The base operating system 64-bit runtime fileset is bos.64bit. Installing bos.64bit also installs the /etc/methods/cfg64 file. The /etc/methods/cfg64 file provides the option of enabling or disabling the 64-bit environment via SMIT, which updates the /etc/inittab file with the load64bit line. (Simply adding the load64bit line does not enable the 64-bit environment).

The command lslpp -l bos.64bit reveals if this fileset is installed. The bos.64bit fileset is on the AIX media; however, installing the bos.64bit fileset does not ensure that you will be able to run 64-bit software. If the bos.64bit fileset is installed on 32-bit hardware, you should be able to compile 64-bit software, but you cannot run 64-bit programs on 32-bit hardware.

The syscalls64 extension must be loaded in order to run a 64-bit executable. This is done from the load64bit entry in the inittab file. You must load the syscalls64 extension even when running a 64-bit kernel on 64-bit hardware.
Hardware Required for 64-Bit Mode

You must have 64-bit hardware to run 64-bit applications. To determine whether your system has 32-bit or 64-bit hardware architecture:

1. Log in as a root user.
2. At the command line, enter bootinfo -y.

This produces the output of either 32 or 64, depending on whether the hardware architecture is 32-bit or 64-bit. In addition, if you enter lsattr -El proc0 at any version of AIX, the type of processor for your server displays.
Kernel Extensions vs. 64-bit Kernel

To determine if the 64-bit kernel extension is loaded, at the command line, enter genkex |grep 64.

Information similar to the following displays:

149bf58 a3ec /usr/lib/drivers/syscalls64.ext

Note:
Having the driver extensions does not mean that the kernel is a 64-bit kernel. A 64-bit kernel became available at the AIX 5.1 level. The driver extensions simply allow the 64-bit application to be compiled by a 32-bit kernel. If the 32-bit kernel has a 64-bit processor, syscalls64.ext allows the 64-bit application to execute. Yet at the AIX 5L level, a 64-bit kernel and a 64-bit processor has better performance with 64-bit applications.
Changing from 32-Bit to 64-Bit

To truly change the kernel to 64-bit, the system must be at the AIX 5.1 or AIX 5.2 levels. To change to a 64-bit kernel, enter the following commands:

ln -sf /usr/lib/boot/unix_64 /unix
ln -sf /usr/lib/boot/unix_64 /usr/lib/boot/unix
lslv -m hd5
bosboot -ad /dev/ipldevice
shutdown -Fr

Changing from 64-Bit to 32-Bit

To change the kernel back to 32-bit, enter the following commands:

ln -sf /usr/lib/boot/unix_mp /unix
ln -sf /usr/lib/boot/unix_mp /usr/lib/boot/unix
lslv -m hd5
bosboot -ad /dev/ipldevice
shutdown -Fr