Modify the interrupt vector

05-Preemptive/trap.c

@@ -0,0 +1,48 @@
+#include "os.h"
+extern void trap_vector();
+
+void trap_init()
+{
+  // set the machine-mode trap handler.
+  w_mtvec((reg_t)trap_vector);
+
+  // enable machine-mode interrupts.
+  w_mstatus(r_mstatus() | MSTATUS_MIE);
+}
+
+reg_t trap_handler(reg_t epc, reg_t cause)
+{
+  reg_t return_pc = epc;
+  reg_t cause_code = cause & 0xfff;
+
+  if (cause & 0x80000000)
+  {
+    /* Asynchronous trap - interrupt */
+    switch (cause_code)
+    {
+    case 3:
+      lib_puts("software interruption!\n");
+      break;
+    case 7:
+      lib_puts("timer interruption!\n");
+      timer_handler();
+      break;
+    case 11:
+      lib_puts("external interruption!\n");
+      break;
+    default:
+      lib_puts("unknown async exception!\n");
+      break;
+    }
+  }
+  else
+  {
+    /* Synchronous trap - exception */
+    lib_puts("Sync exceptions!\n");
+    while (1)
+    {
+      /* code */
+    }
+  }
+  return return_pc;
+}