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Update 7_2_Running_a_Bitcoin_Script.md

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7_2_Running_a_Bitcoin_Script.md
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@ -10,11 +10,11 @@ A Bitcoin Script has three parts: it has a line of input; it has a stack for sto
### Understand the Ordering
Bitcoin Scripts are run from left to right. That sounds easy enough, because it's the same way you read. However, it might actually be the most non-intuitive element of Bitcoin Script, because it means that functions don't look as you'd expect. Instead, _the operands go before the operator._
Bitcoin Scripts are run from left to right. That sounds easy enough, because it's the same way you read. However, it might actually be the most non-intuitive element of Bitcoin Script, because it means that functions don't look like you'd expect. Instead, _the operands go before the operator._
For example, if you were adding together "1" and "2", your Bitcoin Script for that would be `1 2 OP_ADD`, _not_ "1 + 2". Since we know that OP_ADD operator takes two inputs, we know that the two inputs before it are its operands. (At least that's true at the most basic level of Scripting.)
For example, if you were adding together "1" and "2", your Bitcoin Script for that would be `1 2 OP_ADD`, _not_ "1 + 2". Since we know that OP_ADD operator takes two inputs, we know that the two inputs before it are its operands.
> **WARNING:** Technically, everything in Bitcoin Script is an opcode, thus it would be most appropriate to record the above example as `OP_1 OP_2 OP_ADD`. We leave the "OP" prefix off constants, but include it on all actual operators. Some writers prefer to also leave the "OP" prefix off operators, but we have opted not to.
> **WARNING:** Technically, everything in Bitcoin Script is an opcode, thus it would be most appropriate to record the above example as `OP_1 OP_2 OP_ADD`. In our examples, we don't worry about how the constants will be evaluated, as that's a topic of translation, as is explained in [§8.2: Building the Structure of P2SH](8_2_Building_the_Structure_of_P2SH.md). Some writers prefer to also leave the "OP" prefix off operators, but we have opted not to.
### Understand the Stack
@ -22,7 +22,7 @@ It's actually not quite correct to say that an operator applies to the inputs be
_What is a stack?_ A stack is a LIFO (last-in-first-out) data structure. It has two access functions: push and pop. Push places a new object on top of the stack, pushing down everything below it. Pop removes the top object from the stack.
Whenever Bitcoin Script encounters a constant, it pushes it on the stack. So the above example of `1 2 OP_ADD1` would actually look like this as it was processed:
Whenever Bitcoin Script encounters a constant, it pushes it on the stack. So the above example of `1 2 OP_ADD` would actually look like this as it was processed:
```
Script: 1 2 OP_ADD
Stack: [ ]
@ -70,7 +70,7 @@ Stack: [ 1 ]
## Understand the Usage of Bitcoin Script
That's pretty much Bitcoin Scripting ... other than a few intricacies for how this Scripting language interacts with Bitcoin itself.
That's pretty much Bitcoin Scripting ... other than a few intricacies for how this scripting language interacts with Bitcoin itself.
### Understand scriptSig and scriptPubKey
@ -96,11 +96,11 @@ Script: OP_EQUAL
Stack: [ 99 99 ]
Script:
Stack: [ TRUE ]
Stack: [ True ]
```
This abstraction isn't quite accurate: for security reasons, the `scriptSig` is run, then the contents of the stack are transferred for the `scriptPubKey` to run, but it's accurate enough for understanding how the key of `scriptSig` fits into the lock of `scriptPubKey`.
> **WARNING** The above is a non-standard transaction type. It would not actually be accepted by nodes running Bitcoin Core with the standard settings. [§8.1: Building a Bitcoin Script with P2SH](8_1_Building_a_Bitcoin_Script_with_P2SH.md) discusses how you actually _could_ use a Bitcoin Script like this, using the power of P2SH.
> **WARNING** The above is a non-standard transaction type. It would not actually be accepted by nodes running Bitcoin Core with the standard settings. [§8.1: Building a Bitcoin Script with P2SH](8_1_Building_a_Bitcoin_Script_with_P2SH.md) discusses how you actually _could_ use a Bitcoin Script like this, via the power of P2SH.
### Get the Results
@ -109,7 +109,7 @@ Bitcoin will verify a transaction and allow the UTXO to be respent if two criter
1. The execution did not get marked as invalid at any point, for example with a failed OP_VERIFY or the usage of a disabled opcode.
2. The top item in the stack at the end of execution is true (non-zero).
In the above example, the transaction would succeed because the stack has a `TRUE` at its top. But, it would be just as permissible to end with a full stack and the number `42` on top.
In the above example, the transaction would succeed because the stack has a `True` at its top. But, it would be just as permissible to end with a full stack and the number `42` on top.
## Summary: Running a Bitcoin Script