diff --git a/simu/Cargo.toml b/simu/Cargo.toml
index 4c4fe61..18de5a6 100644
--- a/simu/Cargo.toml
+++ b/simu/Cargo.toml
@@ -18,3 +18,7 @@ rgba = []
 rich_keyboard = []
 debug = ["dep:parse_int"]
 futex = ["dep:wait_on_address"]
+
+# [[bench]]
+# name = "bench"
+# harness = false
diff --git a/simu/src/cpu.rs b/simu/src/cpu.rs
index 1f7eb91..08b61a1 100644
--- a/simu/src/cpu.rs
+++ b/simu/src/cpu.rs
@@ -32,17 +32,26 @@ enum Op2 {
 }
 
 struct Reg(u8);
+
+#[allow(unused)] //constructed by transmute
+#[repr(u8)]
 enum Cond {
-    Ifeq,
-    Ifne,
-    Iflt,
-    Ifge,
-    Ifgt,
-    Ifle,
-    Ifult,
-    Ifuge,
-    Ifugt,
-    Ifule,
+    Ifeq = 0b000,
+    Ifne = 0b0001,
+    UK1 = 2,
+    UK2 = 3,
+    UK3 = 4,
+    UK4 = 5,
+    UK5 = 6,
+    UK6 = 7,
+    Iflt = 0b1000,
+    Ifge = 0b1001,
+    Ifgt = 0b1010,
+    Ifle = 0b1011,
+    Ifult = 0b1100,
+    Ifuge = 0b1101,
+    Ifugt = 0b1110,
+    Ifule = 0b1111,
 }
 
 #[derive(Debug)]
@@ -73,20 +82,10 @@ pub enum MMIOInterupt {
 }
 
 impl From<u8> for Cond {
+    #[inline(always)]
     fn from(value: u8) -> Self {
-        match value {
-            0b0000 => Cond::Ifeq,
-            0b0001 => Cond::Ifne,
-            0b1000 => Cond::Iflt,
-            0b1001 => Cond::Ifge,
-            0b1010 => Cond::Ifgt,
-            0b1011 => Cond::Ifle,
-            0b1100 => Cond::Ifult,
-            0b1101 => Cond::Ifuge,
-            0b1110 => Cond::Ifugt,
-            0b1111 => Cond::Ifule,
-            _ => iot(),
-        }
+        //unsafe if called with value >= 15 buuut only ever called on actually an u4 so it's ok
+        unsafe { transmute(value) }
     }
 }
 
@@ -108,8 +107,9 @@ impl From<u32> for InteruptKind {
 }
 
 impl Cond {
-    fn eval(self, a: u32, b: u32) -> bool {
-        match self {
+    #[inline(always)]
+    fn eval(self, a: u32, b: u32) -> Option<bool> {
+        Some(match self {
             Cond::Ifeq => a == b,
             Cond::Ifne => a != b,
             Cond::Iflt => (a as i32) < (b as i32),
@@ -120,7 +120,8 @@ impl Cond {
             Cond::Ifuge => a >= b,
             Cond::Ifugt => a > b,
             Cond::Ifule => a <= b,
-        }
+            _ => return None
+        })
     }
 }
 
@@ -274,6 +275,7 @@ impl Display for Cond {
             Cond::Ifuge => write!(f, "ifuge"),
             Cond::Ifugt => write!(f, "ifugt"),
             Cond::Ifule => write!(f, "ifule"),
+            _ => write!(f, "unknown"),
         }
     }
 }
@@ -354,10 +356,6 @@ impl IndexMut<Reg> for Computer {
     }
 }
 
-fn iot() -> ! {
-    exit(1);
-}
-
 impl Computer {
     pub fn new(filename: String) -> Self {
         let mut new = Self {
@@ -586,8 +584,12 @@ impl Computer {
                         }
                         Instruction::Skip(d, cond, reg, op2) => {
                             self.pc += 1;
-                            if cond.eval(self[reg], self.resolve(op2)) {
-                                self.pc += d as usize
+                            match cond.eval(self[reg], self.resolve(op2)) {
+                                Some(false) => {/*Nothing*/}
+                                Some(true) => {self.pc += d as usize}
+                                None => {
+                                    cold_path();
+                                    self.serve_interupt(InteruptKind::IllegalOpcode, [next_opcode])}
                             }
                         }
                         Instruction::Jump(addr) => {
@@ -743,6 +745,7 @@ impl Computer {
             }
         }
     }
+    #[inline(always)]
     fn resolve(&self, op2: Op2) -> u32 {
         match op2 {
             Op2::Direct(v) => v,
diff --git a/simu/src/main.rs b/simu/src/main.rs
index 105a301..5131663 100644
--- a/simu/src/main.rs
+++ b/simu/src/main.rs
@@ -300,6 +300,8 @@ fn main() -> Result<(), Error> {
             loop {
                 simulation.step(64);
             }
+
+            //ugly debug code, I should improve that using a real TUI crate
             #[cfg(feature = "debug")]
             {
                 let mut input = std::io::stdin().lines();