OP_FROMALTSTACK

INFO

Opcode number: 108
Byte representation: 0x6c
Short description: Moves the top item from the alternate stack to the main stack.

OP_FROMALTSTACK is used for transferring data between the alternate stack and the main stack.

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Operation

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

The alternate stack serves as a secondary storage space during script execution, separate from the main stack. This opcode enables the movement of data from this auxiliary stack back to the main stack.

Use Cases

• Retrieving Data: Scripts can use OP_FROMALTSTACK to retrieve data previously stored on the alternate stack, allowing for more intricate scripting logic.
• Script Organization: By using both the main and alternate stacks, scripts can maintain cleaner and more organized operations, especially in complex scripts.

Notes

• The alternate stack is an essential tool for complex script logic, enabling temporary storage and retrieval of data.
• OP_FROMALTSTACK is commonly used in conjunction with OP_TOALTSTACK for effective data manipulation between the two stacks.

Examples

Example 1

A script that stores a value temporarily and retrieves it later:

# ASM script
OP_1 OP_TOALTSTACK ... OP_FROMALTSTACK

# Raw script
516b ... 6c

# Main stack (start)
1

# Main stack (intermediate)
(empty)

# Alternate stack (intermediate)
1

# Alternate stack (final)
(empty)

# Main stack (final)
1

Example 2

A multi-step script that uses the alternate stack for intermediate calculations:

# ASM script
<value> OP_TOALTSTACK <operations> OP_FROMALTSTACK <further operations>