add: finish basic work

os/src/trap/context.rs

@@ -0,0 +1,29 @@
+use riscv::register::sstatus::{self, Sstatus, SPP};
+
+#[repr(C)]
+pub struct TrapContext {
+    pub x: [usize; 32],
+    pub s_status: Sstatus,
+    pub sepc: usize
+}
+
+impl TrapContext {
+    pub unsafe fn init_application_context(entry: usize, sp: usize) -> Self {
+        let sstatus = sstatus::read();
+        sstatus::set_spp(SPP::User);
+        let mut context = Self {
+            x: [0; 32],
+            s_status: sstatus,
+            sepc: entry
+        };
+
+        context.set_sp(sp);
+        context
+    }
+
+    pub fn set_sp(&mut self, sp: usize) {
+        // x2 is stack pointer register
+        self.x[2] = sp;
+    }
+}
+