1 files changed, 21 insertions, 2 deletions
diff --git a/kernel/trap.c b/kernel/trap.c
index 512c850..eb8009f 100644
--- a/kernel/trap.c
+++ b/kernel/trap.c
@@ -6,6 +6,12 @@
 #include "proc.h"
 #include "defs.h"
 
+/*
+ * Always remember that RISC-V disables interrupts when it starts to take a trap,
+ * so there's no need to call intr_off() at the beginning of trap handling.
+ * Reference: xv6-riscv-book 4.5
+ */
+
 struct spinlock tickslock;
 uint ticks;
 
@@ -76,9 +82,22 @@ usertrap(void)
   if(killed(p))
     exit(-1);
 
-  // give up the CPU if this is a timer interrupt.
-  if(which_dev == 2)
+  if(which_dev == 2){
+    // timer interrupt
+    if(p->alarm_interval > 0 && p->alarm_caninvoke){
+      // record sigalarm
+      p->alarm_tickspassed++;
+      if(p->alarm_tickspassed == p->alarm_interval){
+        // store original trapframe in p->atpfm
+        memmove(p->atpfm, p->trapframe, sizeof(struct trapframe));
+        p->alarm_tickspassed = 0;
+        p->alarm_caninvoke = 0;
+        p->trapframe->epc = p->alarm_handler;
+      }
+    }
+    // give up the CPU.
     yield();
+  }
 
   usertrapret();
 }