TERM_DEF // SCRIPT_AUTHORIZATION / OP_WITHIN
OP_WITHIN
OP_WITHIN. Pops x, min, max; pushes 1 if min ≤ x < max.
This page sits in the Script & Authorization section — The stack VM that decides whether a UTXO is spendable — opcodes, templates, and modern script trees. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
This page sits in the Script & Authorization section — The stack VM that decides whether a UTXO is spendable — opcodes, templates, and modern script trees. Read on for what it is, why it exists, how it works under the hood, and what to watch out for.
WHAT_OP_WITHIN_IS
OP_WITHIN — at a glance
SCRIPT
Why it exists
DESIGN
OP_WITHIN exists because Bitcoin Script needs small integer arithmetic (≤ 4-byte CScriptNum values) for amounts, indices, and comparison results — anything larger is consensus-rejected.
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. OP_WITHIN 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. When OP_WITHIN is reached, it performs its specific stack effect (see below).
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
OP_WITHIN — stack effect + canonical use
EXAMPLE
Opcode : OP_WITHIN
Family : ARITH
Stack effect: x min max → (min ≤ x < max)
Behaviour : Range check, half-open interval.
Open /playground in another tab and search for OP_WITHIN.
Drag the opcode in, watch the stack visualisation step through it
against any combination of inputs you choose.
KEY_PROPERTIES
FAMILY
Belongs to the ARITH family of Script opcodes — siblings share validation rules and historical evolution.
STACK EFFECT
Exactly x min max → (min ≤ x < max). Every full node enforces the same effect, byte-for-byte.
ACTIVE
Active on mainnet today; every spend that uses it is being validated by every full node.
CONSENSUS-CRITICAL
OP_WITHIN's behaviour is consensus rule — a node implementing it differently would fork off the network.
COMMON_PITFALLS
Things that catch people out
PITFALLS
- Arithmetic operands must encode to ≤ 4 bytes; anything larger fails the script (CScriptNum overflow).
RELATED_CONCEPTS
Other terms from Script & Authorization — click any to read its page:
TERMINOLOGY_INDEX
TERMINOLOGY
OP_WITHIN
Pops x, min, max; pushes 1 if min ≤ x < max.
Script
Bitcoin's purpose-built stack language; every locking and unlocking script is a script program.
Stack
The single LIFO data structure all script execution operates on; no variables, no registers.
Locking Script
The scriptPubKey on an output; specifies the spending conditions.
Unlocking Script
The scriptSig/witness in an input; provides the data that satisfies the lock.
Redeem Script
Witness Script
In P2WSH, the script whose SHA256 is committed in the output; placed at the end of the witness stack.
Script Execution
Sequential stepping through opcodes, mutating the stack until either a valid truthy stack remains or the script fails.