OP_ROLL - Moving Nth Item to Top

17. OP_ROLL — Moving Nth Item to Top

Overview

OP_ROLL is the destructive counterpart of OP_PICK. Like OP_PICK, it reads an integer n from the top of the stack and accesses the item n positions deep. However, instead of copying that item, OP_ROLL removes it from its current position and moves it to the top. The rest of the stack closes the gap.

Opcode Reference

Opcode:     OP_ROLL
Hex:        0x7a
Word:       ROLL
Input:      xn ... x1 x0 n
Output:     xn-1 ... x1 x0 xn
Stack Before: [ ..., xn, xn-1, ..., x1, x0, n ]
Stack After:  [ ..., xn-1, ..., x1, x0, xn ]

The item at depth n is removed from its position and placed on top.

Execution Traces

OP_ROLL with n=0 (no change — top item moved to top):

Stack: [ A, B, C, 0 ]

OP_ROLL → Stack: [ A, B, C ]   ← C removed from position 0, placed at top (same)

OP_ROLL with n=1 (equivalent to OP_SWAP):

Stack: [ A, B, C, 1 ]

OP_ROLL → Stack: [ A, C, B ]   ← B extracted from position 1, moved to top

OP_ROLL with n=2 (equivalent to OP_ROT):

Stack: [ A, B, C, 2 ]

OP_ROLL → Stack: [ B, C, A ]   ← A extracted from position 2, moved to top

Stack Size After OP_ROLL

Unlike OP_PICK, OP_ROLL does not increase the stack size:

OP_PICK: Stack before = N items → Stack after = N items (n consumed, copy added)
OP_ROLL: Stack before = N items → Stack after = N-1 items (n consumed, item moved, no copy)

Wait — correction:

Stack before OP_ROLL: N+1 items (N items + n on top)
Stack after OP_ROLL:  N items   (n consumed, item moved to top — no net change in N)

Both OP_PICK and OP_ROLL produce a net stack size of N after removing n, but OP_ROLL achieves this by physically relocating the item rather than copying it.

Practical Use Case: Bringing a Buried Value Forward

Stack: [ sig_A, sig_B, sig_C, pubkey ]

← pubkey buried at depth 3

Push depth: OP_3
OP_ROLL

Stack: [ sig_A, sig_B, sig_C, pubkey ]

Hmm, let's re-index...

Start:   [ sig_A, sig_B, sig_C, pubkey, 3 ]
OP_ROLL: [ sig_B, sig_C, pubkey, sig_A ]
                                 ↑ sig_A moved from depth 3 to top

Failure Conditions

• n must be non-negative.

• n must be within the current stack depth.

• Out-of-range n causes script failure.

Summary

OP_ROLL physically extracts an item from deep within the stack and brings it to the top, reducing the need to rearrange intermediate items manually. It is the destructive version of OP_PICK.