use C-xC-x as my-prefix-map

.emacs.el

@@ -74,7 +74,9 @@
 
 (defvar my-prefix-map
   (make-sparse-keymap))
-(define-key ctl-x-map (kbd "C-x") my-prefix-map)
+(add-hook 'after-init-hook
+          (lambda ()
+            (define-key ctl-x-map (kbd "C-x") my-prefix-map)))
 (define-key my-prefix-map (kbd "C-q") 'quoted-insert)
 (define-key my-prefix-map (kbd "C-z") 'suspend-frame)
 
@@ -1359,7 +1361,7 @@ otherwise, use `pack-default-extension' for pack."
             (define-key dired-mode-map "P" 'my-dired-do-pack-or-unpack)
             (define-key dired-mode-map "a" 'my-dired-display-all-mode)
             (define-key dired-mode-map "h" 'my-dired-display-all-mode)
-            (define-key dired-mode-map "/" 'isearch-forward)
+            (define-key dired-mode-map "/" 'dired-isearch-filenames)
             (substitute-key-definition 'dired-advertised-find-file 'my-dired-find-file dired-mode-map)
             (substitute-key-definition 'dired-up-directory 'my-dired-up-directory dired-mode-map)
             (define-key dired-mode-map (kbd "DEL") 'my-dired-up-directory)