Unfortunately, I am still using a 32-bit kernel using high memory. It was caled "PAE" - physical address extensions.

  $ cat /proc/version
  Linux version 2.6.18-419.0.0.0.2.el5PAE ... (gcc version 4.1.2 20080704 (Red Hat 4.1.2-55)) #1 SMP Wed Jun 28 20:25:21 PDT 2017