What is the alt stack?

Here's where they're defined:

            case OP_TOALTSTACK:
            {
                if (stack.size() < 1)
                    return false;
                altstack.push_back(stacktop(-1));
                popstack(stack);
            }
            break;

            case OP_FROMALTSTACK:
            {
                if (altstack.size() < 1)
                    return false;
                stack.push_back(altstacktop(-1));
                popstack(altstack);
            }
            break;

Example:

Stack:
Alternate Stack:
Script: OP_1 OP_2 OP_TOALTSTACK

Stack: 01
Alternate Stack:
Script: OP_2 OP_TOALTSTACK

Stack: 01 02
Alternate Stack:
Script: OP_TOALTSTACK

Stack: 01
Alternate Stack: 02
Script:

what is it for? Do any specific use cases come to mind?

It's... use-impaired. 99% of the time, you can avoid using OP_(TO|FROM)ALTSTACK by putting things onto the stack in a different order. There are 18 stack manipulation operators, but only one (OP_DUP) is used with any regularity. I'm of the opinion that Bitcoin scripting is overcomplicated for its intended use.

Nick ODell

Posted 2014-10-26T12:45:25.607

Reputation: 26 536

+1, but in stack-based languages, whether or not necessary, it is often convenient to have access to a second stack. For example, in Forth, it is a common idiom to (ab)use the return stack (>R, R>) for storing temporary data. See http://www.forth.com/starting-forth/sf5/sf5.html for an example. The designers may have had this in mind. Anyway, given the difficulty (chain fork) of adding additional features to the scripting language, one can understand that the designers had an incentive to overdesign.

– Nate Eldredge – 2014-10-26T16:08:00.857

@Nick, I see where you're coming from when there are really only a few types of standard transactions, but some think the Bitcoin scripting language is too simple for its intended use. e.g. Ethereum. I think Satoshi found a good balance with the Bitcoin scripting language, making it flexible but also statically analyzable. – morsecoder – 2014-10-27T17:28:07.617

What is the alt stack?

Answers