Memory

Features of memory

• Uses of memory in Solidity

  • Get and return values to external calls

  • Set the function arguments for external calls

  • Revert with an error string

  • Log messages

  • Create other smart contracts

  • Using keccak256

• Memory in Solidity is equivalent to the heap in other languages

  • No garbage collector or free

  • Solidity memory is laid out in 32 byte sequences (like storage)

  • Accessed using [0x00 - 0x20], [0x20 - 0x40], [0x40 - 0x60], [0x60 - 0x80] …….

• Only four instructions that affect memory:

  • mload, mstore, mstore8, msize

• In pure Yul programs, memory is easy to use, but in mixed Solidity/Yul programs, Solidity expects memory to be used in a specific way

Overview

• Gas is charged for each memory access, and how far into the memory array you accessed

• E.g. mload(0xffffffffffffffff) will run out of gas

  • using a hash function to mstore like storage does is not recommended

• mstore(p, v) - stores value v in slot p (just like sload )

• mload(p) retrieves 32 bytes from slot p [p….0x20]

• mstore8(p, v) works similar to mstore but only for one byte

• msize() returns the largest accessed memory index in the transaction

Example

• mstore(0x00, x) → stores x in the first 8 bytes

• mstore(0x01, y) → stores y second 8 bytes (instead of moving forward 32 bytes in storage)

function mstore8() external pure {
        assembly {
            mstore8(0x00, 7) // write to the first byte in memory 0x07 
            mstore(0x00, 7) // overrides the initial store, 0x00..007 (32 bytes) 
        }
    }

How Solidity uses Memory

• 0x00 - 0x3f (64 bytes): scratch space for hashing methods

• 0x40 - 0x5f (32 bytes): currently allocated memory size (aka. free memory pointer, points to the end of the last allocated block)

• 0x60 - 0x7f (32 bytes): empty slot

• memory starts being stored in slot 0x80

• abi.encode and abi.encodePacked - stored in memory

• Structs and arrays (by explicitly using the memory keyword)

• Function arguments

• objects in memory are laid out end to end, arrays have no push unlike storage