diff --git a/src/bin/ijvdisasm.rs b/src/bin/ijvdisasm.rs
new file mode 100644
index 0000000..e095510
--- /dev/null
+++ b/src/bin/ijvdisasm.rs
@@ -0,0 +1,22 @@
+extern crate rustijvm;
+
+use rustijvm::Disassembly;
+use std::env;
+
+
+fn main() {
+    let args: Vec<String> = env::args().collect();
+
+    if args.len() < 2 {
+        println!("Usage: {} ijvmfile", args[0]);
+        return;
+    }
+
+    let ijvmfile = &args[1];
+
+
+    match Disassembly::disassemble_file(ijvmfile) {
+        Err(e) => eprintln!("{}", e),
+        Ok(d) => println!("{}", d),
+    }
+}
\ No newline at end of file
diff --git a/src/disassembler.rs b/src/disassembler.rs
new file mode 100644
index 0000000..61fa49d
--- /dev/null
+++ b/src/disassembler.rs
@@ -0,0 +1,387 @@
+use block::Block;
+use ijvmreader::IJVMReader;
+use binread::{BinRead, BinReadable};
+use machine::MAGIC_HEADER;
+use Result;
+use ops;
+
+use std::clone::Clone;
+use core::fmt;
+
+type OpCode = u8;
+
+#[derive(Debug, Clone)]
+pub struct Instruction {
+    op: OpCode,
+    name: &'static str,
+    params: Vec<i32>,
+    types: Vec<ops::Args>,
+    wide: bool,
+    pos: usize,
+    label: bool,
+}
+
+impl fmt::Display for Instruction {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if self.label {
+            write!(f, "\nLBL_{}:\n", self.pos)?;
+        }
+
+        write!(f, "    {}", self.name)?;
+
+        for (typ, val) in self.types.iter().zip(&self.params) {
+            match *typ {
+                ops::Args::Label => write!(f, " LBL_{}", val)?,
+                ops::Args::Constant => {
+                    if self.op == 0xB6 {
+                        write!(f, " func_{}", val)?;
+                    } else {
+                        write!(f, " CONST_{}", val)?;
+                    }
+                },
+                ops::Args::Var => write!(f, " var{}", val)?,
+                _ => write!(f, " {}", val)?,
+            }
+        }
+
+        write!(f, "\n")?;
+
+        Ok(())
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Constant {
+    value: i32,
+    method: bool,
+}
+
+impl Constant {
+    fn new(value: i32) -> Self {
+        Self {
+            value,
+            method: false,
+        }
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct Method {
+    name: String,
+    pos: usize,
+    args: usize,
+    vars: usize,
+
+    instructions: Vec<Instruction>,
+    labels: Vec<usize>,
+}
+
+impl Method {
+    pub fn new(i: i32, pos: usize, args: usize, vars: usize) -> Self {
+        let name;
+        if pos == 0 {
+            name = "main".to_string();
+        } else {
+            name = format!("func_{}", i);
+        }
+        Self {
+            name,
+            pos,
+            args,
+            vars,
+
+            instructions: Vec::new(),
+            labels: Vec::new(),
+        }
+    }
+
+    pub fn update_vars(&mut self) {
+        let max = self.instructions
+            .iter()
+            .filter(|e| !e.types.is_empty())
+            .filter(|e| {
+                match e.types[0] {
+                    ops::Args::Var => true,
+                    _ => false,
+                }
+            })
+            .map(|e| e.params[0] )
+            .max();
+
+        if let Some(v) = max {
+            self.vars = v as usize + 1;
+        }
+    }
+
+    pub fn mark_labels(&mut self) {
+        for label in &self.labels {
+            let opt = self.instructions.iter_mut().find(|e| e.pos == *label);
+            if let Some(instruction) = opt {
+                instruction.label = true;
+            }
+        }
+    }
+}
+
+impl fmt::Display for Method {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut main_var_off = 0;
+        if self.name == "main" {
+            write!(f, ".main\n")?;
+            main_var_off = 1;
+        } else {
+            write!(f, ".method {}(", self.name)?;
+            let mut comma = "";
+            for i in 1..=self.args {
+                write!(f, "{}var{}", comma, i)?;
+                comma = ", ";
+            }
+            write!(f,")\n")?;
+        }
+
+        if self.vars > 0 {
+            write!(f, "\n.var\n")?;
+            for i in 1..=self.vars {
+                write!(f, "    var{}\n", i + self.args - main_var_off)?;
+            }
+            write!(f, ".end-var\n")?;
+        }
+
+        write!(f, "\n")?;
+
+        for inst in &self.instructions {
+            write!(f, "{}", inst)?;
+        }
+
+
+        if self.name == "main" {
+            write!(f, "\n.end-main\n\n")?;
+        } else {
+            write!(f, "\n.end-method\n\n")?;
+        }
+
+        Ok(())
+    }
+}
+
+#[derive(Debug)]
+pub struct Disassembler {
+    disassembly: Disassembly,
+    text: Block,
+    pool: Block,
+    suspects: Vec<usize>,
+    found: Vec<usize>,
+}
+
+impl Disassembler {
+    pub fn new(text: Block, pool: Block) -> Disassembler {
+        Disassembler {
+            disassembly: Disassembly::default(),
+            text,
+            pool,
+            suspects: Vec::new(),
+            found: Vec::new(),
+        }
+    }
+
+    fn is_method(&self, addr: usize) -> bool {
+        self.suspects.contains(&addr)
+    }
+
+    fn disasm_instruction(&mut self, method: &mut Method) -> Result<()> {
+        let pos = self.text.cur();
+        let mut opcode = self.text.read_u8()?;
+        let mut wide = false;
+        let mut params = Vec::new();
+
+        // WIDE
+        if opcode == 0xC4 {
+            wide = true;
+            opcode = self.text.read_u8()?;
+        }
+
+        let (name, args) = match ops::num_to_op(opcode) {
+            ops::Operation::Invalid(x) => {
+                eprintln!("INVALID OP 0x{:X}\n", x);
+                return Err("Invalid operation");
+            },
+            ops::Operation::Op(name, _, args) => (name, args),
+        };
+
+        for arg in args.clone() {
+            let v = match arg {
+                ops::Args::Byte => self.text.read_i8()? as i32,
+                ops::Args::Short => self.text.read_i16()? as i32,
+                ops::Args::Var => {
+                    if wide {
+                        self.text.read_u16()? as i32
+                    } else {
+                        self.text.read_u8()? as i32
+                    }
+                },
+                ops::Args::Label => {
+                    let offset = self.text.read_i16()?;
+                    let target = pos as i64 + offset as i64;
+                    if target < 0 {
+                        return Err("Invalid jump offset");
+                    }
+                    method.labels.push(target as usize);
+                    target as i32
+                },
+                ops::Args::Constant => self.text.read_u16()? as i32,
+            };
+            params.push(v);
+        }
+
+        let inst = Instruction {
+            op: opcode,
+            name,
+            params,
+            types: args,
+            wide,
+            pos,
+            label: false,
+        };
+
+        // eprintln!("i 0x{:x} ({}) @ {}", opcode, name, pos);
+
+        // INVOKEVIRTUAL
+        if opcode == 0xB6 {
+            let target_addr: usize;
+
+            {
+                let addr = &mut self.disassembly.constants[inst.params[0] as usize];
+                if !addr.method {
+                    addr.method = true;
+                }
+                target_addr = addr.value as usize;
+            }
+
+            if !self.found.contains(&target_addr) {
+                self.found.push(target_addr);
+                let save_pos = self.text.cur();
+                self.text.seek(target_addr)?;
+
+                let arg_count = (self.text.read_u16()? - 1) as usize;
+                let var_count = self.text.read_u16()? as usize;
+
+                let callee = self.disasm_method(inst.params[0], target_addr, arg_count, var_count)?;
+                self.disassembly.methods.push(callee);
+
+                self.text.seek(save_pos)?;
+            }
+        }
+
+        method.instructions.push(inst);
+
+        Ok(())
+    }
+
+    fn disasm_method(&mut self, i: i32, pc: usize, args: usize, vars: usize) -> Result<Method> {
+        let mut method = Method::new(i, pc, args, vars);
+
+        while self.text.has_i8() && !self.is_method(self.text.cur()) {
+            self.disasm_instruction(&mut method)?;
+        }
+
+        method.mark_labels();
+
+        Ok(method)
+    }
+
+    fn find_methods(&mut self) -> Result<Vec<usize>> {
+        let mut res = Vec::new();
+
+        while self.text.has_i8() {
+            let op = self.text.read_u8()?;
+            // INVOKEVIRTUAL
+            if op == 0xB6 {
+                let constant_index = self.text.read_u16()? as usize;
+
+                let constant: &Constant = match self.disassembly.constants.get(constant_index) {
+                    Some(x) => x,
+                    None => continue,
+                };
+
+                let v = self.text[constant.value as usize];
+                if v == 0 {
+                    res.push(constant.value as usize);
+                }
+            }
+        }
+        self.text.seek(0)?;
+
+        Ok(res)
+    }
+
+    pub fn disassemble(&mut self) -> Result<Disassembly> {
+        while self.pool.has_i32() {
+            self.disassembly.constants.push(Constant::new(self.pool.read_i32()?));
+        }
+
+        self.suspects = self.find_methods()?;
+
+        let mut main = self.disasm_method(-1, 0, 0, 0)?;
+        main.update_vars();
+        self.disassembly.methods.insert(0, main);
+
+
+        Ok(self.disassembly.clone())
+    }
+}
+
+#[derive(Debug, Default, Clone)]
+pub struct Disassembly {
+    constants: Vec<Constant>,
+    methods: Vec<Method>,
+}
+
+impl Disassembly {
+    pub fn disassemble_reader(mut reader: IJVMReader) -> Result<Disassembly> {
+        let magic = reader.read_u32()?;
+        if magic != MAGIC_HEADER {
+            return Err("Invalid magic header");
+        }
+        let pool = match reader.read_block() {
+            Ok(a) => a,
+            Err(_) => return Err("Failed to read constants block")
+        };
+        let text = match reader.read_block() {
+            Ok(block) => block,
+            Err(_) => return Err("Failed to read text block")
+        };
+
+        Disassembler::new(text, pool).disassemble()
+    }
+
+    pub fn disassemble_file(file: &str) -> Result<Disassembly> {
+        let reader = IJVMReader::new(file)?;
+        Self::disassemble_reader(reader)
+    }
+
+    pub fn new_from_slice(source: &[u8]) -> Result<Disassembly> {
+        let reader = IJVMReader::new_from_slice(source);
+        Self::disassemble_reader(reader)
+    }
+
+}
+
+impl fmt::Display for Disassembly {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        if self.constants.len() > 0 {
+            write!(f, ".constant\n")?;
+            for (i, c)  in self.constants.iter().enumerate() {
+                if !c.method {
+                    write!(f, "    CONST_{:<6} {}\n", i, c.value)?;
+                }
+            }
+            write!(f, ".end-constant\n\n")?;
+        }
+
+        for method in &self.methods {
+            write!(f, "{}", method)?;
+        }
+
+        Ok(())
+    }
+}
diff --git a/src/lib.rs b/src/lib.rs
index 00e5d8a..caadf4e 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,8 +1,9 @@
+extern crate core;
+
 pub mod ijvmreader;
 pub mod binread;
 pub mod block;
 pub mod ops;
-pub mod machine;
 pub mod stack;
 pub mod pool;
 pub mod frame;
@@ -14,4 +15,8 @@ pub mod heap;
 
 type Result<T> = ::std::result::Result<T, &'static str>;
 
+pub mod machine;
 pub use machine::Machine;
+
+pub mod disassembler;
+pub use disassembler::{Disassembler, Disassembly};
diff --git a/src/machine.rs b/src/machine.rs
index 1482cd5..5a74f64 100644
--- a/src/machine.rs
+++ b/src/machine.rs
@@ -15,7 +15,7 @@ use netstack::NetStack;
 #[cfg(feature = "bonus:heap")]
 use heap::Heap;
 
-const MAGIC_HEADER:u32 = 0x1DEA_DFAD;
+pub const MAGIC_HEADER:u32 = 0x1DEA_DFAD;
 const ANTI_BS_SIZE:usize = 0xFF;
 
 pub struct Machine {
@@ -85,7 +85,7 @@ impl Machine {
 
     pub fn step(&mut self) -> Result<()> {
         match self.block.read_op() {
-            Ok(Operation::Op(a, func)) => {
+            Ok(Operation::Op(a, func, _)) => {
                 if cfg!(feature = "debug:instr") {
                     println!("{}", a);
                     println!("Stack: {:?}", self.cur_frame().stack.data);
diff --git a/src/ops.rs b/src/ops.rs
index b4ce7be..09440af 100644
--- a/src/ops.rs
+++ b/src/ops.rs
@@ -6,8 +6,17 @@ use std::io::{Read};
 
 pub type OpFunc = fn(&mut Machine) -> Result<()>;
 
+#[derive(Debug, Clone)]
+pub enum Args {
+    Byte,
+    Short,
+    Var,
+    Label,
+    Constant,
+}
+
 pub enum Operation {
-    Op(&'static str, OpFunc),
+    Op(&'static str, OpFunc, Vec<Args>),
     Invalid(u8)
 }
 
@@ -15,50 +24,51 @@ const JUMP_OFFSET: i32 = 3;
 
 pub fn num_to_op(op: u8) -> Operation {
     match op {
-        0x00 => Operation::Op("NOP", nop),
-        0x10 => Operation::Op("BIPUSH", bipush),
-        0x13 => Operation::Op("LDC_W", ldc_w),
-        0x15 => Operation::Op("ILOAD", iload),
-        0x36 => Operation::Op("ISTORE", istore),
-        0x57 => Operation::Op("POP", pop),
-        0x59 => Operation::Op("DUP", dup),
-        0x5F => Operation::Op("SWAP", swap),
-        0x60 => Operation::Op("IADD", iadd),
-        0x64 => Operation::Op("ISUB", isub),
-        0x7E => Operation::Op("IAND", iand),
-        0xB0 => Operation::Op("IOR", ior),
-        0x84 => Operation::Op("IINC", iinc),
-        0x99 => Operation::Op("IFEQ", ifeq),
-        0x9b => Operation::Op("IFLT", iflt),
-        0x9F => Operation::Op("IF_ICMPEQ", if_icmpeq),
-        0xA7 => Operation::Op("GOTO", goto),
-        0xAC => Operation::Op("IRETURN", ireturn),
-        0xB6 => Operation::Op("INVOKEVIRTUAL", invokevirtual),
-        0xC4 => Operation::Op("WIDE", wide),
-        0xFC => Operation::Op("IN", _in),
-        0xFD => Operation::Op("OUT", out),
-        0xFF => Operation::Op("HALT", halt),
+        0x00 => Operation::Op("NOP", nop, vec![]),
+        0x10 => Operation::Op("BIPUSH", bipush, vec![Args::Byte]),
+        0x13 => Operation::Op("LDC_W", ldc_w, vec![Args::Constant]),
+        0x15 => Operation::Op("ILOAD", iload, vec![Args::Var]),
+        0x36 => Operation::Op("ISTORE", istore, vec![Args::Var]),
+        0x57 => Operation::Op("POP", pop, vec![]),
+        0x59 => Operation::Op("DUP", dup, vec![]),
+        0x5F => Operation::Op("SWAP", swap, vec![]),
+        0x60 => Operation::Op("IADD", iadd, vec![]),
+        0x64 => Operation::Op("ISUB", isub, vec![]),
+        0x7E => Operation::Op("IAND", iand, vec![]),
+        0xB0 => Operation::Op("IOR", ior, vec![]),
+        0x84 => Operation::Op("IINC", iinc, vec![Args::Var, Args::Byte]),
+        0x99 => Operation::Op("IFEQ", ifeq, vec![Args::Label]),
+        0x9b => Operation::Op("IFLT", iflt, vec![Args::Label]),
+        0x9F => Operation::Op("IF_ICMPEQ", if_icmpeq, vec![Args::Label]),
+        0xA7 => Operation::Op("GOTO", goto, vec![Args::Label]),
+        0xAC => Operation::Op("IRETURN", ireturn, vec![]),
+        0xB6 => Operation::Op("INVOKEVIRTUAL", invokevirtual, vec![Args::Constant]),
+        0xC4 => Operation::Op("WIDE", wide, vec![]),
+        0xFC => Operation::Op("IN", _in, vec![]),
+        0xFD => Operation::Op("OUT", out, vec![]),
+        0xFE => Operation::Op("ERR", err, vec![]),
+        0xFF => Operation::Op("HALT", halt, vec![]),
 
         #[cfg(feature = "extra:sleep")]
-        0xF0 => Operation::Op("SLP", slp),
+        0xF0 => Operation::Op("SLP", slp, vec![Args::Byte]),
 
         #[cfg(feature = "bonus:network")]
-        0xE1 => Operation::Op("NETBIND", netbind),
+        0xE1 => Operation::Op("NETBIND", netbind, vec![]),
         #[cfg(feature = "bonus:network")]
-        0xE2 => Operation::Op("NETCONNECT", netconnect),
+        0xE2 => Operation::Op("NETCONNECT", netconnect, vec![]),
         #[cfg(feature = "bonus:network")]
-        0xE3 => Operation::Op("NETIN", netin),
+        0xE3 => Operation::Op("NETIN", netin, vec![]),
         #[cfg(feature = "bonus:network")]
-        0xE4 => Operation::Op("NETOUT", netout),
+        0xE4 => Operation::Op("NETOUT", netout, vec![]),
         #[cfg(feature = "bonus:network")]
-        0xE5 => Operation::Op("NETCLOSE", netclose),
+        0xE5 => Operation::Op("NETCLOSE", netclose, vec![]),
 
         #[cfg(feature = "bonus:heap")]
-        0xD1 => Operation::Op("NEWARRAY", newarray),
+        0xD1 => Operation::Op("NEWARRAY", newarray, vec![]),
         #[cfg(feature = "bonus:heap")]
-        0xD2 => Operation::Op("IALOAD", iaload),
+        0xD2 => Operation::Op("IALOAD", iaload, vec![]),
         #[cfg(feature = "bonus:heap")]
-        0xD3 => Operation::Op("IASTORE", iastore),
+        0xD3 => Operation::Op("IASTORE", iastore, vec![]),
 
         x => Operation::Invalid(x)
     }
@@ -126,6 +136,12 @@ fn goto(machine: &mut Machine) -> Result<()> {
     machine.block.jump(offset)
 }
 
+fn err(machine: &mut Machine) -> Result<()> {
+    machine.halted = true;
+    eprintln!("MACHINE CALLED ERR");
+    Ok(())
+}
+
 fn halt(machine: &mut Machine) -> Result<()> {
     machine.halted = true;
     Ok(())