diff --git a/bits/Bits64.java b/bits/Bits64.java
new file mode 100644
index 0000000..532b5de
--- /dev/null
+++ b/bits/Bits64.java
@@ -0,0 +1,77 @@
+package ch.epfl.javass.bits;
+
+import static ch.epfl.javass.Preconditions.checkArgument;
+
+public final class Bits64 {   
+    private Bits64() {}
+    
+    /**
+     * Creates a bitmask from a start position to a position defined by start + size -1
+     * @param start the index of the first bit of the bitmask
+     * @param size the length of the bitmask
+     * @throws IllegalArgumentException if start and size don't define a subset of 0 to 63
+     * @return the bitmask
+     */
+    public static long mask(int start, int size) {
+        checkStartSize(start, size);
+        if (size == Long.SIZE)
+            return ~0;
+        else
+            return ((1L << size) - 1) << start;
+    }
+    
+    /**
+     * creates a bitstring which has it's size least significant bits equals to those of bits from start to start + size - 1
+     * @param bits the starting bitstring
+     * @param start the index of the first bit of the bitstring to be extracted
+     * @param size of the bitstring to be extracted
+     * @throws IllegalArgumentException if start and size don't define a subset of 0 to 63
+     * @return a bitstring with the exctracted bitstring from bits as its least significant bits
+     */
+    public static long extract(long bits, int start, int size) {
+        checkStartSize(start, size);
+        return (bits >> (start)) & mask(0, size);
+    }
+    
+    /**
+     * Packs two bitstrings(v1 and v2) together in slots of size s1 and s2
+     * @param vi bitsrings to pack
+     * @param si size of each slot
+     * @throws IllegalArgumentException if vi and si aren't between 1 and 64 and if vi > si and if vi+si>64
+     * @return the final bitstring that packs all of the bitstrings together
+     */
+    public static long pack(long v1, int s1, long v2, int s2) {
+        checkArgument(s1 + s2 <= Long.SIZE);
+        checkPack(v1, s1);
+        checkPack(v2, s2);
+        return (v2 << (s1) ) + v1;
+    }
+    
+    /**
+     * Checks if the arguments of the pack functions are >0 and <Long.SIZE and if the size of the bitstring is less than the slot it's destined to
+     * @param v bitstring to check
+     * @param s size to check
+     * @throws IllegalArgumentException if the conditions are not met
+     */
+    private static void checkPack(long v, int s) {
+        if(!(s <= Long.SIZE 
+                && s >= 1
+                && (mask(0,s) & v)==v ))
+            throw new IllegalArgumentException();
+    }
+    
+    /**
+     * Checks if a given start position and size are valid 
+     * meaning they're positive and less than the max value of {@link Long} or throws an exception.
+     * @param start an int that must be positive and inferior to the max value of {@link Long}.
+     * @param size an int that must be positive and inferior to the max value of {@link Integer}.
+     * @throws IllegalArgumentException if start and size don't define a subset of 0 to 63.
+     */
+    private static void checkStartSize(int start, int size) {
+        checkArgument(size <= Long.SIZE 
+                && size >= 0 
+                && start >= 0 
+                && start <= Long.SIZE 
+                && start + size <= Long.SIZE);
+    }
+}