Opcode Explained

Appearance

On this page

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.

WARNING

This page is not well developed. If you think you can contribute to this page, please consider helping us out by opening a pull request on the repo!

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:

shell
# 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:

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