refund the user for any excess funds he spent on the transaction:
   • Firstly, the postTransaction operation is called to the paymaster.
   • The bootloader asks the operator to provide a refund. During the first VM run without proofs the provide directly inserts the refunds in the memory of the bootloader. During the run for the proved batches, the operator already knows what which values have to be inserted there. You can read more about it in Fee model
   • The bootloader refunds the user.
6. We notify the operator about the refund that was granted to the user. It will be used for the correct displaying of gasUsed for the transaction in explorer.

L1->L2 Transactions

L1->L2 transactions are transactions that were initiated on L1. We assume that from has already authorized the L1→L2 transactions. It also has its L1 pubdata price as well as ergsPrice set on L1.

Most of the steps from the execution of L2 transactions are omitted and we set tx.origin to the from, and ergsPrice to the one provided by transaction. After that, we use mimicCall to provide the operation itself from the name of the sender account.

Note, that for L1→L2 transactions, reserved0 field denotes the amount of ETH that should be minted on L2 as a result of this transaction. reserved1 is the refund receiver address, i.e. the address that would receive the refund for the transaction as well as the msg.value if the transaction fails.

There are two kinds of L1->L2 transactions:

1. Priority operations, initiated by users (they have type 255).
2. Upgrade transactions, that can be initiated during system upgrade (they have type 254).

Read more about differences between the different L1->L2 transaction types.

End of the batch

At the end of the batch we set tx.origin and tx.gasprice context variables to zero to both save L1 gas on calldata and to send the entire Bootloader balance to the operator. This effectively sends all the fees collected by the Bootloader to the operator.

Also, we set the fictive L2 block’s data. Then, we call the system context to ensure that it publishes the timestamp of the L2 block as well as L1 batch. We also reset the txNumberInBlock counter to avoid its state diffs from being published on L1. You can read more about block processing on ZKsync.

After that, we publish the hash as well as the number of priority operations in this batch. Handling L1->L2 ops on ZKsync.

Then, we call the L1Messenger system contract for it to compose the pubdata to be published on L1. You can read more on Handling pubdata.