Opcodes

The complete txscript language includes over 100 operation codes, or opcodes, however only a small number of these are used in the vast majority of the transactions on the Decred blockchain.

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Data Push

These opcodes are used to push commonly used constants onto the stack.

Word	Opcode	Hex	Description
OP_FALSE/OP_0	0	0x00	An empty array of bytes is pushed onto the stack.
OP_DATA_1 - OP_DATA_75	1-75	0x01-0x4b	The next 1-75 bytes is data to be pushed onto the stack.
OP_PUSHDATA1	76	0x4c	The next byte contains the number of bytes to be pushed onto the stack.
OP_PUSHDATA2	77	0x4d	The next two bytes contain the number of bytes to be pushed onto the stack in little endian order.
OP_PUSHDATA4	78	0x4e	The next four bytes contain the number of bytes to be pushed onto the stack in little endian order.
OP_1NEGATE	79	0x4f	The number -1 is pushed onto the stack.
OP_TRUE/OP_1	81	0x51	The number 1 is pushed onto the stack.
OP_2 - OP-16	82-96	0x52-0x60	The number 2-16 is pushed onto the stack.

Flow Control

These opcodes enable conditional logic as well as allowing scripts to stop execution.

Word	Opcode	Hex	Description
OP_IF	99	0x63	If the top stack value is not False, the statements are executed. The top stack value is removed.
OP_NOTIF	100	0x64	If the top stack value is False, the statements are executed. The top stack value is removed.
OP_ELSE	103	0x67	If the preceding OP_IF or OP_NOTIF or OP_ELSE was not executed then these statements are executed, and if the preceding OP_IF or OP_NOTIF or OP_ELSE was executed then these statements are not executed.
OP_ENDIF	104	0x68	Ends an if/else block. All blocks must end, or script execution fails. An OP_ENDIF without OP_IF earlier is also invalid.
OP_VERIFY	105	0x69	Script execution fails if top stack value is not true. The top stack value is removed.
OP_RETURN	106	0x6a	Script execution fails if executed.

Stack

These opcodes allow scripts to modify the state of the stack.

Word	Opcode	Hex	Description
OP_TOALTSTACK	107	0x6b	Puts the input onto the top of the alt stack. Removes it from the main stack.
OP_FROMALTSTACK	108	0x6c	Puts the input onto the top of the main stack. Removes it from the alt stack.
OP_2DROP	109	0x6d	Removes the top two stack items.
OP_2DUP	110	0x6e	Duplicates the top two stack items.
OP_3DUP	111	0x6f	Duplicates the top three stack items.
OP_2OVER	112	0x70	Copies the pair of items two spaces back in the stack to the front.
OP_2ROT	113	0x71	The fifth and sixth items back are moved to the top of the stack.
OP_2SWAP	114	0x72	Swaps the top two pairs of items.
OP_IFDUP	115	0x73	If the top stack value is not 0, duplicate it.
OP_DEPTH	116	0x74	Puts the number of stack items onto the stack.
OP_DROP	117	0x75	Removes the top stack item.
OP_DUP	118	0x76	Duplicates the top stack item.
OP_NIP	119	0x77	Removes the second-to-top stack item.
OP_OVER	120	0x78	Copies the second-to-top stack item to the top.
OP_PICK	121	0x79	The item n back in the stack is copied to the top.
OP_ROLL	122	0x7a	The item n back in the stack is moved to the top.
OP_ROT	123	0x7b	The top three items on the stack are rotated to the left.
OP_SWAP	124	0x7c	The top two items on the stack are swapped.
OP_TUCK	125	0x7d	The item at the top of the stack is copied and inserted before the second-to-top item.

Splice

These opcodes allow a script to perform a limited set of string manipulations.

Word	Opcode	Hex	Description
OP_CAT	126	0x7e	Concatenates two strings.
OP_SUBSTR	127	0x7f	Returns a section of a string.
OP_LEFT	128	0x80	Keeps only characters left of the specified point in a string.
OP_RIGHT	129	0x81	Keeps only characters right of the specified point in a string.
OP_SIZE	130	0x82	Pushes the string length of the top element of the stack (without popping it).

Bitwise Logic

These opcodes allow scripts to manipulate data by performing bit-level logic operations.

Inputs are limited to signed 32-bit integers.

Word	Opcode	Hex	Description
OP_INVERT	131	0x83	Flips all of the bits in the input.
OP_AND	132	0x84	Boolean and between each bit in the inputs.
OP_OR	133	0x85	Boolean or between each bit in the inputs.
OP_XOR	134	0x86	Boolean exclusive or between each bit in the inputs.

Bytewise Comparison

These opcodes compare the top two items on the stack as raw bytes.

Word	Opcode	Hex	Description
OP_EQUAL	135	0x87	Returns 1 if the inputs are exactly equal, 0 otherwise.
OP_EQUALVERIFY	136	0x88	Same as OP_EQUAL, but runs OP_VERIFY afterward.

Bitwise Rotation

These opcodes enable circular shifting of bits. They treat the top two items on the stack as 32-bit integers, the first respresents the number of bits to rotate to the left or right, and the second is the value to be rotated.

Word	Opcode	Hex	Description
OP_ROTR	137	0x89	Shift bits right, sign is not preserved.
OP_ROTL	138	0x8a	Shift bits left, sign is not preserved.

Arithmetic

These opcodes implement common arithmetic operations to be used on integer values.

Arithmetic inputs are limited to signed 32-bit integers, but may overflow their output. For example, it is possible for OP_ADD to have 2^31 - 1 for its two operands resulting 2^32 - 2, which overflows, but is still pushed to the stack as the result of the addition.

Word	Opcode	Hex	Description
OP_1ADD	139	0x8b	1 is added to the input.
OP_1SUB	140	0x8c	1 is subtracted from the input.
OP_2MUL	141	0x8d	The input is multiplied by 2.
OP_2DIV	142	0x8e	The input is divided by 2.
OP_NEGATE	143	0x8f	The sign of the input is flipped.
OP_ABS	144	0x90	The input is made positive.
OP_NOT	145	0x91	If the input is 0 or 1, it is flipped. Otherwise the output will be 0.
OP_0NOTEQUAL	146	0x92	Returns 0 if the input is 0. 1 otherwise.
OP_ADD	147	0x93	Performs addition on the inputs.
OP_SUB	148	0x94	Performs subtraction on the inputs.
OP_MUL	149	0x95	Performs multiplication on the inputs.
OP_DIV	150	0x96	Performs division on the inputs.
OP_MOD	151	0x97	Performs modulo on the inputs.
OP_LSHIFT	152	0x98	Shifts bits left. Zeros are shifted in on the right. Does not preserve sign.
OP_RSHIFT	153	0x99	Shifts bits right. Uses sign extension to preserve sign.
OP_BOOLAND	154	0x9a	If both inputs are not 0, the output is 1. Otherwise 0.
OP_BOOLOR	155	0x9b	If either input is not 0, the output is 1. Otherwise 0.
OP_NUMEQUAL	156	0x9c	Returns 1 if the input numbers are equal, 0 otherwise.
OP_NUMEQUALVERIFY	157	0x9d	Same as OP_NUMEQUAL, but runs OP_VERIFY afterward.
OP_NUMNOTEQUAL	158	0x9e	Returns 1 if the input numbers are not equal, 0 otherwise.
OP_LESSTHAN	159	0x9f	Returns 1 if first input is less than second input, 0 otherwise.
OP_GREATERTHAN	160	0xa0	Returns 1 if first input is greater than second input, 0 otherwise.
OP_LESSTHANOREQUAL	161	0xa1	Returns 1 if first input is less than or equal to second input, 0 otherwise.
OP_GREATERTHANOREQUAL	162	0xa2	Returns 1 if first input is greater than or equal to second input, 0 otherwise.
OP_MIN	163	0xa3	Returns the smaller of two inputs.
OP_MAX	164	0xa4	Returns the larger of two inputs.
OP_WITHIN	165	0xa5	Returns 1 if x is within the specified range (left-inclusive), 0 otherwise.

Cryptography

These opcodes implement cryptographic primitives commonly used in transaction scripts.

The OP_SHA256 opcode is activated by DCP-0002. On mainnet, this agenda was activated on block 189,568 (November 2017). Prior to this agenda activation, OP_SHA256 is treated as OP_UNKNOWN192.

Note that OP_CODESEPARATOR is disabled. Any script including OP_CODESEPARATOR will abort and fail, even if it is within an unexecuted OP_IF branch.

Word	Opcode	Hex	Description
OP_RIPEMD160	166	0xa6	The input is hashed using RIPEMD-160.
OP_SHA1	167	0xa7	The input is hashed using SHA-1.
OP_BLAKE256	168	0xa8	The input is hashed using BLAKE-256.
OP_HASH160	169	0xa9	The input is hashed twice: first with BLAKE-256 and then with RIPEMD-160.
OP_HASH256	170	0xaa	The input is hashed two times with BLAKE-256.
OP_CODESEPARATOR	171	0xab	All of the signature checking words will only match signatures to the data after the most recently-executed OP_CODESEPARATOR. Disabled.
OP_CHECKSIG	172	0xac	Consumes a signature and a full public key, and pushes 1 onto the stack if the transaction data specified by the SIGHASH flag was converted into the signature using the same private key that generated the public key for the given signature type. Otherwise, it pushes 0 onto the stack.
OP_CHECKSIGVERIFY	173	0xad	Same as OP_CHECKSIG, but OP_VERIFY is executed afterward.
OP_CHECKMULTISIG	174	0xae	Compares the first signature against each public key until it finds an ECDSA match. Starting with the subsequent public key, it compares the second signature against each remaining public key until it finds an ECDSA match. The process is repeated until all signatures have been checked or not enough public keys remain to produce a successful result. All signatures need to match a public key. If all signatures are valid, 1 is returned, 0 otherwise.
OP_CHECKMULTISIGVERIFY	175	0xaf	Same as OP_CHECKMULTISIG, but OP_VERIFY is executed afterward.
OP_CHECKSIGALT	190	0xbe	Consumes a signature type, signature and a full public key, and pushes 1 onto the stack if the transaction data specified by the SIGHASH flag was converted into the signature using the same ECDSA private key that generated the public key. Otherwise, it pushes 0 onto the stack.
OP_CHECKSIGALTVERIFY	191	0xbf	Same as OP_CHECKSIGALT, but OP_VERIFY is executed afterward.
OP_SHA256	192	0xc0	The input is hashed using SHA-256. Previously OP_UNKNOWN192.

Lock time

These opcodes are activated by DCP-0003. On mainnet, this agenda was activated on block 189,568 (November 2017). Prior to this agenda activation, these opcodes are treated as OP_NOP2 and OP_NOP3 respectively.

Word	Opcode	Hex	Description
OP_CHECKLOCKTIMEVERIFY	177	0xb1	Script execution fails if the top stack item is greater than the transaction’s nLockTime field, otherwise script evaluation continues as though an OP_NOP was executed. Previously OP_NOP2.
OP_CHECKSEQUENCEVERIFY	178	0xb2	Script execution fails if the relative lock time of the input is not equal to or longer than the value of the top stack item. Previously OP_NOP3.

Proof-of-Stake

These opcodes are used to create Proof-of-Stake transactions.

They are all considered NOPs within the scripting system. Their semantics are enforced outside of the scripting system by the staking consensus rules.

Word	Opcode	Hex	Description
OP_SSTX	186	0xba	Tags an output as a ticket purchase.
OP_SSGEN	187	0xbb	Tags an output as a ticket vote.
OP_SSRTX	188	0xbc	Tags an output as a ticket revocation.
OP_SSTXCHANGE	189	0xbd	Tags an output as ticket purchase change.

No op

As the name implies, these opcodes do nothing, they are reserved for future use. The txscript engine can be passed a flag which will cause usage of these opcodes to return an error.

Following the activation of DCP-0002, OP_UNKNOWN192 is redefined as OP_SHA256.

Following the activation of DCP-0003, OP_NOP2 and OP_NOP3 are redefined as OP_CHECKLOCKTIMEVERIFY and OP_CHECKSEQUENCEVERIFY respectively.

Word	Opcode	Hex	Description
OP_NOP	97	0x61	Does nothing. Does not cause script execution to fail.
OP_NOP1	176	0xb0	Does nothing. Does not cause script execution to fail.
OP_NOP4 - OP_NOP10	179 - 185	0xb3 - 0xb9	Does nothing. Does not cause script execution to fail.
OP_UNKNOWN193 - OP_UNKNOWN248	193 - 248	0xc1 - 0xf8	Does nothing. Does not cause script execution to fail.

Reserved

Word	Opcode	Hex	Description
OP_RESERVED	80	0x50	Script execution fails unless occuring in an unexecuted OP_IF branch.
OP_VER	98	0x62	Script execution fails unless occuring in an unexecuted OP_IF branch.
OP_VERIF	101	0x65	Script execution fails even when occuring in an unexecuted OP_IF branch.
OP_VERNOTIF	102	0x66	Script execution fails even when occuring in an unexecuted OP_IF branch.