update ch4 with more comments

os/src/mm/address.rs

@@ -1,7 +1,10 @@
+//! Implementation of physical and virtual address and page number.
+
 use super::PageTableEntry;
 use crate::config::{PAGE_SIZE, PAGE_SIZE_BITS};
 use core::fmt::{self, Debug, Formatter};
 
+/// physical address
 const PA_WIDTH_SV39: usize = 56;
 const VA_WIDTH_SV39: usize = 39;
 const PPN_WIDTH_SV39: usize = PA_WIDTH_SV39 - PAGE_SIZE_BITS;
@@ -11,12 +14,15 @@ const VPN_WIDTH_SV39: usize = VA_WIDTH_SV39 - PAGE_SIZE_BITS;
 #[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
 pub struct PhysAddr(pub usize);
 
+/// virtual address
 #[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
 pub struct VirtAddr(pub usize);
 
+/// physical page number
 #[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
 pub struct PhysPageNum(pub usize);
 
+/// virtual page number
 #[derive(Copy, Clone, Ord, PartialOrd, Eq, PartialEq)]
 pub struct VirtPageNum(pub usize);
 
@@ -176,6 +182,7 @@ impl StepByOne for VirtPageNum {
 }
 
 #[derive(Copy, Clone)]
+/// a simple range structure for type T
 pub struct SimpleRange<T>
 where
     T: StepByOne + Copy + PartialEq + PartialOrd + Debug,
@@ -208,6 +215,7 @@ where
         SimpleRangeIterator::new(self.l, self.r)
     }
 }
+/// iterator for the simple range structure
 pub struct SimpleRangeIterator<T>
 where
     T: StepByOne + Copy + PartialEq + PartialOrd + Debug,
@@ -238,4 +246,6 @@ where
         }
     }
 }
+
+/// a simple range structure for virtual page number
 pub type VPNRange = SimpleRange<VirtPageNum>;