Better comment and mask representation

src/arch/x86_64/paging/entry.rs

@@ -24,14 +24,14 @@ bitflags! {
 }
 
 pub const ADDRESS_MASK: usize = 0x000f_ffff_ffff_f000;
-pub const COUNTER_MASK: u64 = 0x3ff00000_00000000;
+pub const COUNTER_MASK: u64 = 0x3ff0_0000_0000_0000;
 
 impl Entry {
-    /// Zero entry
+    /// Clear entry
     pub fn set_zero(&mut self) {
         self.0 = 0;
     }
-    
+
     /// Is the entry unused?
     pub fn is_unused(&self) -> bool {
         self.0 == (self.0 & COUNTER_MASK)