Opcode Explained

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OP_LESSTHANOREQUAL ​

INFO

Opcode number: 161
Byte representation: 0xa1
Short description: Pop the top two items; push 1 if the second is less than or equal to the top, 0 otherwise.

OP_LESSTHANOREQUAL compares the top two items on the stack as integers. If the second item is less than or equal to the top item, it pushes 1 (true) onto the stack. If not, it pushes an empty array (false). Both items are removed from the stack after the comparison.

Operation ​

  1. Take the top item (a) and the second item (b) from the stack.
  2. Compare the two items:
    • If b <= a, push 1 (true) onto the stack.
    • Otherwise, push an empty array (false) onto the stack.
  3. Remove the original items (a and b) from the stack.

Notes ​

  • Both items must be valid integers. Bitcoin Script interprets byte arrays up to 4 bytes as integers.
  • An empty array ([]) is treated as 0 when compared.
  • If there are fewer than two items on the stack when OP_GREATERTHAN is executed, the script will fail.

Examples ​

Example 1: Second item is less than the top item ​

shell
# ASM script
OP_2 OP_3 OP_LESSTHANOREQUAL

# Raw script
5253a2

# Stack (before OP_LESSTHANOREQUAL)
3  # top
2

# Stack (after OP_LESSTHANOREQUAL)
1  # true, as 2 <= 3

Example 2: Second item is equal to the top item ​

shell
# ASM script
OP_2 OP_2 OP_LESSTHANOREQUAL

# Raw script
5252a2

# Stack (before OP_LESSTHANOREQUAL)
2  # top
2

# Stack (after OP_LESSTHANOREQUAL)
1  # true, as 2 <= 2