请问如何判断IOS设备的版本 ？

有两个storyboard分别制定给iPhone5和iPhone4的尺寸。如何让代码检测ios版本？

- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
{

    // 1
    UIStoryboard * mainStoryboard = nil ;
    if ( SYSTEM_VERSION_GREATER_THAN_OR_EQUAL_TO ( @ "6.0" ) ) {
        mainStoryboard = [ UIStoryboard storyboardWithName : @ "iPhones6.0" bundle : nil ] ;
    } else {
        mainStoryboard = [ UIStoryboard storyboardWithName : @ "iPhones3-5" bundle : nil ] ;
    }
    // 2
    self.window = [ [ UIWindow alloc ] initWithFrame : [ [ UIScreen mainScreen ] bounds ] ] ;
    self.window.rootViewController = [ mainStoryboard instantiateInitialViewController ] ;
    [ self.window makeKeyAndVisible ] ;
    return YES;
}