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void syscall(void)
{

  int num;
  struct proc *p = myproc();
  num = p->trapframe->a7;
  if (num > 0 && num < NELEM(syscalls) && syscalls[num])
  {
    uint64 arg0 = p->trapframe->a0; // Save first argument BEFORE syscall dispatch

    p->trapframe->a0 = syscalls[num](); // Use num to lookup the system call function for num, call it, and store its return value in p->trapframe->a0


    if (p->traced)
    {

      // Print syscall name, first argument
      printf( "[pid %d] %s(", p->pid, syscall_names[num] );

      if (num == SYS_open || num == SYS_unlink ||  
          num == SYS_chdir || num == SYS_mkdir || num == SYS_link
          /* ignore the second string argument of `link` */)
      {
         // If the syscall is one of these five... 
         char buf[128];
         if ( argstr( 0, buf, sizeof( buf ) ) >= 0 ) {
          printf( "\"%s\"", buf );
         }
         else {
          printf( "\?\"" );
         }
      }
      else if ( num == SYS_exec ){
         // If the syscall is exec... 
         char buf[128];
        if (argstr(0, buf, sizeof(buf)) >= 0)
          printf("exec(\"%s\")", buf);
        else
          printf("\"?\"");
      }
      else
      {
         // If the syscall is any of the others...
         printf("%d", (int)arg0);
      }

      // Print the remaining part.
      printf(") = %d\n", (int)arg0);
    }
  }
  else
  {
    printf("%d %s: unknown sys call %d\n",
           p->pid, p->name, num);
    p->trapframe->a0 = -1;
  }
}