OP_3DUP - Duplicating Top Three Items

5. OP_3DUP — Duplicating Top Three Items

Overview

OP_3DUP extends the duplication family by copying the top three stack items and placing them on top of the existing stack. After execution, the stack contains the original three items plus three new copies above them.

Opcode Reference

Opcode:     OP_3DUP
Hex:        0x6f
Word:       3DUP
Input:      x1 x2 x3
Output:     x1 x2 x3 x1 x2 x3
Stack Before: [ ..., x1, x2, x3 ]   ← x3 is top
Stack After:  [ ..., x1, x2, x3, x1, x2, x3 ]

Execution Trace

Script: OP_1 OP_2 OP_3 OP_3DUP

Step 1: Stack: [ 1 ]

Step 2: Stack: [ 1, 2 ]

Step 3: Stack: [ 1, 2, 3 ]

Step 4 — OP_3DUP: Stack: [ 1, 2, 3, 1, 2, 3 ]

Use Case in Complex Script Verification

OP_3DUP is useful in elliptic curve arithmetic scripts and in scripts that implement custom cryptographic protocols where three values (like x-coordinate, y-coordinate, and a scalar) must each be used twice.

<x> <y> <z>
OP_3DUP
<verify_set_1_operations>
OP_DROP OP_DROP OP_DROP    ← clean up first set result
<verify_set_2_operations>  ← use the original x, y, z

Stack Depth Implications

Using OP_3DUP doubles three items, growing the stack by 3. Since Bitcoin enforces a maximum stack depth of 1000 elements, scripts that use OP_3DUP in loops or repeated patterns must carefully account for stack growth.

Max stack depth: 1000 items
OP_3DUP adds:    3 items per call

Failure Conditions

• Requires at least 3 items on the stack.

Summary

OP_3DUP serves specialized use cases where three items must be reused across multiple operations without being consumed. It is less common than OP_DUP or OP_2DUP but essential in complex script templates involving triple-element state.