diff --git a/hbvm/src/engine/mod.rs b/hbvm/src/engine/mod.rs
index e126928..09865ce 100644
--- a/hbvm/src/engine/mod.rs
+++ b/hbvm/src/engine/mod.rs
@@ -444,15 +444,26 @@ F5-F9 {:016X} {:016X} {:016X} {:016X} {:016X}",
 
                 (255, int) => {
                     trace!("Enviroment Call {}", int);
-                    let ret = self.enviroment_call_table[int as usize](self);
-                    match ret {
-                        Ok(eng) => {
-                            trace!("Resuming execution at {}", eng.index);
+                    let ecall = self.enviroment_call_table[int as usize];
+
+                    match ecall {
+                        Some(call) => {
+                            let ret = call(self);
+
+                            match ret {
+                                Ok(eng) => {
+                                    trace!("Resuming execution at {}", eng.index);
+                                }
+                                Err(err) => {
+                                    return Err(HostError(err));
+                                }
+                            }
                         }
-                        Err(err) => {
-                            return Err(HostError(err));
+                        None => {
+                            return Err(InvalidSystemCall(int));
                         }
                     }
+
                     self.index += 2;
                 }
 
diff --git a/hbvm/src/engine/tests/mod.rs b/hbvm/src/engine/tests/mod.rs
index 6e9aaf2..ff5c6c1 100644
--- a/hbvm/src/engine/tests/mod.rs
+++ b/hbvm/src/engine/tests/mod.rs
@@ -2,6 +2,7 @@ use {
     super::Engine,
     crate::{HaltStatus, RuntimeErrors},
     alloc::vec,
+    RuntimeErrors::*,
 };
 
 #[test]
@@ -30,7 +31,7 @@ fn jump_out_of_bounds() {
     let prog = vec![JUMP as u8, 0, 0, 0, 0, 0, 0, 1, 0];
     let mut eng = Engine::new(prog);
     let ret = eng.run();
-    assert_eq!(ret, Err(RuntimeErrors::InvalidJumpAddress(256)));
+    assert_eq!(ret, Err(InvalidJumpAddress(256)));
 }
 
 #[test]
@@ -38,5 +39,44 @@ fn invalid_system_call() {
     let prog = vec![255, 0];
     let mut eng = Engine::new(prog);
     let ret = eng.run();
-    // assert_eq!(ret, );
+    assert_eq!(ret, Err(InvalidSystemCall(0)));
+}
+
+#[test]
+fn add_u8() {
+    use crate::engine::{Operations::ADD, SubTypes::EightBit};
+
+    let prog = vec![ADD as u8, EightBit as u8, 1, 1, 0xA0];
+    let mut eng = Engine::new(prog);
+    let _ = eng.run();
+    assert_eq!(eng.registers.a0, 2);
+}
+
+#[test]
+fn sub_u8() {
+    use crate::engine::{Operations::SUB, SubTypes::EightBit};
+
+    let prog = vec![SUB as u8, EightBit as u8, 2, 1, 0xA0];
+    let mut eng = Engine::new(prog);
+    let _ = eng.run();
+    assert_eq!(eng.registers.a0, 1);
+}
+#[test]
+fn mul_u8() {
+    use crate::engine::{Operations::MUL, SubTypes::EightBit};
+
+    let prog = vec![MUL as u8, EightBit as u8, 1, 1, 0xA0];
+    let mut eng = Engine::new(prog);
+    let _ = eng.run();
+    assert_eq!(eng.registers.a0, 2);
+}
+
+#[test]
+fn div_u8() {
+    use crate::engine::{Operations::DIV, SubTypes::EightBit};
+
+    let prog = vec![DIV as u8, EightBit as u8, 1, 1, 0xA0];
+    let mut eng = Engine::new(prog);
+    let _ = eng.run();
+    assert_eq!(eng.registers.a0, 2);
 }
diff --git a/hbvm/src/lib.rs b/hbvm/src/lib.rs
index 705c25c..ebed013 100644
--- a/hbvm/src/lib.rs
+++ b/hbvm/src/lib.rs
@@ -12,6 +12,7 @@ pub enum RuntimeErrors {
     HostError(u64),
     PageNotMapped(u64),
     InvalidJumpAddress(u64),
+    InvalidSystemCall(u8),
 }
 
 // If you solve the halting problem feel free to remove this
diff --git a/hbvm/src/main.rs b/hbvm/src/main.rs
index 3c38426..583383a 100644
--- a/hbvm/src/main.rs
+++ b/hbvm/src/main.rs
@@ -7,28 +7,28 @@ use hbvm::{
 fn main() -> Result<(), RuntimeErrors> {
     #[rustfmt::skip]
         let prog: Vec<u8> = vec![
-            NOP as u8, NOP as u8,
-            255, 10,
+            // NOP as u8, NOP as u8,
+            // 255, 10,
             ADD as u8, EightBit as u8, 100, 20, 0xA7,
-            ADD as u8,
-                EightBit as u8, 1, 0, 0xB0,
-            ADD as u8,
-                SixtyFourBit as u8,
-                    0, 0, 0, 0, 0, 0, 0, 0,
-                    0, 0, 0, 0, 0, 0, 0, 2, 0xD0,
-            SUB as u8, EightBit as u8, 255, 0, 0xA7,
-            DIV as u8, EightBit as u8, 12, 5, 0xA8,
-            ADD as u8, Register8 as u8, 0xA7, 0xB0, 0xA7,
-            LOAD as u8, AddrToReg as u8,
-                        0, 0, 0, 0, 0, 0, 0, 2,
-                        0xA0,
-            JUMP as u8, 0, 0, 0, 0, 0, 0, 0, 0,
+            // ADD as u8,
+                // EightBit as u8, 1, 0, 0xB0,
+            // ADD as u8,
+                // SixtyFourBit as u8,
+                    // 0, 0, 0, 0, 0, 0, 0, 0,
+                    // 0, 0, 0, 0, 0, 0, 0, 2, 0xD0,
+            // SUB as u8, EightBit as u8, 255, 0, 0xA7,
+            // DIV as u8, EightBit as u8, 12, 5, 0xA8,
+            // ADD as u8, Register8 as u8, 0xA7, 0xB0, 0xA7,
+            // LOAD as u8, AddrToReg as u8,
+                        // 0, 0, 0, 0, 0, 0, 0, 2,
+                        // 0xA0,
+            // JUMP as u8, 0, 0, 0, 0, 0, 0, 0, 0,
 
         ];
 
     let mut eng = Engine::new(prog);
     // eng.set_timer_callback(time);
-    eng.enviroment_call_table[10] = print_fn;
+    eng.enviroment_call_table[10] = Some(print_fn);
     eng.run()?;
     eng.dump();
     println!("{:#?}", eng.registers);