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Overview

Opcode: 0x08 Introduced: Frontier (EVM genesis) ADDMOD performs modular addition (a + b) % N where all operands are 256-bit unsigned integers. Unlike standard ADD followed by MOD, ADDMOD computes the result using wider arithmetic to prevent intermediate overflow, making it essential for cryptographic operations. Division by zero (N = 0) returns 0 rather than throwing an exception.

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Specification

Stack Input: 
a (top)
b
N (modulus)
Stack Output: 
(a + b) % N
Gas Cost: 8 (GasMidStep) Operation: 
if N == 0:
  result = 0
else:
  result = (a + b) % N

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Behavior

ADDMOD pops three values from the stack (a, b, N), computes (a + b) mod N, and pushes the result back:
  • Normal case: Result is (a + b) % N
  • N = 0: Returns 0 (EVM convention)
  • No intermediate overflow: Uses 512-bit arithmetic internally
The key advantage over ADD then MOD is that ADDMOD avoids intermediate overflow when a + b >= 2^256.

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Examples

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Basic Modular Addition

import { addmod } from '@tevm/voltaire/evm/arithmetic';
import { createFrame } from '@tevm/voltaire/evm/Frame';

// (5 + 10) % 3 = 15 % 3 = 0
const frame = createFrame({ stack: [5n, 10n, 3n] });
const err = addmod(frame);

console.log(frame.stack); // [0n]
console.log(frame.gasRemaining); // Original - 8

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Overflow-Safe Addition

// MAX + MAX would overflow in ADD, but ADDMOD handles it
const MAX_U256 = (1n << 256n) - 1n;
const frame = createFrame({ stack: [MAX_U256, MAX_U256, 100n] });
const err = addmod(frame);

// (MAX + MAX) % 100 = (2^256 - 2) % 100
const expected = ((MAX_U256 + MAX_U256) % 100n);
console.log(frame.stack); // [expected]

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Zero Modulus

// Division by zero returns 0
const frame = createFrame({ stack: [5n, 10n, 0n] });
const err = addmod(frame);

console.log(frame.stack); // [0n]

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Modulus of 1

// Any number mod 1 is 0
const frame = createFrame({ stack: [999n, 888n, 1n] });
const err = addmod(frame);

console.log(frame.stack); // [0n]

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Large Modulus

// Result when sum < modulus
const frame = createFrame({ stack: [5n, 10n, 1000n] });
const err = addmod(frame);

console.log(frame.stack); // [15n] (no reduction needed)

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Gas Cost

Cost: 8 gas (GasMidStep) ADDMOD costs more than basic ADD due to wider arithmetic requirements: Comparison:
  • ADD/SUB: 3 gas
  • MUL/DIV/MOD: 5 gas
  • ADDMOD/MULMOD: 8 gas
  • EXP: 10 + 50 per byte
Despite higher cost, ADDMOD is more efficient than separate ADD + MOD operations when dealing with potential overflow.

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Edge Cases

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Maximum Values

const MAX = (1n << 256n) - 1n;

// MAX + MAX mod 7
const frame = createFrame({ stack: [MAX, MAX, 7n] });
addmod(frame);

const expected = (MAX + MAX) % 7n;
console.log(frame.stack); // [expected]

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Identity Elements

// a + 0 = a (mod N)
const frame1 = createFrame({ stack: [42n, 0n, 17n] });
addmod(frame1);
console.log(frame1.stack); // [42n % 17n = 8n]

// 0 + 0 = 0 (mod N)
const frame2 = createFrame({ stack: [0n, 0n, 17n] });
addmod(frame2);
console.log(frame2.stack); // [0n]

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Stack Underflow

// Not enough stack items
const frame = createFrame({ stack: [5n, 10n] });
const err = addmod(frame);

console.log(err); // { type: "StackUnderflow" }

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Out of Gas

// Insufficient gas
const frame = createFrame({ stack: [5n, 10n, 3n], gasRemaining: 7n });
const err = addmod(frame);

console.log(err); // { type: "OutOfGas" }
console.log(frame.gasRemaining); // 0n

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Common Usage

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Elliptic Curve Point Addition

// secp256k1 field arithmetic (p = 2^256 - 2^32 - 977)
assembly {
    let p := 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F

    // Add two field elements
    let x1 := mload(0x00)
    let x2 := mload(0x20)
    let sum := addmod(x1, x2, p)
}

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Modular Ring Operations

// Ring arithmetic mod N
function addInRing(uint256 a, uint256 b, uint256 N)
    pure returns (uint256)
{
    assembly {
        mstore(0x00, addmod(a, b, N))
        return(0x00, 0x20)
    }
}

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Hash Computations

// Polynomial rolling hash
assembly {
    let hash := 0
    let base := 31
    let mod := 1000000007

    // hash = (hash * base + char) % mod
    hash := addmod(mulmod(hash, base, mod), char, mod)
}

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Schnorr/BLS Signature Math

// s = (k + e*x) mod n (Schnorr signature)
assembly {
    let n := 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEBAAEDCE6AF48A03BBFD25E8CD0364141
    let k := mload(0x00)
    let e_x := mulmod(e, x, n)
    let s := addmod(k, e_x, n)
}

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Implementation

/**
 * ADDMOD opcode (0x08) - Addition modulo N
 */
export function addmod(frame: FrameType): EvmError | null {
  // Consume gas (GasMidStep = 8)
  frame.gasRemaining -= 8n;
  if (frame.gasRemaining < 0n) {
    frame.gasRemaining = 0n;
    return { type: "OutOfGas" };
  }

  // Pop operands: a, b, N
  if (frame.stack.length < 3) return { type: "StackUnderflow" };
  const a = frame.stack.pop();
  const b = frame.stack.pop();
  const n = frame.stack.pop();

  // Compute result
  let result: bigint;
  if (n === 0n) {
    result = 0n;
  } else {
    // BigInt handles arbitrary precision - no overflow
    result = (a + b) % n;
  }

  // Push result
  if (frame.stack.length >= 1024) return { type: "StackOverflow" };
  frame.stack.push(result);

  // Increment PC
  frame.pc += 1;

  return null;
}

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Testing

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Test Coverage

import { describe, it, expect } from 'vitest';
import { addmod } from './0x08_ADDMOD.js';

describe('ADDMOD (0x08)', () => {
  it('computes (a + b) % N', () => {
    const frame = createFrame([5n, 10n, 3n]);
    expect(addmod(frame)).toBeNull();
    expect(frame.stack).toEqual([0n]); // 15 % 3 = 0
  });

  it('returns 0 when N is 0', () => {
    const frame = createFrame([5n, 10n, 0n]);
    expect(addmod(frame)).toBeNull();
    expect(frame.stack).toEqual([0n]);
  });

  it('handles large values without overflow', () => {
    const MAX = (1n << 256n) - 1n;
    const frame = createFrame([MAX, MAX, 7n]);
    expect(addmod(frame)).toBeNull();
    expect(frame.stack).toEqual([(MAX + MAX) % 7n]);
  });

  it('returns StackUnderflow with insufficient stack', () => {
    const frame = createFrame([5n, 10n]);
    expect(addmod(frame)).toEqual({ type: 'StackUnderflow' });
  });

  it('returns OutOfGas when insufficient gas', () => {
    const frame = createFrame([5n, 10n, 3n], 7n);
    expect(addmod(frame)).toEqual({ type: 'OutOfGas' });
  });
});

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Edge Cases Tested

  • Basic modular addition (15 % 3 = 0)
  • Zero modulus (returns 0)
  • Modulus of 1 (always returns 0)
  • Large values (MAX + MAX)
  • Overflow-safe computation
  • Identity elements (a + 0, 0 + 0)
  • Stack underflow (< 3 items)
  • Out of gas (< 8 gas)

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Security

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Cryptographic Importance

ADDMOD is critical for implementing cryptographic operations that require modular arithmetic: Elliptic Curve Operations: 
// secp256k1 field addition
uint256 constant FIELD_P = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFEFFFFFC2F;

function fieldAdd(uint256 a, uint256 b) pure returns (uint256) {
    return addmod(a, b, FIELD_P);
}
BLS12-381 Group Operations: 
// BLS12-381 field modulus
uint256 constant BLS_P = 0x1a0111ea397fe69a4b1ba7b6434bacd764774b84f38512bf6730d2a0f6b0f6241eabfffeb153ffffb9feffffffffaaab;

function blsFieldAdd(uint256 a, uint256 b) pure returns (uint256) {
    return addmod(a, b, BLS_P);
}

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Timing Safety

ADDMOD operations complete in constant time regardless of operand values, preventing timing side-channel attacks in cryptographic implementations.

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Overflow Protection

Unlike ADD then MOD, ADDMOD prevents intermediate overflow: Vulnerable pattern: 
// Can overflow if a + b >= 2^256
uint256 sum = a + b;
uint256 result = sum % N;  // Wrong result if overflow occurred
Safe pattern: 
// Always correct
uint256 result = addmod(a, b, N);

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References

  • ADD - Basic addition with wrapping
  • MULMOD - Modular multiplication
  • MOD - Unsigned modulo operation