TERM_DEF // MODULE_1_THE_STACK / OPCODE
OPCODE
Opcode. A single instruction in a Bitcoin Script program.
An opcode (operation code) is one instruction in a Bitcoin Script program. Each opcode is encoded as a single byte in the raw script. The Script interpreter executes opcodes one at a time, left to right, maintaining the stack as it goes. There are roughly 80 active opcodes in Bitcoin Script.
This page sits in the Module 1 — The Stack section — Vocabulary introduced in the The Stack module. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
An opcode (operation code) is one instruction in a Bitcoin Script program. Each opcode is encoded as a single byte in the raw script. The Script interpreter executes opcodes one at a time, left to right, maintaining the stack as it goes. There are roughly 80 active opcodes in Bitcoin Script.
This page sits in the Module 1 — The Stack section — Vocabulary introduced in the The Stack module. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_OPCODE_IS
Opcode — at a glance
MODULE 1
Opcode is part of Bitcoin Script, the small stack-based programming language that decides whether any given output can be spent. A single instruction in a Bitcoin Script program. Script is intentionally simple — no loops, no recursion, no global state — because every node/">full node must execute every script in every transaction, and any complexity becomes a denial-of-service vector.
An opcode (operation code) is one instruction in a Bitcoin Script program. Each opcode is encoded as a single byte in the raw script. The Script interpreter executes opcodes one at a time, left to right, maintaining the stack as it goes. There are roughly 80 active opcodes in Bitcoin Script.
Why it exists
DESIGN
Bitcoin needed programmable money. A flat "pay address X" rule would have been too rigid — no multisig, no time-locks, no hashed commitments, no Lightning. Script is the answer: a tiny, deterministic, non-Turing-complete bytecode that lets coins be locked behind arbitrary spending conditions while keeping validation cheap and predictable.
HOW_IT_WORKS
Mechanism
HOW IT WORKS
Every UTXO is locked by a scriptPubKey — the output's locking script. To spend it, you provide a scriptSig (or witness) containing data that satisfies the lock. The node concatenates them, runs the combined script on a stack machine, and accepts the spend if and only if execution finishes with a single truthy value on the stack. Opcode contributes a specific stack effect within that process — opcodes either push, pop, copy, hash, branch, or verify, and they do so left-to-right deterministically.
1. The script is parsed into a sequence of opcodes and push-data items.
2. Execution starts with an empty stack and an empty alt-stack.
3. Each opcode runs in order — push opcodes add to the stack, others consume the top items and may push results.
4. Conditional opcodes (OP_IF/OP_NOTIF/OP_ELSE/OP_ENDIF) branch execution.
5. Final state: a single non-zero (truthy) value on top → the spend is authorised. Anything else (empty stack, false, error) → the script fails and the tx is rejected.
WORKED_EXAMPLE
A canonical P2PKH script — the most common locking script in Bitcoin
EXAMPLE
scriptPubKey (locking script — sits in the output):
OP_DUP OP_HASH160 <20-byte pubkey hash> OP_EQUALVERIFY OP_CHECKSIG
scriptSig (unlocking script — provided by the spender):
<signature> <pubkey>
Combined execution, left to right:
push signature → [sig]
push pubkey → [sig pubkey]
OP_DUP → [sig pubkey pubkey]
OP_HASH160 → [sig pubkey hash160(pubkey)]
push expected_hash → [sig pubkey hash160(pubkey) expected]
OP_EQUALVERIFY → [sig pubkey] (fails if hashes differ)
OP_CHECKSIG → [1] (true if signature is valid)
End state: single truthy value on top → spend authorised.
KEY_PROPERTIES
STACK-BASED
Every operation reads and writes the top of a single shared LIFO stack. No registers, no variables, no heap.
DETERMINISTIC
No randomness, no clocks. Every node executes the same script the same way — divergence would fork the network.
NON-TURING-COMPLETE
No loops, no recursion. Every script halts in bounded time, so validation cost is predictable.
CONSENSUS-CRITICAL
A misbehaving Script implementation forks its node off the network. The reference implementation is the de-facto spec.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- OP_RETURN makes an output provably unspendable — useful for data commitments, ruinous if used accidentally.
- Several opcodes were disabled in 2010 after security incidents (OP_MUL, OP_DIV, OP_SUBSTR, …) and have never been re-enabled.
- Number encoding (CScriptNum) is sign-magnitude, not two's complement. -1 is 0x81, not 0xff — a frequent source of bugs.
- OP_CHECKMULTISIG has a historical off-by-one bug — it pops one extra dummy item from the stack. Always prefix the sigs with an OP_0.
RELATED_CONCEPTS
Other terms from Module 1 — The Stack — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
Opcode
A single instruction in a Bitcoin Script program.
Stack
Last-in, first-out scratch pad the Script VM uses.
LIFO
Last In, First Out — the ordering rule of a stack.
Push
Place a value on top of the stack.
Pop
Remove the top value from the stack.
OP_DUP
Duplicate the top stack item.
Script
The bytecode program that locks and unlocks Bitcoin.