How do I get a cr4 register using C?

mrjbom · **Joined:** Sun Jul 21, 2019 7:34 am **Posts:** 300

Hi.
I try to get it with this kind of code

Code:

uint32_t cr4 = 0;
__asm__ (
    "mov %%cr4, %0" : "=r"(cr4)
);
dprintf("cr4 = %I\n", cr4);
for(int i = 0; i < 32; ++i) {
    dprintf("%I", (cr4 >> i) & 1);
}

but I get 0, for cr0 it works.

kzinti · **Joined:** Mon Feb 02, 2015 7:11 pm **Posts:** 898

Could it be that CR4 is indeed 0?

mrjbom · **Joined:** Sun Jul 21, 2019 7:34 am **Posts:** 300

kzinti wrote:

Could it be that CR4 is indeed 0?

No, I have PAE enabled(GRUB includes it), so 5 bits should be equal to 1.

iansjack · **Posted:** Tue Jun 02, 2020 3:04 pm

Are you sure? I'm probably wrong, but I thought PAE was something that was enabled in the kernel, not the boot loader.

mrjbom · **Joined:** Sun Jul 21, 2019 7:34 am **Posts:** 300

iansjack wrote:

Are you sure? I'm probably wrong, but I thought PAE was something that was enabled in the kernel, not the boot loader.

Hm.
the memory_map that GRUB gives me sees memory above 4 GB, I think it does it with PAE and expect to see it with this call.
Perhaps it is really disabled in the core.

kzinti · **Joined:** Mon Feb 02, 2015 7:11 pm **Posts:** 898

No, Grub doesn't enable PAE for you. In fact, paging is not even enabled.

The fact that it reports memory above 4 GB has nothing to do with PAE being enabled or not.

You have to setup page tables for PAE and enable it yourself if you want to use it.

See https://www.pvv.ntnu.no/~steinarh/grub/ ... html#SEC12:

Quote:

Paging must be turned off.

mrjbom · **Joined:** Sun Jul 21, 2019 7:34 am **Posts:** 300

kzinti wrote:

No, Grub doesn't enable PAE for you. In fact, paging is not even enabled.

The fact that it reports memory above 4 GB has nothing to do with PAE being enabled or not.

You have to setup page tables for PAE and enable it yourself if you want to use it.

See https://www.pvv.ntnu.no/~steinarh/grub/ ... html#SEC12:

Quote:

Paging must be turned off.

I understood. Thanks.

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How do I get a cr4 register using C?

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