Ethereum: What is an alt stack?

Understanding Ethereum’s Alternative Stack: OP_TOALTSTACK and OP_FROMALTSTACK Explained

In the world of cryptocurrency, it’s crucial for developers, investors, and anyone interested in the underlying technology to understand the inner workings of different blockchain platforms. We were recently browsing the Bitcoin Wiki’s Scripts page ( where we came across two opcodes that piqued our curiosity: OP_TOALTSTACK and OP_FROMALTSTACK. In this article, we’ll delve deeper into these mysterious opcodes and their meaning in the Ethereum altcoin.

OP_TOALTSTACK

The first opcode, OP_TOALTSTACK, is often referred to as the “alt-stack” opcode. This opcode is used when a script attempts to access or manipulate data from an alt-chain (a separate blockchain that operates independently of the main Bitcoin network). When an alt-chain script tries to call a function with an argument that is not in local memory, it must use the OP_TOALTSTACK operation code.

Here is a simple example:

0x40 < Bitcoin Script > // Define a new script (Bitcoin)
0x28 < uint256("myAltChain") // Declare a variable "myAltChain" with the value 123
| 0x20 < callscript(0x40, 0x30) // Call the function "myFunction" in the Bitcoin script
/ 0x8 < OP_TOALTSTACK // To access the data of the altchain, use the opcode OP_TOALTSTACK
/ 0x90 < uint256("myAltChain") // Call a function with an argument (myAltChain)
/ 0x88 < return op < 0x1F // Return the value of myAltChain

In this example, the script is trying to call a function named “myFunction” in the Bitcoin script. Since we declared a variable “myAltChain” with a value of 123 earlier in the script, we can use it as an argument when calling the function.

OP_FROMALTSTACK

The second opcode, OP_FROMALTSTACK, is used when a script needs to access data from an alternative stack that was previously stored in its local memory. This opcode is most commonly used when interacting with scripts on different chains using smart contracts or other mechanisms.

Here is another example:

0x40 < Bitcoin Script > // Define a new script (Bitcoin)
0x28 < uint256("myAltChain") // Declare a variable "myAltChain" with the value 123
| 0x20 < callscript(0x40, 0x30) // Call the function "myFunction" in the Bitcoin script
/ 0x8 < OP_FROMALTSTACK // To access data from an alternative chain, use the OP_FROMALTSTACK opcode
/ 0x90 < uint256("myAltChain") // Call a function with an argument (myAltChain)
/ 0x88 < return op < 0x1F // Return the value of myAltChain

In this example, the script writes the value of “myAltChain” to its local memory and then uses it to call another function. The OP_FROMALTSTACK opcode is used to access stored data.

Conclusion

The OP_TOALTSTACK and OP_FROMALTSTACK operation codes are a core part of Ethereum’s alternative stack, allowing scripts on different chains to interact with each other using smart contracts and other mechanisms. Understanding these transaction codes can help developers and investors appreciate the complexity and potential for interoperability between different blockchain platforms.

In summary, by mastering the use of OP_TOALTSTACK and OP_FROMALTSTACK, users can open up new possibilities for building decentralized applications (dApps) that seamlessly interact with scripts on different alternative networks.

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