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+title = "Nockmas 2025: Day 9"
+date = "2026-01-02"
+description = "12 days of Nockmas: Extend, opcode 8"
+# aliases = []
+
+[extra]
+# author = ""
+ship = "~lagrev-nocfep"
+image = "https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/day+9/nockmas-day-9-extend-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 9 extend image](https://s3.us-east-1.amazonaws.com/urbit.orgcontent/Blog/day+9/nockmas-day-9-extend-social.png)
+
+On this ninth day of Nockmas, we will explore opcode 8, Extend and how Nock implements variable binding.
+
+# Opcode 8: Extend
+
+## Syntax
+
+```nock
+*[a 8 b c]          *[[*[a b] a] c]
+```
+
+## Explanation
+
+Opcode 8 pins a new value to the head of the subject, then evaluate the body.  This is how Nock implements variable binding—the new value becomes accessible at address 2.
+
+1. Evaluate `b` against subject to produce a new value.
+2. Construct `[new-value subject]` as the extended subject.
+3. Evaluate `c` against the extended subject.
+
+After opcode 8, addresses shift:
+
+* Address 2 → the new pinned value
+* Address 3 → the original subject
+* Address 6 → what was address 2
+* Address 7 → what was address 3
+
+Opcode 8 extends the subject, while [opcode 7](https://nock.is/content/specification/opcode-7.html) replaces it:
+
+```nock
+*[a 7 b c]  →  *[*[a b] c]       :: New subject is *[a b]
+*[a 8 b c]  →  *[[*[a b] a] c]   :: New subject is [*[a b] a]
+```
+
+There are two primary patterns for using opcode 8.  In the first, a local variable is bound for use in a computation:
+
+```nock
+*[data [8 [compute-value] [body-using-value-at-+2]]]
+```
+
+We start with our subject:
+```nock
+:subject 42
+```
+
+**Output:**
+```
+Subject set to: 42
+```
+
+And call a Nock 8 with the following formula:
+```nock
+[8 [1 41] [[0 2] [0 3]]]
+```
+
+**Output:**
+```
+[41 42]
+```
+
+The other, more complex, pattern is to pin a [core](https://docs.urbit.org/build-on-urbit/hoon-school/f-cores) and its [arms](https://docs.urbit.org/build-on-urbit/hoon-school/f-cores#arms) to the subject for invocation later.  We'll see this pattern in detail in [opcode 9](https://nock.is/content/specification/opcode-9.html)'s discussion.
+
+```nock
+*[context [8 [1 battery] [9 2 [0 1]]]]
+```
+
+Join us tomorrow when we cover Nock 9, Invoke.
+
+> *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.*