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I was trying to install Ubuntu 12.04 in VirtualBox 4.2.12r84980. I see

this kernel requires an x86-64 CPU, but only detects an i686 CPU, unable to boot

But I am using a 64 bit Windows 8, and trying same .iso for trying Ubuntu. Then what is the problem?

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Try installing the 32-bit Ubuntu ISO – edwin Jun 16 '13 at 17:12
It does not matter that your host system is 64bit. The whole purpose of a virtual machine is that it is a separate and autonomous machine albeit a virtual one. Edit the virtual machine configuration and change its CPU type to 64bit. – hmayag Jun 16 '13 at 21:22

To be able to run a 64-bit OS in Virtual Box we have to make sure the virtual machine's architecture is set to 64-bit too.

  • Choose Ubuntu 64-bit in General -> Basic settings on creation of your VM

    enter image description here

  • In addition, for running 64-bit guests it is recommended to enable the Input/Output APIC in the System -> Motherboard settings for your virtual machine:

  • In the System -> Acceleration tab we may want to enable the hardware virtualization features VT-x/AMD-V of your CPU.

  • On the host system we may have to enable hardware virtualization in the BIOS.

  • In some systems a host BIOS option to prevent non-trusted execution needs to be turned off (source).

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it worked : thank you – aluph Jun 17 '13 at 16:36
it worked like charm .. Thanks alot. – Anil Chahal Feb 8 at 8:52

Open the virtual machine settings. Go to the Version dropdown box (General > Basic > Version).

If you don't see "Ubuntu(64 bit)" as an option in the dropdown, it means that virtualization is not enabled on the host.

Reboot the host, go in to BIOS and enable Virtualization. Exit the BIOS, making sure you save changes.

Now, you should see all the 64 bit options in the Version dropdown box.

Virtual machine general settings

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For those of you who are like me and don't have an option on your virtual box to select the version: Ubuntu(64 bit):

Do this: download the 32 bit Ubuntu iso. ESPECIALLY if when downloading Ubuntu on your virtual box, you chose to allocate less than 2GB of memory.

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