diff --git a/kernel/proc.h b/kernel/proc.h
index 8ae5cf0..d48fd3f 100644
--- a/kernel/proc.h
+++ b/kernel/proc.h
@@ -111,4 +111,5 @@ struct proc {
   struct file      *ofile[NOFILE]; // Open files
   struct inode     *cwd;           // Current directory
   char              name[16];      // Process name (debugging)
+  int               mask;          // Process mask
 };
diff --git a/kernel/syscall.c b/kernel/syscall.c
index 31b0326..075fb36 100644
--- a/kernel/syscall.c
+++ b/kernel/syscall.c
@@ -101,6 +101,7 @@ extern u64 sys_unlink(void);
 extern u64 sys_link(void);
 extern u64 sys_mkdir(void);
 extern u64 sys_close(void);
+extern u64 sys_trace(void);
 
 // An array mapping syscall numbers from syscall.h
 // to the function that handles the system call.
@@ -110,7 +111,7 @@ static u64 (*syscalls[])(void) = {
   [SYS_chdir] = sys_chdir, [SYS_dup] = sys_dup,       [SYS_getpid] = sys_getpid, [SYS_sbrk] = sys_sbrk,
   [SYS_sleep] = sys_sleep, [SYS_uptime] = sys_uptime, [SYS_open] = sys_open,     [SYS_write] = sys_write,
   [SYS_mknod] = sys_mknod, [SYS_unlink] = sys_unlink, [SYS_link] = sys_link,     [SYS_mkdir] = sys_mkdir,
-  [SYS_close] = sys_close,
+  [SYS_close] = sys_close, [SYS_trace] = sys_trace,
 };
 
 void
diff --git a/kernel/syscall.h b/kernel/syscall.h
index 7d671f0..dfc9645 100644
--- a/kernel/syscall.h
+++ b/kernel/syscall.h
@@ -22,3 +22,4 @@
 #define SYS_link   19
 #define SYS_mkdir  20
 #define SYS_close  21
+#define SYS_trace  22
diff --git a/kernel/sysproc.c b/kernel/sysproc.c
index 4274a9d..5645c72 100644
--- a/kernel/sysproc.c
+++ b/kernel/sysproc.c
@@ -89,3 +89,10 @@ sys_uptime(void)
   release(&tickslock);
   return xticks;
 }
+
+u64
+sys_trace(void)
+{
+  /* TODO: Implement sys_trace */
+  return 0;
+}
diff --git a/user/usys.pl b/user/usys.pl
index 01e426e..9c97b05 100755
--- a/user/usys.pl
+++ b/user/usys.pl
@@ -36,3 +36,4 @@ entry("getpid");
 entry("sbrk");
 entry("sleep");
 entry("uptime");
+entry("trace");