Print nvram settings
# nvram -p
Set boot-args to use 64 bit kernel
# nvram boot-args="arch=x86_64"
Don't forget to add in your existing boot-args to the command above. Using nvram means you don't need to have to remember to hold down 6 or 4 during booting each time. To go back to the 32 bit kernel, just set the boot-args using nvram without the arch=x86_64 option.
Check if running 64 bit kernel
To check if you're using the 64 bit kernel, you can run uname -v and check if there is mention of x86_64 in the returned string.
When I type your following hint, I got the an error message: nvram: Error setting variable - 'boot-args': (iokit/common) general error .... Please tell me what to do? I know my MacBook 4,1 dualcore is able to run 64 bit.....
I don't know what else to say, according to System Profiler I have a MacBookPro4,1 and the command is the one I use without any problems.
# man nvram
This will return the options for nvram and one of the examples I get is setting boot-args. Are you running nvram as a superuser?
You need to be root to change the nvram. That's why the author shows a "#' at the prompt.
Try this instead:
$ sudo nvram boot-args="arch=x86_64"
When prompted for a password, enter your own password (Not root's)
You can also edit a string file in com.apple.Boot.plist Navigate to /Library/Preferences/SystemConfiguration/com.apple.Boot.plist and open the boot.plist with a text editor and find the following string.
[key]Kernel Flags[/key]
[string][/string]
and change it to
[key]Kernel Flags[/key]
[string]arch=x86_64[/string]
(Courtesy of http://www.ihackintosh.com/2009/08/snow-leopard-106-and-64-bit-what-you-need-to-know/)
There's a nice little app called Startup Mode Selector that detects if your machine is supported with a 64 bit kernel and allows you to change between 32 and 64 bit.