Opcode Explained

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OP_TOALTSTACK ​

INFO

Opcode number: 107
Byte representation: 0x6b
Short description: Move the top item from the main stack to the alternate stack.

OP_TOALTSTACK is used to manipulate stack items between the main stack and an alternate stack.

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Operation ​

  1. Pop the top item from the main stack.
  2. Push this item onto the alternate stack.

The alternate stack is a secondary stack that scripts can use to store data temporarily. It operates independently of the main stack, allowing for more complex script operations.

Use Cases ​

  • Temporary Storage: Scripts can use OP_TOALTSTACK to temporarily store data that might be needed later in the script execution. This helps in organizing data and maintaining a clear main stack.
  • Complex Script Logic: In scripts that require intricate logic and multiple steps, OP_TOALTSTACK allows for better management of stack items and cleaner script execution.

Notes ​

  • The alternate stack is not visible in script execution unless accessed explicitly.
  • OP_TOALTSTACK is often used in conjunction with OP_FROMALTSTACK to move items back to the main stack when needed.

Examples ​

Example 1 ​

A script that requires temporarily setting aside a value:

shell
# ASM script
OP_1 OP_TOALTSTACK

# Raw script
516b

# Main stack
(empty)

# Alternate stack
1

Example 2 ​

A multi-step verification script where intermediate values are stored:

shell
# ASM script
<value> OP_TOALTSTACK <other operations> <value retrieval from alternate stack>