diff --git a/src/machine.rs b/src/machine.rs
index 4afbd8a..558f936 100644
--- a/src/machine.rs
+++ b/src/machine.rs
@@ -1,24 +1,28 @@
 use std::io::{BufReader, BufWriter, Read, Write};
-
 use crate::instruction::{Instruction, InstructionType};
 
+const CELL_SIZE: u8 = 255;
+const ADDRESS_SIZE: usize = 30000;
+
 pub struct Machine<'a> {
     ip: usize,
     mp: usize,
     buf: [u8; 1],
-    memory: [u32; 50000],
+    memory: [u8; ADDRESS_SIZE],
     code: Vec<Instruction>,
     input: &'a mut BufReader<Box<dyn Read>>,
     output: &'a mut BufWriter<Box<dyn Write>>,
 }
 
 impl<'a> Machine<'a> {
-    pub fn new(code: Vec<Instruction>, input: &'a mut BufReader<Box<dyn Read>>, output: &'a mut BufWriter<Box<dyn Write>>) -> Self {
+    pub fn new(code: Vec<Instruction>,
+               input: &'a mut BufReader<Box<dyn Read>>,
+               output: &'a mut BufWriter<Box<dyn Write>>) -> Self {
         Machine {
             ip: 0,
             mp: 0,
             buf: [0; 1],
-            memory: [0; 50000],
+            memory: [0; ADDRESS_SIZE],
             code,
             input,
             output,
@@ -30,7 +34,7 @@ impl<'a> Machine<'a> {
         while self.ip < self.code.len() {
             // get current instruction
             let instruction = &self.code[self.ip];
-            println!("{:?}", instruction);
+            println!("{:?} ; {}", instruction, self.mp);
             // match type and execute
             match instruction.itype {
                 InstructionType::Increment => self.do_increment(instruction.argument),
@@ -55,21 +59,41 @@ impl<'a> Machine<'a> {
             self.ip += 1;
         }
     }
-
+    
+    // safely increment cell value
     pub fn do_increment(&mut self, argument: usize){
-        self.memory[self.mp] += argument as u32
+        if((self.memory[self.mp] + argument as u8) >= CELL_SIZE ) {
+            self.memory[self.mp] = CELL_SIZE - (self.memory[self.mp] + argument as u8);
+        } else {
+            self.memory[self.mp] += argument as u8;
+        }
     }
-
+    
+    // safely decrement cell value
     pub fn do_decrement(&mut self, argument: usize){
-        self.memory[self.mp] -= argument as u32
-    }
-
-    pub fn do_leftshift(&mut self, argument: usize){
-        self.mp += argument
+        if( self.memory[self.mp] < argument as u8 ){
+            self.memory[self.mp] = CELL_SIZE - argument as u8;
+        } else {
+            self.memory[self.mp] -= argument as u8;
+        }
     }
 
+    // safely increment memory pointer address
     pub fn do_rightshift(&mut self, argument: usize){
-        self.mp -= argument
+        if((self.mp + argument) >= ADDRESS_SIZE ){
+            self.mp = ADDRESS_SIZE - (self.mp + argument);
+        } else {
+            self.mp += argument;
+        }
+    }
+
+    // safely decrement memory pointer address
+    pub fn do_leftshift(&mut self, argument: usize){
+        if( self.mp < argument ) {
+            self.mp = ADDRESS_SIZE - argument;
+        } else {
+            self.mp -= argument;
+        }
     }
 
     pub fn do_get_char(&mut self, argument: usize){
@@ -83,7 +107,7 @@ impl<'a> Machine<'a> {
                 panic!("input read error")
             }
 
-            self.memory[self.mp] = self.buf[0] as u32;
+            self.memory[self.mp] = self.buf[0] as u8;
         }
     }