diff --git a/hbvm/src/bytecode.rs b/hbvm/src/bytecode.rs
index 12129b36..a9770c63 100644
--- a/hbvm/src/bytecode.rs
+++ b/hbvm/src/bytecode.rs
@@ -9,7 +9,7 @@ macro_rules! constmod {
 }
 
 constmod!(pub opcode(u8) {
-    NOP = 0, // _
+    NOP = 0, // N
     ADD = 1, // RRR
     SUB = 2, // RRR
     MUL = 3, // RRR
@@ -37,8 +37,8 @@ constmod!(pub opcode(u8) {
 
     JMP     = 100, // I
     JMPCOND = 101, // I
-    RET     = 103, // _
-    ECALL   = 255, // _
+    RET     = 103, // N
+    ECALL   = 255, // N
 });
 
 #[repr(packed)] pub struct ParamRRR(pub u8, pub u8, pub u8);
diff --git a/hbvm/src/main.rs b/hbvm/src/main.rs
index 27895f39..5ee03afe 100644
--- a/hbvm/src/main.rs
+++ b/hbvm/src/main.rs
@@ -1,13 +1,18 @@
-use hbvm::{vm::Vm, RuntimeErrors};
+use hbvm::{validate::validate, vm::Vm, RuntimeErrors};
 
 fn main() -> Result<(), RuntimeErrors> {
     // TODO: Grab program from cmdline
     #[rustfmt::skip]
     let prog = &[];
 
-    unsafe {
-        let mut vm = Vm::new_unchecked(prog);
-        vm.run();
+    if let Err(e) = validate(prog) {
+        eprintln!("Program validation error: {e:?}");
+        return Ok(());
+    } else {
+        unsafe {
+            let mut vm = Vm::new_unchecked(prog);
+            vm.run();
+        }
     }
     Ok(())
 }
diff --git a/hbvm/src/validate.rs b/hbvm/src/validate.rs
index e69de29b..d6aacaa4 100644
--- a/hbvm/src/validate.rs
+++ b/hbvm/src/validate.rs
@@ -0,0 +1,68 @@
+macro_rules! bail {
+    ($kind:ident, $start:expr, $curr:expr, $offset:expr) => {
+        return Err(Error {
+            kind: ErrorKind::$kind,
+            index: ($curr.as_ptr() as usize) - ($start.as_ptr() as usize) + $offset
+        })
+    };
+    ($kind:ident, $start:expr, $curr:expr) => {
+        bail!($kind, $start, $curr, 0)
+    };
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum ErrorKind {
+    InvalidInstruction,
+    InvalidRegister,
+    Unimplemented,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub struct Error {
+    kind:  ErrorKind,
+    index: usize,
+}
+
+pub fn validate(mut program: &[u8]) -> Result<(), Error> {
+    use crate::bytecode::opcode::*;
+
+    #[inline]
+    fn reg(regs: &[u8]) -> Option<usize> {
+        regs.iter()
+            .enumerate()
+            .filter_map(|(n, &r)| (r > 59).then_some(n))
+            .next()
+    }
+
+    let start = program;
+    loop {
+        program = match program {
+            [] => return Ok(()),
+            // N
+            [NOP | RET | ECALL, rest @ ..] => rest,
+            // RRR
+            [ADD..=XOR | ADDF..=DIVF, _, _, _, rest @ ..] => {
+                if let Some(n) = reg(&program[1..=3]) {
+                    bail!(InvalidRegister, start, program, n + 1)
+                }
+                rest
+            }
+            // RR
+            [NOT, _, _, rest @ ..] => {
+                if let Some(n) = reg(&program[1..=2]) {
+                    bail!(InvalidRegister, start, program, n + 1)
+                }
+                rest
+            }
+            // RI
+            [LI, reg, _, _, _, _, _, _, _, _, rest @ ..] => {
+                if *reg > 59 {
+                    bail!(InvalidRegister, start, program, 1)
+                }
+                rest
+            }
+            [LD | ST, ..] => bail!(Unimplemented, start, program),
+            _ => bail!(InvalidInstruction, start, program),
+        }
+    }
+}