---
eip: 7958
title: EVM64 - Little endian opcodes
description: Little endian opcodes `BYTE64`, `MLOAD64`, `MSTORE64`, and `PUSH*64` for EVM64
author: Wei Tang (@sorpaas)
discussions-to: https://ethereum-magicians.org/t/eip-7937-64-bit-mode-evm-opcodes-evm64/23794
status: Stagnant
type: Standards Track
category: Core
created: 2025-05-26
requires: 7937
---

## Abstract

This EIP defines additional little endian opcodes that can be deployed alongside [EIP-7937](./eip-7937.md).

## Motivation

The core EIP that defines EVM64 (EIP-7937) is endianness-independent. This EIP defines those additional opcodes that must expose endianness. They are the bitwise opcode `BYTE64`, memory opcodes `MLOAD64` and `MSTORE64`, and stack opcodes `PUSH*64`.

## Specification

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 and RFC 8174.

We define the following gas cost constant:

* `G_VERYLOW64`: 2

### `BYTE64`

`BYTE64` (0xc01a) is defined as `(x >> i * 8) & 0xFF`. Note that the definition is changed from big endian to little endian.

The gas cost is `G_VERYLOW64`.

### `MLOAD64` and `MSTORE64`

`MLOAD64` (0xc051) will load a 64-bit integer in little endian onto the stack. `MSTORE64` (0xc052) will read an 64-bit integer from the stack, and store it to memory in little endian.

The gas cost for both opcodes is `G_VERYLOW64`. The memory resizing costs count as 8 bytes.

As an example, `[0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08]` is an array of 8 bytes (64 bits). Calling `MLOAD64` to load this 8-byte value to stack will read it in little endian, resulting in 64-bit integer `0x0807060504030201`. 64-bit mode always operate on only the least significant 64 bits. When another 256-bit opcodes encounter this value on stack, it will be `0x00..00 0807060504030201`. Calling `MSTORE64` to store this value to memory will result in the array `[0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08]`.

### `PUSH*64`

`PUSH2_64` (0xc061) to `PUSH8_64` (0xc067) follows 2-byte to 8-byte literal. The literal is read little endian and pushed onto the stack.

The gas cost for them is `G_VERYLOW64`.

As an example, `0xc0 0x67 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08` will result in a stack with single value `0x00..00 0807060504030201`.

## Rationale

For detailed rationale discussion, please see the core EVM64 definition [EIP-7937](./eip-7937.md).

## Backwards Compatibility

No backward compatibility issues found.

## Test Cases

To be added.

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## Reference Implementation

To be added.

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## Security Considerations

Needs discussion.

## Copyright

Copyright and related rights waived via [CC0](../LICENSE.md).