diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..2f7896d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1 @@
+target/
diff --git a/binary-serialize-derive/Cargo.lock b/binary-serialize-derive/Cargo.lock
new file mode 100644
index 0000000..94b9d96
--- /dev/null
+++ b/binary-serialize-derive/Cargo.lock
@@ -0,0 +1,47 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "binary-serialize-derive"
+version = "0.1.0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "proc-macro2"
+version = "1.0.101"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "89ae43fd86e4158d6db51ad8e2b80f313af9cc74f5c0e03ccb87de09998732de"
+dependencies = [
+ "unicode-ident",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.40"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1885c039570dc00dcb4ff087a89e185fd56bae234ddc7f056a945bf36467248d"
+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "syn"
+version = "2.0.106"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ede7c438028d4436d71104916910f5bb611972c5cfd7f89b8300a8186e6fada6"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5a5f39404a5da50712a4c1eecf25e90dd62b613502b7e925fd4e4d19b5c96512"
diff --git a/binary-serialize-derive/Cargo.toml b/binary-serialize-derive/Cargo.toml
new file mode 100644
index 0000000..1ba97b3
--- /dev/null
+++ b/binary-serialize-derive/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "binary-serialize-derive"
+version = "0.1.0"
+edition = "2021"
+
+[lib]
+proc-macro = true
+
+[dependencies]
+proc-macro2 = "1.0.101"
+quote = "1.0.40"
+syn = { version = "2.0.106", features = ["full"] }
diff --git a/binary-serialize-derive/src/lib.rs b/binary-serialize-derive/src/lib.rs
new file mode 100644
index 0000000..57aeadf
--- /dev/null
+++ b/binary-serialize-derive/src/lib.rs
@@ -0,0 +1,36 @@
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+#[proc_macro_derive(BinarySerializable)]
+pub fn derive_binary_serializable(input: TokenStream) -> TokenStream {
+    let input = parse_macro_input!(input as DeriveInput);
+
+    let name = input.ident;
+
+    let fields = match input.data {
+        syn::Data::Struct(s) => s.fields,
+        _ => panic!("#[derive(BinarySerializable)] only works on structs"),
+    };
+
+    let field_writes = fields.iter().map(|f| {
+        let ident = f.ident.as_ref().unwrap();
+        quote! {
+            BinarySerializable::write_to(&self.#ident, writer)?;
+        }
+    });
+
+    let expanded = quote! {
+        impl BinarySerializable for #name {
+            fn write_to<W: std::io::Write>(&self, writer: &mut W) -> std::io::Result<()> {
+                #(#field_writes)*
+                return Ok(());
+            }
+        }
+    };
+
+    TokenStream::from(expanded)
+}
+
diff --git a/logicworld-subassembly/Cargo.lock b/logicworld-subassembly/Cargo.lock
new file mode 100644
index 0000000..b0d05f3
--- /dev/null
+++ b/logicworld-subassembly/Cargo.lock
@@ -0,0 +1,80 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "binary-serialize-derive"
+version = "0.1.0"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
+name = "logicworld-subassembly"
+version = "0.1.0"
+dependencies = [
+ "binary-serialize-derive",
+ "quaternion",
+ "vecmath",
+]
+
+[[package]]
+name = "piston-float"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ad78bf43dcf80e8f950c92b84f938a0fc7590b7f6866fbcbeca781609c115590"
+
+[[package]]
+name = "proc-macro2"
+version = "1.0.101"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "89ae43fd86e4158d6db51ad8e2b80f313af9cc74f5c0e03ccb87de09998732de"
+dependencies = [
+ "unicode-ident",
+]
+
+[[package]]
+name = "quaternion"
+version = "2.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a6a23cd933813dea6a8e24391c80634deba6a70ca5a14b2b944b8cb6d871e071"
+dependencies = [
+ "vecmath",
+]
+
+[[package]]
+name = "quote"
+version = "1.0.40"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1885c039570dc00dcb4ff087a89e185fd56bae234ddc7f056a945bf36467248d"
+dependencies = [
+ "proc-macro2",
+]
+
+[[package]]
+name = "syn"
+version = "2.0.106"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ede7c438028d4436d71104916910f5bb611972c5cfd7f89b8300a8186e6fada6"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "unicode-ident"
+version = "1.0.18"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "5a5f39404a5da50712a4c1eecf25e90dd62b613502b7e925fd4e4d19b5c96512"
+
+[[package]]
+name = "vecmath"
+version = "1.0.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "956ae1e0d85bca567dee1dcf87fb1ca2e792792f66f87dced8381f99cd91156a"
+dependencies = [
+ "piston-float",
+]
diff --git a/logicworld-subassembly/Cargo.toml b/logicworld-subassembly/Cargo.toml
new file mode 100644
index 0000000..999980f
--- /dev/null
+++ b/logicworld-subassembly/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "logicworld-subassembly"
+version = "0.1.0"
+edition = "2021"
+
+[dependencies]
+binary-serialize-derive = { path = "../binary-serialize-derive" }
+quaternion = "2.0.0"
+vecmath = "1.0.0"
diff --git a/logicworld-subassembly/src/lib.rs b/logicworld-subassembly/src/lib.rs
new file mode 100644
index 0000000..d2e9e70
--- /dev/null
+++ b/logicworld-subassembly/src/lib.rs
@@ -0,0 +1,128 @@
+use std::{
+    fs::{self, File}, i32, io::{self, Write}, path::Path
+};
+
+use crate::lw::{BinarySerializable, ComponentIdMap, ModVersion, Version};
+
+pub mod lw;
+
+enum SaveType {
+    _World = 1,
+    Subassembly = 2,
+}
+
+pub fn create_subassembly(
+    name: String,
+    path: &Path,
+    components: &[lw::Component],
+    wires: &[lw::Wire],
+) -> io::Result<()> {
+    let directory = Path::join(path, &name);
+    fs::create_dir_all(&directory)?;
+
+    let mut file = File::create(Path::join(&directory, "data.partialworld"))?;
+    write_subassembly_file(&mut file, &components, &wires)?;
+
+    let mut placing_info_file = File::create(Path::join(&directory, "LastMainPlacingInfo.jecs"))?;
+    write_placing_info_file(&mut placing_info_file)?;
+
+    let mut placements_file = File::create(Path::join(&directory, "placements.jecs"))?;
+    write_placements_file(&mut placements_file)?;
+
+    let mut meta_file = File::create(Path::join(&directory, "meta.jecs"))?;
+    write_meta_file(&mut meta_file, &name)?;
+
+    return Ok(());
+}
+
+fn write_subassembly_file(
+    file: &mut impl Write,
+    components: &[lw::Component],
+    wires: &[lw::Wire],
+) -> io::Result<()> {
+    // Header:
+    file.write_all(b"Logic World save")?; // Magic bytes
+    file.write_all(&[7 as u8])?; // Version 7
+    Version::new(0, 92, 0, 455).write_to(file)?; // Game Version
+    file.write_all(&[SaveType::Subassembly as u8])?;
+
+    (components.len() as i32).write_to(file)?;
+    (wires.len() as i32).write_to(file)?;
+
+    // Body:
+    let mods = vec![ModVersion {
+        id: "MHG".to_owned(),
+        version: Version::new(0, 91, 4, -1),
+    }];
+    mods.write_to(file)?;
+
+    let component_id_map = vec![ComponentIdMap {
+        numeric_id: 15,
+        text_id: "MHG.CircuitBoard".to_owned(),
+    }];
+    component_id_map.write_to(file)?;
+
+    for component in components {
+        component.write_to(file)?;
+    }
+
+    for wire in wires {
+        wire.write_to(file)?;
+    }
+
+    0i32.write_to(file)?; // We don't have any turned on nets
+
+    // Footer:
+    file.write_all(b"redstone sux lol")?;
+
+    return Ok(());
+}
+
+fn write_placing_info_file(file: &mut impl Write) -> io::Result<()> {
+    file.write_all(
+        b"PlacingInfo:
+    RaiseOnChangedEvents: true
+    Flipped: false
+    RotationAboutUpVector: 180
+    Offset:
+        x: 0
+        y: 0
+    BoardIsFlat: true
+    BoardRotationState: 0
+",
+    )
+}
+fn write_placements_file(file: &mut impl Write) -> io::Result<()> {
+    file.write_all(
+        b"Placements:
+    -
+        CornerMidpointPositionBeforeFlipping:
+            x: 0
+            y: 0
+            z: 0
+        RotationBeforeFlipping:
+            x: 0
+            y: 0
+            z: 0
+            w: 1
+        PlacementType: OnTerrain
+        Flipped: false
+        BoardRotationState: 0
+        Type: Standard
+data_format_version: 1
+",
+    )
+}
+
+fn write_meta_file(file: &mut impl Write, title: &str) -> io::Result<()> {
+    file.write_all(
+        format!(
+            "Title: {}
+Column: null
+Category: null
+",
+            title
+        )
+        .as_bytes(),
+    )
+}
diff --git a/logicworld-subassembly/src/lw.rs b/logicworld-subassembly/src/lw.rs
new file mode 100644
index 0000000..10d9cde
--- /dev/null
+++ b/logicworld-subassembly/src/lw.rs
@@ -0,0 +1,149 @@
+use std::io::{self, Write};
+
+use binary_serialize_derive::BinarySerializable;
+use quaternion::Quaternion;
+use vecmath::Vector3;
+
+pub trait BinarySerializable {
+    fn write_to<W: Write>(&self, writer: &mut W) -> io::Result<()>;
+}
+
+#[repr(u8)]
+#[derive(Copy, Clone)]
+pub enum PegType {
+    Input = 1,
+    Output = 2,
+}
+
+pub type Int = i32;
+pub type Float = f32;
+pub type ComponentAddress = u32;
+
+#[derive(BinarySerializable)]
+pub struct Version {
+    pub major: Int,
+    pub minor: Int,
+    pub build: Int,
+    pub revision: Int,
+}
+impl Version {
+    pub fn new(major: Int, minor: Int, build: Int, revision: Int) -> Self {
+        Self {
+            major,
+            minor,
+            build,
+            revision,
+        }
+    }
+}
+
+#[derive(BinarySerializable)]
+pub struct ModVersion {
+    pub id: String,
+    pub version: Version,
+}
+
+#[derive(BinarySerializable)]
+pub struct Input {
+    circuit_state_id: Int,
+}
+#[derive(BinarySerializable)]
+pub struct Output {
+    circuit_state_id: Int,
+}
+
+#[derive(BinarySerializable)]
+pub struct ComponentIdMap {
+    pub numeric_id: i16,
+    pub text_id: String,
+}
+
+#[derive(BinarySerializable)]
+pub struct PegAddress {
+    peg_type: PegType,
+    component_address: ComponentAddress,
+    index: Int,
+}
+
+#[derive(BinarySerializable)]
+pub struct Component {
+    pub address: ComponentAddress,
+    pub parent: ComponentAddress,
+    pub numeric_id: u16,
+    pub position: Vector3<Int>,
+    pub rotation: Quaternion<Float>,
+    pub inputs: Vec<Input>,
+    pub outputs: Vec<Output>,
+    pub custom_data: Vec<u8>,
+}
+
+#[derive(BinarySerializable)]
+pub struct Wire {
+    first_point: PegAddress,
+    second_point: PegAddress,
+    circuit_state_id: Int,
+    wire_rotation: Float,
+}
+
+impl<T: BinarySerializable> BinarySerializable for Vec<T> {
+    fn write_to<W: Write>(&self, writer: &mut W) -> io::Result<()> {
+        writer.write_all(&(self.len() as Int).to_le_bytes())?;
+        for value in self {
+            value.write_to(writer)?;
+        }
+        return Ok(());
+    }
+}
+
+impl BinarySerializable for Vec<u8> {
+    fn write_to<W: Write>(&self, writer: &mut W) -> io::Result<()> {
+        writer.write_all(&(self.len() as Int).to_le_bytes())?;
+        writer.write_all(&self)?;
+        return Ok(());
+    }
+}
+
+impl BinarySerializable for PegType {
+    fn write_to<W: Write>(&self, writer: &mut W) -> io::Result<()> {
+        writer.write_all(&[*self as u8])
+    }
+}
+
+impl BinarySerializable for String {
+    fn write_to<W: Write>(&self, writer: &mut W) -> io::Result<()> {
+        writer.write_all(&(self.len() as Int).to_le_bytes())?;
+        writer.write_all(self.as_bytes())?;
+        return Ok(());
+    }
+}
+
+impl BinarySerializable for Vector3<Int> {
+    fn write_to<W: Write>(&self, writer: &mut W) -> io::Result<()> {
+        self[0].write_to(writer)?; // x
+        self[1].write_to(writer)?; // y
+        self[2].write_to(writer)?; // z
+        return Ok(());
+    }
+}
+
+impl BinarySerializable for Quaternion<Float> {
+    fn write_to<W: Write>(&self, writer: &mut W) -> io::Result<()> {
+        self.1[0].write_to(writer)?; // x
+        self.1[1].write_to(writer)?; // y
+        self.1[2].write_to(writer)?; // z
+        self.0.write_to(writer)?; // w
+        return Ok(());
+    }
+}
+
+macro_rules! impl_binary_num {
+    ($($t:ty),*) => {
+        $(impl BinarySerializable for $t {
+            fn write_to<W: std::io::Write>(&self, writer: &mut W) -> std::io::Result<()> {
+                writer.write_all(&self.to_le_bytes())
+            }
+        })*
+    };
+}
+
+impl_binary_num!(i16, u16, i32, u32, f32);