diff --git a/assembler/src/error.rs b/assembler/src/error.rs
index c5ad53b..c5627cc 100644
--- a/assembler/src/error.rs
+++ b/assembler/src/error.rs
@@ -34,14 +34,49 @@ impl AssembleError {
             kind,
         }
     }
+
+    /// Prints a parser error to the screen.
+    fn print_parser_error(
+        &self,
+        f: &mut std::fmt::Formatter<'_>,
+        parse_error: &ParserError,
+    ) -> std::fmt::Result {
+        let Some(source_info) = &self.source_info else {
+            write!(
+                f,
+                "Parse error thrown with no source information. Error: {parse_error}"
+            )?;
+
+            return Ok(());
+        };
+
+        write!(f, "Parser error, {parse_error} at {source_info}")?;
+
+        // Prints out the context for our error.
+        source_info.print_context_with_underline().map_err(|e| {
+            _ = writeln!(f, "Print context error: {e}");
+
+            std::fmt::Error {}
+        })?;
+
+        Ok(())
+    }
 }
 
 impl Display for AssembleError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         if let Some(info) = &self.source_info {
             write!(f, "at {info}")?;
+
+            match &self.kind {
+                AssembleErrorKind::Parser(err) => self.print_parser_error(f, err)?,
+                _ => write!(f, "{}", self.kind)?,
+            }
+
+            return Ok(());
         }
 
+        // Handle errors without SourceInfo.
         write!(f, "{}", self.kind)?;
 
         Ok(())
@@ -68,13 +103,7 @@ pub enum AssembleErrorKind {
 }
 
 #[derive(Debug, Clone)]
-pub struct ParserError {
-    error_type: ParserErrorType,
-    source_info: SourceInfo,
-}
-
-#[derive(Debug, Clone)]
-pub enum ParserErrorType {
+pub enum ParserError {
     UnexpectedToken,
     MissingOperand,
     InvalidInstruction,
@@ -82,7 +111,7 @@ pub enum ParserErrorType {
     DuplicateLabel,
 }
 
-impl Display for ParserErrorType {
+impl Display for ParserError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         match self {
             Self::UnexpectedToken => write!(f, "unexpected token"),
@@ -94,28 +123,6 @@ impl Display for ParserErrorType {
     }
 }
 
-impl Display for ParserError {
-    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
-        // TODO: Print the path/to/filename.dsa:line_no, column col_no.
-        write!(
-            f,
-            "Parser error, {} at {}",
-            self.error_type, self.source_info
-        )?;
-
-        // Prints out the context for our error.
-        self.source_info
-            .print_context_with_underline()
-            .map_err(|e| {
-                _ = writeln!(f, "Print context error: {e}");
-
-                std::fmt::Error {}
-            })?;
-
-        Ok(())
-    }
-}
-
 #[derive(Debug, Clone)]
 pub enum SymbolError {
     Undefined,
diff --git a/assembler/src/source.rs b/assembler/src/source.rs
index 96a054e..2f455a7 100644
--- a/assembler/src/source.rs
+++ b/assembler/src/source.rs
@@ -9,6 +9,7 @@ use std::{
 use crate::error::AssembleError;
 
 pub mod lines;
+pub mod opcode;
 pub mod source_info;
 pub mod token;
 pub mod token_info;
diff --git a/assembler/src/source/opcode.rs b/assembler/src/source/opcode.rs
new file mode 100644
index 0000000..5056515
--- /dev/null
+++ b/assembler/src/source/opcode.rs
@@ -0,0 +1,349 @@
+//! This module contains instructions for tokenisation.
+
+use std::{fmt, str::FromStr};
+
+use common::prelude::{ITypeArgs, Instruction, Interrupt, RTypeArgs};
+
+use crate::{
+    error::{AssembleError, AssembleErrorKind},
+    source::source_info::SourceInfo,
+};
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+pub enum Opcode {
+    Nop,
+    Mov,
+    Movs,
+    Ldb,
+    Ldbs,
+    Ldh,
+    Ldhs,
+    Ldw,
+    Stb,
+    Sth,
+    Stw,
+    Lli,
+    Lui,
+    Jmp,
+    Jeq,
+    Jne,
+    Jgt,
+    Jge,
+    Jlt,
+    Jle,
+    Cmp,
+    Inc,
+    Dec,
+    Shl,
+    Shr,
+    Add,
+    Sub,
+    And,
+    Or,
+    Not,
+    Xor,
+    Nand,
+    Nor,
+    Xnor,
+    Int,
+    Irt,
+    Hlt,
+    AddI,
+    SubI,
+
+    // Pseudo-instructions
+    Db,
+    Dh,
+    Dw,
+    Resb,
+    Resh,
+    Resw,
+    Push,
+    Pop,
+    Pusha,
+    Popa,
+    Lwi,
+    Call,
+    Return,
+
+    // Meta instructions (these aren't present in the binary as instructions)
+    Include,
+    Data,
+    Segment,
+}
+
+#[derive(Debug)]
+pub enum OpcodeFromStrError {
+    InvalidRegister(&'static str),
+    InvalidOpcode(String),
+}
+
+impl std::fmt::Display for OpcodeFromStrError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::InvalidRegister(reg) => write!(f, "register does not exist: {reg}"),
+            Self::InvalidOpcode(op) => write!(f, "instruction does not exist: {op}"),
+        }
+    }
+}
+
+impl std::error::Error for OpcodeFromStrError {}
+
+impl Opcode {
+    pub const OPCODES: &[&str] = &[
+        // Real instructions (0x00-0x26)
+        "nop", "mov", "movs", "ldb", "ldbs", "ldh", "ldhs", "ldw", "stb", "sth", "stw",
+        "lli", "lui", "jmp", "jeq", "jne", "jgt", "jge", "jlt", "jle", "cmp", "inc",
+        "dec", "shl", "shr", "add", "sub", "and", "or", "not", "xor", "nand", "nor",
+        "xnor", "int", "irt", "hlt", "addi", "subi", // Pseudo-instructions
+        "db", "dh", "dw", "resb", "resh", "resw", "push", "pop", "lwi", "call", "return",
+        "pusha", "popa", // meta instructions
+        "include",
+    ];
+
+    pub fn to_instruction(
+        &self,
+        source_info: SourceInfo,
+    ) -> Result<Instruction, AssembleError> {
+        match self {
+            Self::Nop => Ok(Instruction::Nop),
+            Self::Mov => Ok(Instruction::Mov(RTypeArgs::default())),
+            Self::Movs => Ok(Instruction::MovSigned(RTypeArgs::default())),
+            Self::Ldb => Ok(Instruction::LoadByte(ITypeArgs::default())),
+            Self::Ldbs => Ok(Instruction::LoadByteSigned(ITypeArgs::default())),
+            Self::Ldh => Ok(Instruction::LoadHalfword(ITypeArgs::default())),
+            Self::Ldhs => Ok(Instruction::LoadHalfwordSigned(ITypeArgs::default())),
+            Self::Ldw => Ok(Instruction::LoadWord(ITypeArgs::default())),
+            Self::Stb => Ok(Instruction::StoreByte(ITypeArgs::default())),
+            Self::Sth => Ok(Instruction::StoreHalfword(ITypeArgs::default())),
+            Self::Stw => Ok(Instruction::StoreWord(ITypeArgs::default())),
+            Self::Lli => Ok(Instruction::LoadLowerImmediate(ITypeArgs::default())),
+            Self::Lui => Ok(Instruction::LoadUpperImmediate(ITypeArgs::default())),
+            Self::Jmp => Ok(Instruction::Jump(ITypeArgs::default())),
+            Self::Jeq => Ok(Instruction::JumpEq(ITypeArgs::default())),
+            Self::Jne => Ok(Instruction::JumpNeq(ITypeArgs::default())),
+            Self::Jgt => Ok(Instruction::JumpGt(ITypeArgs::default())),
+            Self::Jge => Ok(Instruction::JumpGe(ITypeArgs::default())),
+            Self::Jlt => Ok(Instruction::JumpLt(ITypeArgs::default())),
+            Self::Jle => Ok(Instruction::JumpLe(ITypeArgs::default())),
+            Self::Cmp => Ok(Instruction::Compare(RTypeArgs::default())),
+            Self::Inc => Ok(Instruction::Increment(RTypeArgs::default())),
+            Self::Dec => Ok(Instruction::Decrement(RTypeArgs::default())),
+            Self::Shl => Ok(Instruction::ShiftLeft(RTypeArgs::default())),
+            Self::Shr => Ok(Instruction::ShiftRight(RTypeArgs::default())),
+            Self::Add => Ok(Instruction::Add(RTypeArgs::default())),
+            Self::Sub => Ok(Instruction::Sub(RTypeArgs::default())),
+            Self::And => Ok(Instruction::And(RTypeArgs::default())),
+            Self::Or => Ok(Instruction::Or(RTypeArgs::default())),
+            Self::Not => Ok(Instruction::Not(RTypeArgs::default())),
+            Self::Xor => Ok(Instruction::Xor(RTypeArgs::default())),
+            Self::Nand => Ok(Instruction::Nand(RTypeArgs::default())),
+            Self::Nor => Ok(Instruction::Nor(RTypeArgs::default())),
+            Self::Xnor => Ok(Instruction::Xnor(RTypeArgs::default())),
+            Self::Int => Ok(Instruction::Interrupt(Interrupt::default())),
+            Self::Irt => Ok(Instruction::IntReturn),
+            Self::Hlt => Ok(Instruction::Halt),
+            Self::AddI => Ok(Instruction::AddImmediate(ITypeArgs::default())),
+            Self::SubI => Ok(Instruction::SubImmediate(ITypeArgs::default())),
+            Self::Segment => Ok(Instruction::Segment(0)),
+            _ => Err(AssembleError::new_source_error(
+                source_info,
+                AssembleErrorKind::Unimplemented(
+                    "Opcode::to_instruction called on an instruction that does not exist in common.",
+                ),
+            )),
+        }
+    }
+
+    #[must_use]
+    pub const fn to_opcode_value(&self) -> Option<u8> {
+        match self {
+            Self::Nop => Some(0x00),
+            Self::Mov => Some(0x01),
+            Self::Movs => Some(0x02),
+            Self::Ldb => Some(0x03),
+            Self::Ldbs => Some(0x04),
+            Self::Ldh => Some(0x05),
+            Self::Ldhs => Some(0x06),
+            Self::Ldw => Some(0x07),
+            Self::Stb => Some(0x08),
+            Self::Sth => Some(0x09),
+            Self::Stw => Some(0x0A),
+            Self::Lli => Some(0x0B),
+            Self::Lui => Some(0x0C),
+            Self::Jmp => Some(0x0D),
+            Self::Jeq => Some(0x0E),
+            Self::Jne => Some(0x0F),
+            Self::Jgt => Some(0x10),
+            Self::Jge => Some(0x11),
+            Self::Jlt => Some(0x12),
+            Self::Jle => Some(0x13),
+            Self::Cmp => Some(0x14),
+            Self::Inc => Some(0x15),
+            Self::Dec => Some(0x16),
+            Self::Shl => Some(0x17),
+            Self::Shr => Some(0x18),
+            Self::Add => Some(0x19),
+            Self::Sub => Some(0x1A),
+            Self::And => Some(0x1B),
+            Self::Or => Some(0x1C),
+            Self::Not => Some(0x1D),
+            Self::Xor => Some(0x1E),
+            Self::Nand => Some(0x1F),
+            Self::Nor => Some(0x20),
+            Self::Xnor => Some(0x21),
+            Self::Int => Some(0x22),
+            Self::Irt => Some(0x23),
+            Self::Hlt => Some(0x24),
+            Self::AddI => Some(0x25),
+            Self::SubI => Some(0x26),
+            // TODO: Maybe recombine pseudos?
+            Self::Segment => Some(0x27),
+            // Pseudo-instructions don't have opcode values
+            _ => None,
+        }
+    }
+
+    #[must_use]
+    pub const fn is_pseudo_instruction(&self) -> bool {
+        matches!(
+            self,
+            Self::Db
+                | Self::Dh
+                | Self::Dw
+                | Self::Resb
+                | Self::Resh
+                | Self::Resw
+                | Self::Push
+                | Self::Pop
+                | Self::Lwi
+        )
+    }
+}
+
+impl FromStr for Opcode {
+    type Err = OpcodeFromStrError;
+
+    fn from_str(s: &str) -> Result<Self, Self::Err> {
+        match s.to_lowercase().as_str() {
+            "nop" => Ok(Self::Nop),
+            "mov" => Ok(Self::Mov),
+            "movs" => Ok(Self::Movs),
+            "ldb" => Ok(Self::Ldb),
+            "ldbs" => Ok(Self::Ldbs),
+            "ldh" => Ok(Self::Ldh),
+            "ldhs" => Ok(Self::Ldhs),
+            "ldw" => Ok(Self::Ldw),
+            "stb" => Ok(Self::Stb),
+            "sth" => Ok(Self::Sth),
+            "stw" => Ok(Self::Stw),
+            "lli" => Ok(Self::Lli),
+            "lui" => Ok(Self::Lui),
+            "jmp" => Ok(Self::Jmp),
+            "jeq" => Ok(Self::Jeq),
+            "jne" => Ok(Self::Jne),
+            "jgt" => Ok(Self::Jgt),
+            "jge" => Ok(Self::Jge),
+            "jlt" => Ok(Self::Jlt),
+            "jle" => Ok(Self::Jle),
+            "cmp" => Ok(Self::Cmp),
+            "inc" => Ok(Self::Inc),
+            "dec" => Ok(Self::Dec),
+            "shl" => Ok(Self::Shl),
+            "shr" => Ok(Self::Shr),
+            "add" => Ok(Self::Add),
+            "sub" => Ok(Self::Sub),
+            "and" => Ok(Self::And),
+            "or" => Ok(Self::Or),
+            "not" => Ok(Self::Not),
+            "xor" => Ok(Self::Xor),
+            "nand" => Ok(Self::Nand),
+            "nor" => Ok(Self::Nor),
+            "xnor" => Ok(Self::Xnor),
+            "int" => Ok(Self::Int),
+            "irt" => Ok(Self::Irt),
+            "hlt" => Ok(Self::Hlt),
+            "addi" => Ok(Self::AddI),
+            "subi" => Ok(Self::SubI),
+            "db" => Ok(Self::Db),
+            "dh" => Ok(Self::Dh),
+            "dw" => Ok(Self::Dw),
+            "resb" => Ok(Self::Resb),
+            "resh" => Ok(Self::Resh),
+            "resw" => Ok(Self::Resw),
+            "push" => Ok(Self::Push),
+            "pop" => Ok(Self::Pop),
+            "lwi" => Ok(Self::Lwi),
+            "include" => Ok(Self::Include),
+            "call" => Ok(Self::Call),
+            "return" => Ok(Self::Return),
+            "pusha" => Ok(Self::Pusha),
+            "popa" => Ok(Self::Popa),
+            _ => Err(OpcodeFromStrError::InvalidOpcode(s.to_string())),
+        }
+    }
+}
+
+impl fmt::Display for Opcode {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::Nop => write!(f, "nop"),
+            Self::Mov => write!(f, "mov"),
+            Self::Movs => write!(f, "movs"),
+            Self::Ldb => write!(f, "ldb"),
+            Self::Ldbs => write!(f, "ldbs"),
+            Self::Ldh => write!(f, "ldh"),
+            Self::Ldhs => write!(f, "ldhs"),
+            Self::Ldw => write!(f, "ldw"),
+            Self::Stb => write!(f, "stb"),
+            Self::Sth => write!(f, "sth"),
+            Self::Stw => write!(f, "stw"),
+            Self::Lli => write!(f, "lli"),
+            Self::Lui => write!(f, "lui"),
+            Self::Jmp => write!(f, "jmp"),
+            Self::Jeq => write!(f, "jeq"),
+            Self::Jne => write!(f, "jne"),
+            Self::Jgt => write!(f, "jgt"),
+            Self::Jge => write!(f, "jge"),
+            Self::Jlt => write!(f, "jlt"),
+            Self::Jle => write!(f, "jle"),
+            Self::Cmp => write!(f, "cmp"),
+            Self::Inc => write!(f, "inc"),
+            Self::Dec => write!(f, "dec"),
+            Self::Shl => write!(f, "shl"),
+            Self::Shr => write!(f, "shr"),
+            Self::Add => write!(f, "add"),
+            Self::Sub => write!(f, "sub"),
+            Self::And => write!(f, "and"),
+            Self::Or => write!(f, "or"),
+            Self::Not => write!(f, "not"),
+            Self::Xor => write!(f, "xor"),
+            Self::Nand => write!(f, "nand"),
+            Self::Nor => write!(f, "nor"),
+            Self::Xnor => write!(f, "xnor"),
+            Self::Int => write!(f, "int"),
+            Self::Irt => write!(f, "irt"),
+            Self::Hlt => write!(f, "hlt"),
+            Self::AddI => write!(f, "addi"),
+            Self::SubI => write!(f, "subi"),
+            Self::Db => write!(f, "db"),
+            Self::Dh => write!(f, "dh"),
+            Self::Dw => write!(f, "dw"),
+            Self::Resb => write!(f, "resb"),
+            Self::Resh => write!(f, "resh"),
+            Self::Resw => write!(f, "resw"),
+            Self::Push => write!(f, "push"),
+            Self::Pop => write!(f, "pop"),
+            Self::Lwi => write!(f, "lwi"),
+            Self::Call => write!(f, "call"),
+            Self::Return => write!(f, "return"),
+            Self::Pusha => write!(f, "pusha"),
+            Self::Popa => write!(f, "popa"),
+
+            // meta instructions
+            Self::Include => write!(f, "include"),
+            Self::Data => write!(f, "data"),
+            Self::Segment => write!(f, "[SEGMENT]"),
+        }
+    }
+}
diff --git a/assembler/src/source/pseudo_opcode.rs b/assembler/src/source/pseudo_opcode.rs
new file mode 100644
index 0000000..1f75b1e
--- /dev/null
+++ b/assembler/src/source/pseudo_opcode.rs
@@ -0,0 +1,4 @@
+//! This module contains code for handling pseudo opcodes.
+
+/// Pseudo instructions that cannot simply be lowered to ISA instructions.
+pub enum PseudoOpcode {}
diff --git a/assembler/src/source/token.rs b/assembler/src/source/token.rs
index 2303961..899de4d 100644
--- a/assembler/src/source/token.rs
+++ b/assembler/src/source/token.rs
@@ -5,10 +5,9 @@
 use common::prelude::*;
 
 use crate::source::{
+    opcode::Opcode,
     source_info::SourceInfo,
-    token_info::{
-        DirectiveToken, InstructionToken, LabelToken, RegisterToken, SymbolToken,
-    },
+    token_info::{DirectiveToken, LabelToken, RegisterToken, SymbolToken},
 };
 
 #[derive(Debug, Clone, PartialEq, Eq, Hash)]
@@ -22,7 +21,7 @@ pub enum TokenType {
     /// String literal (e.g., `"hello world"`).
     String(String),
     /// Assembly instruction (e.g., `add`, `jmp`, `nop`).
-    Instruction(InstructionToken),
+    Instruction(Opcode),
     /// Label definition (e.g., `loop_start:`).
     Label(LabelToken),
     /// Assembler directive (e.g., `.global`, `.section`, `.dw`, `.resb`).
@@ -65,11 +64,8 @@ impl Token {
     }
 
     #[must_use]
-    pub const fn instruction(mnemonic: String, source_info: SourceInfo) -> Self {
-        Self::new(
-            TokenType::Instruction(InstructionToken { mnemonic }),
-            source_info,
-        )
+    pub const fn instruction(op: Opcode, source_info: SourceInfo) -> Self {
+        Self::new(TokenType::Instruction(op), source_info)
     }
 
     #[must_use]
diff --git a/assembler/src/source/token_info.rs b/assembler/src/source/token_info.rs
index 1d5dc81..557e74b 100644
--- a/assembler/src/source/token_info.rs
+++ b/assembler/src/source/token_info.rs
@@ -21,14 +21,14 @@ pub struct RegisterToken {
 }
 
 impl RegisterToken {
+    #[must_use]
+    pub const fn new(reg: Register) -> Self {
+        Self { reg }
+    }
+
     /// Returns the name of a valid [`Register`]
     #[must_use]
     pub fn name(&self) -> String {
         self.reg.to_string()
     }
 }
-
-#[derive(Debug, Clone, PartialEq, Eq, Hash)]
-pub struct InstructionToken {
-    pub mnemonic: String,
-}
diff --git a/assembler/src/source/tokeniser.rs b/assembler/src/source/tokeniser.rs
index 1266a83..08c0ba0 100644
--- a/assembler/src/source/tokeniser.rs
+++ b/assembler/src/source/tokeniser.rs
@@ -3,6 +3,7 @@
 
 use std::{
     path::{Path, PathBuf},
+    str::FromStr,
     sync::Arc,
 };
 
@@ -15,13 +16,12 @@ use crate::{
     error::{AssembleError, AssembleErrorKind, IoError, IoErrorKind},
     model::module::Module,
     source::{
-        lines::lines_with_spans,
+        lines::{LineSpan, lines_with_spans},
         load_source_bytes,
+        opcode::Opcode,
         source_info::SourceInfo,
         token::{Token, TokenType},
-        token_info::{
-            DirectiveToken, LabelToken, RegisterToken, SymbolToken,
-        },
+        token_info::{DirectiveToken, LabelToken, RegisterToken, SymbolToken},
     },
 };
 
@@ -38,10 +38,10 @@ pub struct Tokeniser {
 
     // Pre-compiled regex patterns
     label_regex: Regex,
-    // register_regex: Regex,
+    register_regex: Regex,
     immediate_regex: Regex,
     directive_regex: Regex,
-    // instruction_regex: Regex,
+    instruction_regex: Regex,
     symbol_regex: Regex,
     string_regex: Regex,
     comment_regex: Regex,
@@ -56,23 +56,25 @@ impl Tokeniser {
 
             label_regex: Regex::new(r"^([a-zA-Z_][a-zA-Z0-9_]*):")
                 .expect("Failed to compile label regex pattern"),
-            // register_regex: Regex::new(r"^(r[0-9]+|sp|fp|pc)")
-            //     .expect("Failed to compile register regex pattern"),
+            register_regex: Regex::new(
+                r"^(rg([0-9]|[a-f])|acc|spr|bpr|ret|idr|mmr|zero|noreg|pcx)\b",
+            )
+            .expect("Failed to compile register regex pattern"),
             immediate_regex: Regex::new(
                 r"^(0x[0-9a-fA-F_]+|0b[0-1_]+|0o[0-7_]+|[0-9_]+)",
             )
             .expect("Failed to compile immediate regex pattern"),
-            directive_regex: Regex::new(r"^\.([a-zA-Z]+)")
+            directive_regex: Regex::new(r"^(res[bwh]|d[bwh]|include|section|global|local)\b")
                 .expect("Failed to compile directive regex pattern"),
-            // instruction_regex: Regex::new(
-            //     r"^(add|sub|mul|div|jmp|call|ret|lli|nop|halt)",
-            // )
-            // .expect("Failed to compile instruction regex pattern"),
+            instruction_regex: Regex::new(
+                r"^(nop|movs?|ld[bhw]s?|st[bhw]|l[lu]i|j(mp|[egl][qte])|cmp|[id]nc|sh[lr]|add[i]?|sub[i]?|x?n?or|and|not|i[rd]t|hlt|lhwmm|lidt|push[a]?|pop[a]?|lwi|return|call)\b",
+            )
+            .expect("Failed to compile instruction regex pattern"),
             symbol_regex: Regex::new(r"^([a-zA-Z_][a-zA-Z0-9_]*)")
                 .expect("Failed to compile symbol regex pattern"),
             string_regex: Regex::new(r#"^"([^"]*)"#)
                 .expect("Failed to compile string regex pattern"),
-            comment_regex: Regex::new("//.*")
+            comment_regex: Regex::new("^//.*")
                 .expect("Failed to compile comment regex pattern"),
         }
     }
@@ -137,7 +139,7 @@ impl Tokeniser {
 
     fn tokenize_line(
         &self,
-        line_span: &crate::source::lines::LineSpan,
+        line_span: &LineSpan,
         module: &Arc<Module>,
     ) -> Result<Vec<Token>, AssembleError> {
         let mut tokens = Vec::new();
@@ -188,9 +190,11 @@ impl Tokeniser {
     }
 
     fn try_match_register(&self, input: &str) -> Option<(TokenType, usize)> {
-        _ = self;
+        let caps = self.register_regex.captures(input)?;
+        let reg = caps.get(1)?.as_str();
+        let len = caps.get(0)?.len();
 
-        let reg = match Register::try_from(input) {
+        let reg = match Register::try_from(reg) {
             Ok(reg) => reg,
             Err(_why) => {
                 // Probably ignore the error.
@@ -198,8 +202,6 @@ impl Tokeniser {
             }
         };
 
-        let len = input.len();
-
         Some((TokenType::Register(RegisterToken { reg }), len))
     }
 
@@ -234,15 +236,14 @@ impl Tokeniser {
         Some((TokenType::Directive(DirectiveToken { directive }), len))
     }
 
-    const fn try_match_instruction(&self, _input: &str) -> Option<(TokenType, usize)> {
-        _ = self;
+    fn try_match_instruction(&self, input: &str) -> Option<(TokenType, usize)> {
+        let caps = self.instruction_regex.captures(input)?;
+        let mnemonic = caps.get(1)?.as_str().to_string();
+        let len = caps.get(0)?.len();
 
-        // let instruction =
-        // Some((TokenType::Instruction(InstructionToken { mnemonic }), len))
+        let op = Opcode::from_str(&mnemonic).ok()?;
 
-        // TODO: fix me.
-
-        None
+        Some((TokenType::Instruction(op), len))
     }
 
     fn try_match_symbol(&self, input: &str) -> Option<(TokenType, usize)> {
@@ -262,6 +263,14 @@ impl Tokeniser {
     }
 
     fn match_token(&self, input: &str) -> Result<(TokenType, usize), AssembleError> {
+        if let Some(m) = self.try_match_directive(input) {
+            return Ok(m);
+        }
+
+        if let Some(m) = self.try_match_instruction(input) {
+            return Ok(m);
+        }
+
         if let Some(m) = self.try_match_comment(input) {
             return Ok(m);
         }
@@ -278,14 +287,6 @@ impl Tokeniser {
             return Ok(m);
         }
 
-        if let Some(m) = self.try_match_directive(input) {
-            return Ok(m);
-        }
-
-        if let Some(m) = self.try_match_instruction(input) {
-            return Ok(m);
-        }
-
         if let Some(m) = self.try_match_string(input) {
             return Ok(m);
         }
diff --git a/assembler/src/source/tokeniser/error.rs b/assembler/src/source/tokeniser/error.rs
index 84127ed..f238713 100644
--- a/assembler/src/source/tokeniser/error.rs
+++ b/assembler/src/source/tokeniser/error.rs
@@ -3,6 +3,8 @@
 #[derive(Debug, Clone, Copy)]
 pub enum TokeniserError {}
 
+impl TokeniserError {}
+
 impl std::fmt::Display for TokeniserError {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         write!(f, "TODO!!!!!!")
diff --git a/assembler/src/source/tokeniser/tests.rs b/assembler/src/source/tokeniser/tests.rs
index da73fba..6aab304 100644
--- a/assembler/src/source/tokeniser/tests.rs
+++ b/assembler/src/source/tokeniser/tests.rs
@@ -1,9 +1,13 @@
 //! Unit tests for the tokenizer
 
+use common::prelude::Register;
+
 use crate::{
     context::AssemblerContext,
     source::{
+        opcode::Opcode,
         token::{Token, TokenType},
+        token_info::RegisterToken,
         tokeniser::Tokeniser,
     },
 };
@@ -65,7 +69,7 @@ fn test_single_instruction() {
             .any(|t| matches!(t, TokenType::Instruction(_)))
     );
     if let TokenType::Instruction(instr) = &tokens[0].token_type {
-        assert_eq!(instr.mnemonic, "add");
+        assert_eq!(instr.to_string(), "add");
     } else {
         panic!("Expected instruction token");
     }
@@ -73,15 +77,13 @@ fn test_single_instruction() {
 
 #[test]
 fn test_all_instructions() {
-    let instructions = [
-        "add", "sub", "mul", "div", "jmp", "call", "ret", "lli", "nop", "halt",
-    ];
+    let instructions = ["add", "sub", "jmp", "call", "return", "lli", "nop", "hlt"];
 
     for instr in &instructions {
         let tokens = tokenize_source(instr).expect("Failed to tokenize instruction");
 
         if let TokenType::Instruction(parsed_instr) = &tokens[0].token_type {
-            assert_eq!(parsed_instr.mnemonic, *instr);
+            assert_eq!(parsed_instr.to_string(), *instr);
         } else {
             panic!("Expected instruction token for {instr}");
         }
@@ -90,7 +92,7 @@ fn test_all_instructions() {
 
 #[test]
 fn test_registers() {
-    let test_cases = [("rg0", "r0"), ("rgf", "rgf"), ("pcx", "pcx")];
+    let test_cases = [("rg0", "rg0"), ("rgf", "rgf"), ("pcx", "pcx")];
 
     for (input, expected) in &test_cases {
         let tokens = tokenize_source(input).expect("Failed to tokenize register");
@@ -149,10 +151,9 @@ fn test_labels() {
 #[test]
 fn test_directives() {
     let test_cases = [
-        (".global", "global"),
-        (".section", "section"),
-        (".data", "data"),
-        (".text", "text"),
+        ("global", "global"),
+        ("section", "section"),
+        ("local", "local"),
     ];
 
     for (input, expected) in &test_cases {
@@ -185,3 +186,52 @@ fn test_symbols() {
         }
     }
 }
+
+#[test]
+fn test_complex_instruction_line() {
+    let source = "addi rg1, rg2, 0xFF";
+    let tokens = tokenize_source(source).expect("Failed to tokenise complex instruction");
+
+    // Should have: instruction, register, comma, register, comma, immediate, newline, EOF
+    assert!(tokens.len() >= 6);
+    assert!(matches!(tokens[0].token_type, TokenType::Instruction(_)));
+    assert!(matches!(tokens[1].token_type, TokenType::Register(_)));
+    assert!(matches!(tokens[2].token_type, TokenType::Comma));
+    assert!(matches!(tokens[3].token_type, TokenType::Register(_)));
+    assert!(matches!(tokens[4].token_type, TokenType::Comma));
+    assert!(matches!(tokens[5].token_type, TokenType::Immediate(_)));
+}
+
+#[test]
+fn test_multiline_with_comments() {
+    const EXPECTED_TOKEN_TYPES: [TokenType; 11] = [
+        TokenType::Instruction(Opcode::Add),
+        TokenType::Register(RegisterToken::new(Register::Rg0)),
+        TokenType::Comma,
+        TokenType::Register(RegisterToken::new(Register::Rg1)),
+        TokenType::Newline,
+        TokenType::Instruction(Opcode::SubI),
+        TokenType::Register(RegisterToken::new(Register::Rg2)),
+        TokenType::Comma,
+        TokenType::Immediate(10),
+        TokenType::Newline,
+        TokenType::Eof,
+    ];
+
+    const SOURCE: &str = r"add rg0, rg1 // Another comment
+        subi rg2, 10";
+
+    let tokens =
+        tokenize_source(SOURCE).expect("Failed to tokenise source with comments");
+    let token_types = extract_token_types(&tokens);
+
+    assert_eq!(
+        token_types.len(),
+        EXPECTED_TOKEN_TYPES.len(),
+        "{token_types:#?}"
+    );
+
+    for (expected, got) in EXPECTED_TOKEN_TYPES.iter().zip(token_types.iter()) {
+        assert!(!(expected != *got), "Expected {expected:?}, got {got:?}");
+    }
+}
diff --git a/common/src/instructions.rs b/common/src/instructions.rs
index deeaddb..64ea686 100644
--- a/common/src/instructions.rs
+++ b/common/src/instructions.rs
@@ -1,9 +1,10 @@
 use crate::{instructions::encode::Encode, prelude::*};
 
-#[derive(Copy, Clone, Debug, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, PartialEq, Eq, Default)]
 pub enum Interrupt {
     Software(u8),
     Breakpoint,
+    #[default]
     HardFault,
 }
 
diff --git a/common/src/instructions/args.rs b/common/src/instructions/args.rs
index 664c31a..1dc440a 100644
--- a/common/src/instructions/args.rs
+++ b/common/src/instructions/args.rs
@@ -1,4 +1,5 @@
-//! Various types of arguments that instructions can take, alongside encoding and decoding logic.
+//! Various types of arguments that instructions can take, alongside encoding and decoding
+//! logic.
 
 use crate::{
     instructions::{RegisterParseError, encode::Encode},
@@ -35,18 +36,20 @@ impl std::fmt::Display for ArgsDecodeError {
 
 impl std::error::Error for ArgsDecodeError {}
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
 /// Used by instructions with 2 registers and an immediate argument.
 pub struct ITypeArgs {
     pub immediate: u16,
     pub r1: Register,
-    /// May not actually be used by some instructions taking an immediate e.g. LUI. This is solved by making the constructor take Options.
+    /// May not actually be used by some instructions taking an immediate e.g. LUI. This
+    /// is solved by making the constructor take Options.
     pub r2: Register,
 }
 
 impl ITypeArgs {
     #[must_use]
-    /// Creates a new [`ITypeArgs`]. If r1 or r2 is unset, they will be replaced with [`Register::NoReg`].
+    /// Creates a new [`ITypeArgs`]. If r1 or r2 is unset, they will be replaced with
+    /// [`Register::NoReg`].
     pub fn new(immediate: u16, r1: Option<Register>, r2: Option<Register>) -> Self {
         let r1 = r1.unwrap_or_default();
         let r2 = r2.unwrap_or_default();
@@ -56,8 +59,8 @@ impl ITypeArgs {
 }
 
 impl Encode for ITypeArgs {
-    /// Encodes an I-type instruction from its fields. These must have some unused high-order
-    /// bits set to 0 else the bit shifting logic gets fucked.
+    /// Encodes an I-type instruction from its fields. These must have some unused
+    /// high-order bits set to 0 else the bit shifting logic gets fucked.
     fn encode(self, opcode: u8) -> u32 {
         let opcode = u32::from(opcode);
         let r1 = self.r1 as u32;
@@ -84,7 +87,7 @@ impl TryFrom<u32> for ITypeArgs {
 }
 
 /// Used by instructions not using immediates (besides 5 bit shift values).
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
 pub struct RTypeArgs {
     pub sr1: Register,
     pub sr2: Register,
@@ -95,7 +98,8 @@ pub struct RTypeArgs {
 
 impl RTypeArgs {
     #[must_use]
-    /// Creates a new [`RTypeArgs`]. If any registers are unset, they will be replaced with [`Register::NoReg`]. If `shamt` is unset, it will be set to 0.
+    /// Creates a new [`RTypeArgs`]. If any registers are unset, they will be replaced
+    /// with [`Register::NoReg`]. If `shamt` is unset, it will be set to 0.
     pub fn new(
         sr1: Option<Register>,
         sr2: Option<Register>,
@@ -122,7 +126,8 @@ impl Encode for RTypeArgs {
     ///
     /// # Arguments
     ///
-    /// - `shamt`: The amount to shift value (used only in shift instructions, otherwise 0).
+    /// - `shamt`: The amount to shift value (used only in shift instructions, otherwise
+    ///   0).
     fn encode(self, opcode: u8) -> u32 {
         let opcode = u32::from(opcode);
         let sr1 = self.sr1 as u32;