Common Bitcoin Script opcodes explained

Opcodes are Bitcoin Script's instructions: duplicate, hash, compare, check signatures and times. A clear reference to the opcodes you will see most.

The everyday set

Opcode	Byte	What it does
`OP_DUP`	0x76	Copies the top item
`OP_HASH160`	0xa9	Replaces the top item with its HASH160
`OP_EQUAL`	0x87	Pushes TRUE if the top two items are equal
`OP_EQUALVERIFY`	0x88	Same, but fails immediately if they're not
`OP_CHECKSIG`	0xac	Checks a signature against a public key and the transaction
`OP_CHECKMULTISIG`	0xae	Checks m signatures against n public keys
`OP_CHECKLOCKTIMEVERIFY`	0xb1	Fails if the transaction's locktime is too early
`OP_CHECKSEQUENCEVERIFY`	0xb2	Fails if the relative timelock isn't met
`OP_RETURN`	0x6a	Marks the output as unspendable, used for data
`OP_0` to `OP_16`	0x00, 0x51–0x60	Push small numbers

"VERIFY" versions

Two opcodes in action: OP_DUP copies the top item, OP_HASH160 hashes the copy

Opcodes ending in VERIFY check something and stop the script with failure if it's false, instead of pushing FALSE. That keeps scripts short.

Disabled opcodes

Early on, Satoshi disabled opcodes such as OP_CAT and OP_MUL after bugs were found. Any script using a disabled opcode fails. Re-enabling some of them is a regular topic of Bitcoin proposals, which would need a soft fork.

Reading a real script

OP_DUP OP_HASH160 <20-byte hash> OP_EQUALVERIFY OP_CHECKSIG
76     a9         14 <…20 bytes…>   88             ac

That's the classic Pay to public key hash (P2PKH) lock. The 14 means "push the next 20 bytes".

Opcodes added by soft forks

Opcode	Added	Purpose
OP_CHECKLOCKTIMEVERIFY	2015, BIP 65	Absolute timelocks
OP_CHECKSEQUENCEVERIFY	2016, BIP 112	Relative timelocks
OP_CHECKSIGADD	2021, BIP 342	Efficient multisig in Tapscript

New opcodes are added by redefining "no-op" opcodes, so old nodes still accept the scripts, which keeps upgrades as soft forks.

OP_SUCCESS in Tapscript

Taproot reserved many unused opcode numbers as OP_SUCCESSx, which make a script succeed today. Future soft forks can give them real meanings safely.

Key takeaways

Opcodes are single-byte instructions in Script.
VERIFY variants fail immediately instead of pushing false.
Some opcodes were disabled early for safety.

Common questions

What does OP_CHECKSIG actually sign?

A hash of the spending transaction, built according to the signature's sighash flag.

Are there opcodes for loops?

No. Script has conditionals (OP_IF) but no loops.

How many opcodes does Bitcoin have?

Around 100 are defined and usable; several more are disabled or reserved.

Can new opcodes be added?

Yes, through soft forks that redefine reserved opcodes, as with CLTV and CSV.