update comments

os/src/trap/context.rs

@@ -1,25 +1,30 @@
 use riscv::register::sstatus::{self, Sstatus, SPP};
-
+/// Trap Context
 #[repr(C)]
 pub struct TrapContext {
+    /// general regs[0..31]
     pub x: [usize; 32],
+    /// CSR sstatus      
     pub sstatus: Sstatus,
+    /// CSR sepc
     pub sepc: usize,
 }
 
 impl TrapContext {
+    /// set stack pointer to x_2 reg (sp)
     pub fn set_sp(&mut self, sp: usize) {
         self.x[2] = sp;
     }
+    /// init app context
     pub fn app_init_context(entry: usize, sp: usize) -> Self {
-        let mut sstatus = sstatus::read();
-        sstatus.set_spp(SPP::User);
+        let mut sstatus = sstatus::read(); // CSR sstatus
+        sstatus.set_spp(SPP::User); //previous privilege mode: user mode
         let mut cx = Self {
             x: [0; 32],
             sstatus,
-            sepc: entry,
+            sepc: entry, // entry point of app
         };
-        cx.set_sp(sp);
-        cx
+        cx.set_sp(sp); // app's user stack pointer
+        cx // return initial Trap Context of app
     }
 }