# Script
Bitcoin Cash transactions make use of a scripting language to authorize and secure transfers.
While, colloquially, there is a tendency to refer to transactions as "sending" Bitcoin Cash to "an address", that is merely an abstraction.
In fact, the only thing that permits the spending of existing [UTXOs](/protocol/blockchain/transaction#transaction-output) is the successful execution of a script.
The only thing preventing the spending of newly created UTXOs is the difficulty of producing a successfully executing script.
Through the use of cryptographic signatures and hash functions, such scripts are often designed specifically to be difficult to produce unless you are the intended spender of a given UTXO, though that need not necessarily be the case.
For more information on how Transactions are commonly secured, see [Locking Script](/protocol/blockchain/transaction/locking-script).
This page will focus on how the scripts are run, what they are capable of, and what limitations they have.
## Script Execution
Scripts are executed using a stack-based memory model and have an intentionally restricted set of available operations.
Unlike the common general-purpose programming languages your are probably aware of, Script (the term for the language itself) does not allow for loops, persistent state/memory across script executions, or the definition of functions.
Instead, scripts are expected to contain whatever data they need and use the available operations to prove transaction validity.
### Features
In addition to the primary stack ("the stack"), there is a secondary stack, referred to as the "alt-stack", which data can be moved to temporarily.
Any data left on the alt-stack is lost when a given sub-script finishes execution.
In effect, any data moved to the alt-stack by an unlocking script is not present when the locking script runs.
There are a large number of op-codes that support everything from simple stack-manipulation, to mathematical calculations, to complex cryptographic processes. In terms of control structures there are only basic conditional branching (IF/ELSE) operations available.
### Transaction Validation
Scripts are run when validating transactions, and successful execution of all of the scripts defined by the transaction is a necessary, but not sufficient, condition for transaction validity.
See [Transaction Validation](/protocol/blockchain/transaction-validation) for more details.
As a part of validating a transaction, a script is built for each input spent by the transaction.
Each script is the sequential execution (carrying over the same stack, but not alt-stack) of the [unlocking script](/protocol/blockchain/transaction/unlocking-script) provided with the input definition (which is used that the beginning of the script) and the locking script provided by the [previous output](/protocol/blockchain/transaction#transaction-output) being referenced.
The exception to this is [pay to script hash](/protocol/blockchain/transaction/locking-script#standard-scripts), which has an altered execution workflow.
In general, though, this combined unlocking/locking script is then executed and considered successful if and only if the following conditions are met:
- **Non-Zero Value** - after execution the top of the stack must contain a non-zero (TRUE) value.
- **No Stack Overflows** - no operation should attempt to pop a value from the stack when the stack is empty. An overflow of the alt-stack is also disallowed.
- **Clean Stack** - after execution the stack must only contain a single value, which must be non-zero (TRUE). Added in [HF-20181115](/protocol/forks/hf-20181115). The alt-stack is exempt from this.
Additionally, in order for the combined script to be valid, the following must be true:
- **Non-Empty Scripts** - both the locking and unlocking scripts must be non-empty.
- **Max Script Length** - the locking and unlocking script must each be less than the max script length of 10,000 bytes (for a combined script maximum of 20,000 bytes).
- **Contained Control Flow** - an IF/ELSE block cannot start in the unlocking script and end in the locking script, the script must be in the top-level scope when the locking script execution begins.
- **Permitted Operations Only** - the locking script must not include operations that are disallowed and must not execute operations that are disabled..
- **Push Only** - the unlocking script must contain only push operations (i.e. those with op codes 0x60 or less). Added in [HF-20181115](/protocol/forks/hf-20181115).
## Op Codes
The table below lists the currently allocated op codes. Op codes marked with **(ignored)** are permitted but will do nothing when executed.
Op codes marked with **(disabled)** are permitted in scripts so long as they are not executed.
Op codes marked with **(do not use)** are disallowed and will make a transaction invalid merely be being present.
| Op Code Range | Name |
|--|--|
| 0x00 | [OP_0](/protocol/blockchain/script/op-codes/op-x), OP_FALSE |
| 0x01 - 0x4B | [OP_DATA_X](/protocol/blockchain/script/op-codes/op-data-x) |
| 0x4C | OP_PUSHDATA1 |
| 0x4D | OP_PUSHDATA2 |
| 0x4E | OP_PUSHDATA4 |
| 0x4F | OP_1NEGATE |
| 0x50 | OP_RESERVED **(disabled)** |
| 0x51 - 0x60 | [OP_1](/protocol/blockchain/script/op-codes/op-x) - OP_16, OP_TRUE |
| 0x61 | OP_NOP |
| 0x62 | OP_VER **(disabled)** |
| 0x63 | OP_IF |
| 0x64 | OP_NOTIF |
| 0x65 | OP_VERIF **(do not use)** |
| 0x66 | OP_VERNOTIF **(do not use)** |
| 0x67 | OP_ELSE |
| 0x68 | OP_ENDIF |
| 0x69 | OP_VERIFY |
| 0x6A | OP_RETURN |
| 0x6B | OP_TOALTSTACK |
| 0x6C | OP_FROMALTSTACK |
| 0x6D | OP_2DROP |
| 0x6E | OP_2DUP |
| 0x6F | OP_3DUP |
| 0x70 | OP_2OVER |
| 0x71 | OP_2ROT |
| 0x72 | OP_2SWAP |
| 0x73 | OP_IFDUP |
| 0x74 | OP_DEPTH |
| 0x75 | OP_DROP |
| 0x76 | OP_DUP |
| 0x77 | OP_NIP |
| 0x78 | OP_OVER |
| 0x79 | OP_PICK |
| 0x7A | OP_ROLL |
| 0x7B | OP_ROT |
| 0x7C | OP_SWAP |
| 0x7D | OP_TUCK |
| 0x7E | OP_CAT |
| 0x7F | OP_SUBSTR |
| 0x80 | OP_LEFT |
| 0x81 | OP_RIGHT |
| 0x82 | OP_SIZE |
| 0x83 | OP_INVERT |
| 0x84 | OP_AND |
| 0x85 | OP_OR |
| 0x86 | OP_XOR |
| 0x87 | OP_EQUAL |
| 0x88 | OP_EQUALVERIFY |
| 0x89 | OP_RESERVED1 **(do not use)** |
| 0x8A | OP_RESERVED2 **(do not use)** |
| 0x8B | OP_1ADD |
| 0x8C | OP_1SUB |
| 0x8D | OP_2MUL |
| 0x8E | OP_2DIV |
| 0x8F | OP_NEGATE |
| 0x90 | OP_ABS |
| 0x91 | OP_NOT |
| 0x92 | OP_0NOTEQUAL |
| 0x93 | OP_ADD |
| 0x94 | OP_SUB |
| 0x95 | OP_MUL |
| 0x96 | OP_DIV |
| 0x97 | OP_MOD |
| 0x98 | OP_LSHIFT |
| 0x99 | OP_RSHIFT |
| 0x9A | OP_BOOLAND |
| 0x9B | OP_BOOLOR |
| 0x9C | OP_NUMEQUAL |
| 0x9D | OP_NUMEQUALVERIFY |
| 0x9E | OP_NUMNOTEQUAL |
| 0x9F | OP_LESSTHAN |
| 0xA0 | OP_GREATERTHAN |
| 0xA1 | OP_LESSTHANOREQUAL |
| 0xA2 | OP_GREATERTHANOREQUAL |
| 0xA3 | OP_MIN |
| 0xA4 | OP_MAX |
| 0xA5 | OP_WITHIN |
| 0xA6 | OP_RIPEMD160 |
| 0xA7 | OP_SHA1 |
| 0xA8 | OP_SHA256 |
| 0xA9 | OP_HASH160 |
| 0xAA | OP_HASH256 |
| 0xAB | OP_CODESEPARATOR |
| 0xAC | OP_CHECKSIG |
| 0xAD | OP_CHECKSIGVERIFY |
| 0xAE | OP_CHECKMULTISIG |
| 0xAF | OP_CHECKMULTISIGVERIFY |
| 0xB0 | OP_NOP1 **(ignored)** |
| 0xB1 | OP_CHECKLOCKTIMEVERIFY |
| 0xB2 | OP_CHECKSEQUENCEVERIFY |
| 0xB3 | OP_NOP4 **(ignored)** |
| 0xB4 | OP_NOP5 **(ignored)** |
| 0xB5 | OP_NOP6 **(ignored)** |
| 0xB6 | OP_NOP7 **(ignored)** |
| 0xB7 | OP_NOP8 **(ignored)** |
| 0xB8 | OP_NOP9 **(ignored)** |
| 0xB9 | OP_NOP10 **(ignored)** |
| 0xBA | OP_CHECKDATASIG |
| 0xBB | OP_CHECKDATASIGVERIFY |
| 0xBC - 0xFF | Unused **(disabled)** |
### Node-Specific Behavior
Some node implementations define custom op codes.
#### bchd
| Op Code Range | Name |
|--|--|
| 0xFA | OP_SMALLINTEGER 
|
| 0xFB | OP_PUBKEYS |
| 0xFD | OP_PUBKEYHASH |
| 0xFE | OP_PUBKEY |
| 0xFF | OP_INVALIDOPCODE |
#### Bitcoin ABC
| Op Code Range | Name |
|--|--|
| 0xF0 | OP_PREFIX_BEGIN |
| 0xF7 | OP_PREFIX_END |
| 0xFF | OP_INVALIDOPCODE |
#### Bitcoin Unlimited
| Op Code Range | Name |
|--|--|
| 0xF0 | OP_BIGINTEGER |
| 0xF1 | OP_DATA |
| 0xFA | OP_SMALLINTEGER |
| 0xFB | OP_PUBKEYS |
| 0xFD | OP_PUBKEYHASH |
| 0xFE | OP_PUBKEY |
| 0xFF | OP_INVALIDOPCODE |
#### Flowee the Hub
| Op Code Range | Name |
|--|--|
| 0xFA | OP_SMALLINTEGER |
| 0xFB | OP_PUBKEYS |
| 0xFD | OP_PUBKEYHASH |
| 0xFE | OP_PUBKEY |
| 0xFF | OP_INVALIDOPCODE |