Setting of PE bit reboots the system

[Swapnil Pimpale <swapnil.pict@gmail.com>]

Hi all,

I am working on Intel Core 2 Duo architecture. As a part of my
academic project, I need to bring up the second core from the
bootloader (GRUB).
When the second core comes up, I need to switch it from real mode to
protected mode.
For this, I am trying to set the PE bit of cr0 register. But, whenever
I set the PE bit the system reboots.
For switching from real to protected mode I use the following steps:
1) Clear Interrupts
2) Load GDT
3) Set PE bit of cr0
4) Long Jump to Protected mode code

Is there any thing that I am missing? Should the gate A20 line be
turned on from the second core also?
Is there any solution on the problem of setting PE bit?

Thanks,
Swapnil Pimpale.