Opcode Explained

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OP_TUCK ​

INFO

Opcode number: 125
Byte representation: 0x7d
Short description: Copy the top item and insert it below the second item.

OP_TUCK is used to duplicate the top item on the stack and insert the duplicate below the second item. This effectively "tucks" the top item into the third position while leaving a copy of it on the top.

Operation ​

  1. Copy the top item on the stack.
  2. Insert the copied item below the second item (i.e., into the third position).

Notes ​

  • OP_TUCK is useful when you need to use the top stack item twice, both at the top and immediately below the second item.
  • If there are fewer than two items on the stack when OP_TUCK is executed, the script will fail.

Example ​

Tucking the top item into the second position:

shell
# ASM script
OP_1 OP_2 OP_3 OP_TUCK

# Raw script
5152537d

# Stack (before OP_TUCK)
3
2
1

# Stack (after OP_TUCK)
3
2
3 # tucked below the 2nd item
1