# Working with Memory

When working with the EVM, your bytecode has read/write access to a fresh, isolated, and arbitrary string of memory.

In this chapter we cover:

  1. How Memory Works
  2. Memory Use Cases
  3. Memory Expansion
  4. Opcodes that use Memory

# How Memory Works

When working with assembly-level code, in most machine environments you would need to allocate memory before using it. In other words, you would need to ask the environment for permission to have more memory. If granted, your program continues. Otherwise, it errors out.

However, the EVM is different in that you don’t need to manually allocate memory. Instead of asking the environment for more memory, you simply directly use the memory locations you need.

As you use more memory, you automatically pay for the additional memory allocation. Read on for more details.

# Basic Opcodes

The basic opcodes that interact with memory are:

  • MSTORE - Writes 32 bytes into memory at a specified memory location.
  • MSTORE8 - Writes 1 byte of data (the 8 rightmost bits of the top item on the stack) into memory at a specified memory location.
  • MLOAD - Copies 32 bytes from a specified memory location onto the stack.

Each of these opcodes takes two arguments:

  1. The memory location – a 32-byte index of where to write to
  2. The data itself – 32 bytes (or 1 byte in the case of MSTORE8) of what to write.

While the data to write is simply what you put on the stack, the memory location is more complicated in concept and gas cost, as we’ll see in the next few sections.

# Memory Layout

Memory on the EVM is essentially a 2^256 length string:

  • A memory location behaves like a string index.
  • Each index holds one byte of data, initialized to 0x00.
  • You can read or write to any index you want! …in theory, at least 🙂 (see next section, “Accessing Memory”).

Every EVM function call gets a fresh, isolated string of memory:

  • Fresh as in, each index in memory is set to zero.
  • Isolated as in, each function call has its own memory; it cannot access the memory of other function calls.

# Accessing Memory

Memory is accessible via a 32-byte index. That’s an enormous key space!

But you can’t access that entire space. You can in theory, but not in practice.

The reason for this is memory expansion gas costs. Starting from zero, any time you access memory you haven’t before (within the same function call), you pay for that newly-accessed memory.

On the other hand, if you access memory you’ve already accessed within the same function call, then you do not pay any extra gas, because there is no memory expansion in that scenario.

In other words, reusing memory saves you gas.

See the gas cost breakdown in Memory Expansion.

# Memory Use Cases

There are three main reasons your EVM programs will need to use memory:

  1. Returning data. Any data that your code wants to return needs to be in memory first so that the RETURN opcode to access it.
  2. Longer data lifespans. If you’re manipulating a piece of data over many opcodes or ⚙️ Internal Function Calls (coming soon), having a stable location in memory can make your opcode logic much simpler, especially considering the 16 swap/dup reach limitation.
  3. Larger data. As large as the EVM’s word size is, not all working values can fit into 32 bytes. You will need to use memory as a workspace for larger amounts of data.

# 1. Returning Data

The RETURN opcodes cannot return an item from the stack. Instead, you must place your data into memory, and then return it.

Here is an example of returning the value 7:

PUSH1 0x07
PUSH1 0x50

PUSH1 0x20
PUSH1 0x50

In the above example:

  • The MSTORE opcode takes two arguments: The memory location and the value to store.
    • The memory location we are using is 0x50. In practice, we’d want to choose a location that does not clash with other data in memory (note that in this rudimentary example, it doesn’t matter which value we pick, so we choose a unique one to make the code easier to read).
    • The value we are storing is 0x07. Note that MSTORE always writes 32 bytes of data.
  • The RETURN opcode takes two arguments: The memory location of the where the data-to-return resides, and the length of that data in bytes.
    • The memory location we are using (0x50) is the same location we wrote to using MSTORE.
    • The length is 32 bytes (0x20) to return the full data we wrote using MSTORE.

Once the RETURN opcodes executes, the EVM stops execution of the function call, passing the return data to the parent context. This parent context may be the end of the transaction, or another function call that used a CALL opcode.

For more information on how returning data works, see ☎️ How EVM Function Calls Work (coming soon).

# 2. Longer data lifespans

Sometimes there will be data that you need to keep track of and continuously update throughout the lifespan of your function call, but not something you need to persist across function calls. Memory is the perfect use for this.

Arguably, the most common example is Solidity’s “free memory pointer”. This value exists at memory location 0x40. Semantically, it represents “the next location in memory that hasn’t been used yet” (read more at 💠 How Solidity Manages Memory (coming soon)).

This is useful not just for Solidity, but also for any bytecode architecture that allocates memory on a dynamic basis. It allows you to always have a handle on where to put new data, without conflicting with or overriding existing data.

# 3. Larger Data

Stack items are always 32 bytes in length. Memory is not limited in this respect.

A common example is contract deployment. During the contract deployment process, your bytecode needs to return the final state of the new contract’s bytecode.

This means your deployment bytecode is returning runtime bytecode, as a value in memory. This runtime bytecode will surely be larger than 32 bytes – up to 24 kilobytes on Layer 1. This is much larger than what can fit onto the stack.

More info on this topic at 🚀 How Contract Deployment Works (coming soon).

# Memory Expansion

Any time you reference a memory address n, the EVM will automatically allocate any memory that hasn’t already been allocated from 0 to n (inclusive).

In other words, any memory location you use will cause the EVM to ensure all memory locations below it are also allocated.

This is called memory expansion.

Every byte of memory that gets allocated incurs a memory expansion gas cost. On the flip side, reusing already-allocated memory does not incur memory expansion gas cost.

In addition, the more memory you use, the more expensive it gets.

You can check the size of your allocated memory at any time using the MSIZE opcode.

# The Formula

The formula for memory expansion gas cost is as follows (note that / in this context is integer devision):

expansion_cost(bytes) = words^2 / 512 + words * 3
    words = (bytes + 31) / 32

Note that this assumes nothing has been allocated.

To accommodate for previously allocated bytes, you need to calculate twice: one for the total bytes used, and one for the previous total bytes used.

real_cost(current, prev) = expansion_cost(current) - expansion_cost(prev)

# Memory Expansion Example 1

PUSH1 0x01
PUSH1 0x40

Assuming this is the only bytecode that runs in a function call…

The MSTORE operation causes the EVM to allocate a total of 96 bytes.

  • Although MSTORE only writes 32 bytes, in this example it writes to position 0x40, which is memory location 64 (in decimal).
  • The highest memory location we reference is 0x5f, or 95 in decimal. This is because we write 32 bytes starting from memory location 0x40.
  • In total, this example has a memory expansion gas cost of expansion_cost(96) = 3^2 / 512 + 3 * 3 = 9 gas.

# Memory Expansion Example 2

PUSH1 0x01
PUSH1 0x20

PUSH1 0x02
PUSH1 0x40

Assuming this is the only bytecode that runs in a function call…

The MSTORE operation causes the EVM to allocate a total of 96 bytes (again!).

  • Although we write 32 bytes more than the previous example, the highest memory location we use is still 0x5f.
  • Therefore, we still only pay for a total of 96 bytes of memory expansion gas cost.

# Opcodes that use Memory

The following opcodes access memory, and therefore have the potential to expand memory (and thus incur memory expansion gas cost):

  • Opcodes that write to memory:
    • MSTORE / MSTORE8 – Writes data from the stack into memory.
    • CALLDATACOPY – Copies data from calldata into memory.
    • CODECOPY – Copies bytecode from the currently running contract into memory.
    • EXTCODECOPY – Copies bytecode from an external contract address into memory.
    • RETURNDATACOPY – Copies data returned from the last external call into memory.
  • Opcodes that read from memory:
    • MLOAD – Copies 32 bytes of memory onto the stack.
    • RETURN – Returns a specified slice of memory.
    • SHA3 – Performs a keccak hash of a specified slice of memory, then puts the result onto the stack.
    • LOG0 - LOG4 – Reads a specified slice of memory and emits it as part of an event.
    • REVERT – Reverts with a specified slice of memory as the revert message.
    • CREATE / CREATE2 – Creates a new contract with a specified slice of memory as the new contract’s deployment bytecode.
  • Opcodes that both read and write:
    • CALL / STATICCALL / DELEGATECALL – Reads a slice of memory as calldata, calls an external contract, then writes the returned data to a slice of memory.

For more details on these opcodes, see wolflo's excellent opcode reference page.

# Conclusion

The EVM makes working with memory simpler than most environments. There’s no need to manually allocate memory, and each function call gets its own, isolated string of memory.

Many opcodes use memory, and understanding the gas costs associated with memory expansion is important for managing gas costs.

Ultimately, memory is only temporary. To write real programs, we will need persistent data, which we cover in the next chapter.