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app/content/blog/nockmas-2025-day-10.md

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+title = "Nockmas 2025: Day 10"
+date = "2026-01-03"
+description = "12 days of Nockmas: Invoke, opcode 9"
+# aliases = []
+
+[extra]
+# author = ""
+ship = "~lagrev-nocfep"
+image = "https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/day+10/nockmas-day-10-invoke-social.png"
+# imageCard = "https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/Blog_Building+on+Urbit_Social+16x9.png"
+# imageIndex = "https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/Blog_Building+on+Urbit_Tall+Banner.png"
+# imageDark =
+# imageCardDark =
+# imageIndexDark =
+tags =  ["nock", "nockmas", ""]
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+
+![nockmas day 10 invoke image](https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/day+10/nockmas-day-10-invoke-social.png)
+
+On this tenth day of Nockmas, we explore opcode 9, Invoke, continuing the pattern of pinning a core and its arms which we saw in yesterday's discussion. 
+
+# Opcode 9: Invoke
+
+## Syntax
+
+```nock
+*[a 9 b c]          *[*[a c] 2 [0 1] 0 b]
+```
+
+## Explanation
+
+Opcode 9 enables the invocation pattern.  Produce a core (code + data bundle) from formula c, then execute the arm (code) at axis b within that core, with the core itself as subject.
+
+Briefly, a core in Nock/Hoon is a cell `[battery payload]`:
+
+* `battery` (address 2):  one or more formulas (the “arms”) as a noun.
+* `payload` (address 3):  the data upon which the formulas operate.
+
+Opcode 9 is how you call a core's arm.
+
+1. Evaluate `c` against the subject to produce a new core.
+2. Look up the formula at address `b` in the core (the arm).
+3. Evaluate that formula with the core as subject.
+4. Return the result.
+
+(Note, however, that there is nothing intrinsic to Nock about the core, `[battery payload]`.  It's a convenient pattern to repeat and reason over, but Nock itself does not enforce any special semantics on it.  See [Detailed Examples](https://nock.is/content/specification/detailed-examples.html) and [Idioms, Gates, Patterns](https://nock.is/content/examples/idioms-gates-patterns.html) for further discussion.)
+
+While understanding cores is key to understanding [Hoon](https://docs.urbit.org/hoon/why-hoon) and [Jock](https://jock.org), we will defer deeper discussion of cores and arms. Visit [nock.is](https://nock.is) or [docs.urbit.org](https://docs.urbit.org) if you are eager to learn more.
+
+This example subject pins a bit of operational code as a constant for later use:
+
+```nock
+:subject [1 4 1 0]
+```
+
+**Output:**
+```
+Subject set to: [1 4 1 0]
+```
+
+This code retrieves and evaluates it.
+
+```nock
+[9 1 [0 3]]
+```
+
+**Output:**
+```
+1
+```
+
+There are more complicated patterns to build on top of this once you look at cores in detail, but we will leave it here for our initial introduction. 
+
+Join us tomorrow when we cover Nock 10, Edit.
+
+> *12 Days of Nockmas is an exploration of Nock, Urbit's instruction set architecture. This ISA is used by both Urbit and Nockchain, [has interpreters written in many languages](https://docs.urbit.org/nock/implementations), with production versions in both C and Rust. The content of this series is drawn from the [Nock language site](https://nock.is/). Visit the site for interactive code examples and more Nock related content.*