BA-06 · LESSON 01

Stack-machine execution

KEY DEFINITIONS

Terms used in this lesson

OP_ADD: A Bitcoin Script operation that removes two script numbers and pushes their sum.
OP_EQUAL: A Bitcoin Script operation that compares two byte strings and pushes true or false.
Witness: Signatures, scripts, or other data supplied to satisfy a Bitcoin output’s spending condition.
OP_VERIFY: A Bitcoin Script operation that consumes a value and immediately fails if it is false.
SegWit: Segregated Witness: a Bitcoin upgrade that separates authorization data from the transaction identifier and introduces versioned witness programs.
Tapscript: The version of Bitcoin Script used by a Taproot script-path spend.
UTXO: Unspent transaction output: one discrete chunk of bitcoin that can be used as a transaction input.

Script is a deliberately constrained stack language whose success depends on final truth and consensus limits.

35 min Difficulty 2/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · script stack traceExecute a Script one opcode at a timeSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered validation pipeline

01
WITNESS<preimage 736563726574>
Initial stack items are supplied by the spending transaction.
02
OP_SHA2562bb80d...a25b
The top byte vector is replaced by its SHA-256 digest.
03
PUSH EXPECTED2bb80d...a25b
The locking program pushes the committed digest.
04
OP_EQUAL1
The two byte vectors are removed and replaced by false or true.
05
RESULTClean true stack
Consensus accepts only the successful execution under the active Script rules.

Rule shown: Script is deterministic: every opcode consumes and produces exact stack items, and a failure stops the input immediately.

STAGE 1 OF 5WITNESS: <preimage 736563726574>

Initial stack items are supplied by the spending transaction.

Governing relationship: Script is deterministic: every opcode consumes and produces exact stack items, and a failure stops the input immediately. Complete path: 1. WITNESS, <preimage 736563726574>. Initial stack items are supplied by the spending transaction. 2. OP_SHA256, 2bb80d...a25b. The top byte vector is replaced by its SHA-256 digest. 3. PUSH EXPECTED, 2bb80d...a25b. The locking program pushes the committed digest. 4. OP_EQUAL, 1. The two byte vectors are removed and replaced by false or true. 5. RESULT, Clean true stack. Consensus accepts only the successful execution under the active Script rules.

1. State transitionWhat exists, what changes, and what results

BEFORE

Initial script tokens are 2 3 OP_ADD 5 OP_EQUAL, with an empty starting stack.

MECHANISM

For modern outputs, witness items are placed on the initial stack in their serialized order under the witness-program rules.

AFTER

A Script explanation is complete only when every stack mutation and failure point can be traced.

Text equivalent: Starting from Initial script tokens are 2 3 OP_ADD 5 OP_EQUAL, with an empty starting stack. The mechanism begins when For modern outputs, witness items are placed on the initial stack in their serialized order under the witness-program rules. The resulting lesson is A Script explanation is complete only when every stack mutation and failure point can be traced.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Load unlocking data

For modern outputs, witness items are placed on the initial stack in their serialized order under the witness-program rules.

Complete path: 1. Load unlocking data: For modern outputs, witness items are placed on the initial stack in their serialized order under the witness-program rules. 2. Execute left to right: Each opcode reads the top stack items, performs its defined operation, and may push a result. Data pushes place byte strings without interpreting them as global variables. 3. Stop on execution errors: Stack underflow, failed VERIFY operations, invalid encodings under applicable rules, and disabled operations terminate the script as invalid. 4. Interpret boolean results: Bitcoin Script treats specified byte encodings as false and other values as true. Equality operations can push a boolean, while VERIFY-style operations fail immediately on false. 5. Require the final condition: The relevant legacy, SegWit, or Tapscript evaluation rules determine final-stack requirements, including clean-stack behavior for witness programs.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

A versioned byte interpreter executes each opcode with exact stack order, Bitcoin CastToBool, resource bounds, and the applicable final or clean-stack rule.

CHANGED CONDITION

Underflow stops at the first opcode; a well-formed false result is separately classified and neither outcome consumes the UTXO.

Important distinction: Bitcoin Script executes once when an output is created. The output commits a condition. The relevant script is evaluated later by validating nodes when a transaction attempts to spend that output.

Text equivalent: The accepted path is enforced by A versioned byte interpreter executes each opcode with exact stack order, Bitcoin CastToBool, resource bounds, and the applicable final or clean-stack rule. If the required condition changes, Underflow stops at the first opcode; a well-formed false result is separately classified and neither outcome consumes the UTXO.

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Learn the idea

Bitcoin Script is a stack-based language for evaluating whether a transaction input satisfies the output it spends. A stack is a last-in, first-out list of byte strings. An opcode is an instruction that pushes, removes, compares, or verifies stack items. The applicable Script rules succeed only when execution finishes without error and with the required true result.

GUIDED EXPLANATION1/5 ideas inspected

Load unlocking data

For modern outputs, witness items are placed on the initial stack in their serialized order under the witness-program rules.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Bitcoin Script executes once when an output is created.”

The output commits a condition. The relevant script is evaluated later by validating nodes when a transaction attempts to spend that output.

FINISH LEARNING FIRST

The questions unlock after every required learning activity

Inspect every guided idea, open the worked example, rebuild its mechanism, and complete the deterministic lesson tool. Your progress is saved automatically.