diff --git a/src/vm/error.rs b/src/vm/error.rs
index dc52452..f0841df 100644
--- a/src/vm/error.rs
+++ b/src/vm/error.rs
@@ -1,8 +1,11 @@
-use crate::vm::vm::*;
+use crate::vm::{vm::*, reg::Reg};
 use snafu::Snafu;
 
 #[derive(Snafu, Debug, Clone)]
 pub enum VmError {
+    #[snafu(display("illegal register: 0x{:02x}", reg))]
+    IllegalReg { reg: Reg },
+    #[snafu(display("memory address out of bounds: 0x{:016x}", addr))]
     MemOutOfBounds { addr: Addr },
 }
 
diff --git a/src/vm/obj/assemble/mod.rs b/src/vm/obj/assemble/mod.rs
index 01bb26f..6425827 100644
--- a/src/vm/obj/assemble/mod.rs
+++ b/src/vm/obj/assemble/mod.rs
@@ -54,7 +54,6 @@ impl<'a> Assemble<'a> {
                     });
                 }
             }
-
             self.symbols.globals_mut().extend(exports);
         }
 
@@ -151,6 +150,8 @@ impl<'a> Assemble<'a> {
                         }
                     }
                 }
+                // TODO : make sure we aren't trying to export anything that doesn't exist
+
                 // only return exports if specified
                 if export {
                     labels.retain(|k, _| exports.contains(k));
diff --git a/src/vm/tick.rs b/src/vm/tick.rs
index d6ef601..4a2dd0b 100644
--- a/src/vm/tick.rs
+++ b/src/vm/tick.rs
@@ -10,7 +10,7 @@ impl Vm {
             ($mapping:expr) => {{
                 let (r1, r2) = cursor.next_regs()?;
                 next_ip = cursor.position();
-                let value = ($mapping)(self.get_reg(r1), self.get_reg(r2));
+                let value = ($mapping)(self.get_reg_checked(r1)?, self.get_reg_checked(r2)?);
                 self.set_reg(r1, value);
             }};
         }
@@ -25,7 +25,7 @@ impl Vm {
             INV => {
                 let r1 = cursor.next_reg()?;
                 next_ip = cursor.position();
-                let value = self.get_reg(r1);
+                let value = self.get_reg_checked(r1)?;
                 self.set_reg(r1, !value);
             }
             NOT => {
@@ -80,8 +80,8 @@ impl Vm {
             LOAD => {
                 let (r1, r2) = cursor.next_regs()?;
                 next_ip = cursor.position();
-                let value = self.load(r2);
-                self.set_reg(r1, value);
+                let value = self.load(r2)?;
+                self.set_reg_checked(r1, value)?;
             }
             REGCOPY => math_inst!(|_: u64, w2: u64| w2),
             STOREIMM64 => {
@@ -101,14 +101,14 @@ impl Vm {
             MEMCOPY => {
                 let (r1, r2) = cursor.next_regs()?;
                 next_ip = cursor.position();
-                let value = self.load(r2);
-                self.store(r1, value);
+                let value = self.load(r2)?;
+                self.store(r1, value)?;
             }
             STORE => {
                 let (r1, r2) = cursor.next_regs()?;
                 next_ip = cursor.position();
                 let value = self.get_reg(r1);
-                self.store(r2, value);
+                self.store(r2, value)?;
             }
             HALT => {
                 next_ip = cursor.position();
diff --git a/src/vm/vm.rs b/src/vm/vm.rs
index 4d9fb17..53ab09b 100644
--- a/src/vm/vm.rs
+++ b/src/vm/vm.rs
@@ -1,5 +1,5 @@
 use crate::vm::{error::*, flags::*, mem::*, reg::*};
-use byteorder::{ReadBytesExt, WriteBytesExt, LE};
+use byteorder::{WriteBytesExt, LE};
 use std::{io::Cursor, mem};
 
 pub type Word = u64;
@@ -18,7 +18,6 @@ impl Vm {
     }
 
     pub fn mem_cursor(&self, at: usize) -> MemCursor {
-        // TODO : MemCursor
         MemCursor::new(&self.mem[at..])
     }
 
@@ -37,59 +36,62 @@ impl Vm {
         self.flags().contains(Flags::HALT)
     }
 
-    pub fn get_word(&self, addr: Word) -> Word {
-        let mut reader = Cursor::new(&self.mem[(addr as usize)..]);
-        reader
-            .read_u64::<LE>()
-            .expect("word outside of address range")
+    pub fn get_word(&self, addr: Addr) -> Result<Word> {
+        self.check_read(addr, 8)?;
+        Ok(self.mem_cursor(addr as usize).next_u64().unwrap())
     }
 
-    pub fn get_halfword(&self, addr: Word) -> HalfWord {
-        let mut reader = Cursor::new(&self.mem[(addr as usize)..]);
-        reader
-            .read_u32::<LE>()
-            .expect("word outside of address range")
+    pub fn get_halfword(&self, addr: Addr) -> Result<HalfWord> {
+        self.check_read(addr, 4)?;
+        Ok(self.mem_cursor(addr as usize).next_u32().unwrap())
     }
 
-    pub fn get_byte(&self, addr: Word) -> u8 {
-        let mut reader = Cursor::new(&self.mem[(addr as usize)..]);
-        reader.read_u8().expect("word outside of address range")
+    pub fn get_byte(&self, addr: Addr) -> Result<u8> {
+        self.check_addr(addr)?;
+        Ok(self.mem_cursor(addr as usize).next_u8().unwrap())
     }
 
-    pub fn set_word(&mut self, addr: Word, value: Word) {
-        let mut writer = Cursor::new(&mut self.mem[(addr as usize)..]);
-        writer
-            .write_u64::<LE>(value)
-            .expect("word outside of address range");
+    pub fn set_word(&mut self, addr: Addr, value: Word) -> Result<()> {
+        self.check_read(addr, 8)?;
+        let mut cursor = Cursor::new(&mut self.mem[addr as usize..]);
+        Ok(cursor.write_u64::<LE>(value).unwrap())
     }
 
-    pub fn set_halfword(&mut self, addr: Word, value: HalfWord) {
-        let mut writer = Cursor::new(&mut self.mem[(addr as usize)..]);
-        writer
-            .write_u32::<LE>(value)
-            .expect("word outside of address range");
+    pub fn set_halfword(&mut self, addr: Addr, value: HalfWord) -> Result<()> {
+        self.check_read(addr, 4)?;
+        let mut cursor = Cursor::new(&mut self.mem[addr as usize..]);
+        Ok(cursor.write_u32::<LE>(value).unwrap())
     }
 
-    pub fn set_byte(&mut self, addr: Word, value: u8) {
-        let mut writer = Cursor::new(&mut self.mem[(addr as usize)..]);
-        writer
-            .write_u8(value)
-            .expect("word outside of address range");
+    pub fn set_byte(&mut self, addr: Addr, value: u8) -> Result<()> {
+        self.check_addr(addr)?;
+        let mut cursor = Cursor::new(&mut self.mem[addr as usize..]);
+        Ok(cursor.write_u8(value).unwrap())
     }
 
-    pub fn load(&self, reg: Reg) -> Word {
-        self.get_word(self.get_reg(reg))
+    pub fn load(&self, reg: Reg) -> Result<Word> {
+        self.get_word(self.get_reg_checked(reg)?)
     }
 
-    pub fn store(&mut self, reg: Reg, value: Word) {
-        let addr = self.get_reg(reg);
-        self.set_word(addr, value);
+    pub fn store(&mut self, reg: Reg, value: Word) -> Result<()> {
+        let addr = self.get_reg_checked(reg)?;
+        self.set_word(addr, value)
+    }
+
+    pub fn get_reg_checked(&self, reg: Reg) -> Result<Word> {
+        self.check_reg(reg)?;
+        Ok(self.get_reg(reg))
     }
 
     pub fn get_reg(&self, reg: Reg) -> Word {
         self.registers[reg as usize]
     }
 
+    pub fn set_reg_checked(&mut self, reg: Reg, value: Word) -> Result<Word> {
+        self.check_reg(reg)?;
+        Ok(self.set_reg(reg, value))
+    }
+
     pub fn set_reg(&mut self, reg: Reg, value: Word) -> Word {
         mem::replace(&mut self.registers[reg as usize], value)
     }
@@ -119,4 +121,25 @@ impl Vm {
     pub fn set_flags(&mut self, flags: Flags) {
         self.set_reg(FLAGS, flags.bits());
     }
+
+    fn check_addr(&self, addr: Addr) -> Result<()> {
+        if addr >= (self.mem.len() as u64) {
+            Err(VmError::MemOutOfBounds { addr })
+        } else {
+            Ok(())
+        }
+    }
+
+    fn check_read(&self, addr: Addr, len: Word) -> Result<()> {
+        self.check_addr(addr)
+            .and_then(|_| self.check_addr(addr + len - 1))
+    }
+
+    fn check_reg(&self, reg: Reg) -> Result<()> {
+        if reg > LAST_REG {
+            Err(VmError::IllegalReg { reg })
+        } else {
+            Ok(())
+        }
+    }
 }