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Posted: **Thu Jun 06, 2013 11:44 am**

I've been following bran's kernel development tutorial for setting up my GDT.

I first tried doing it his way, in C, with the ASM stub that loads the gdt. That didn't work, so i tried loading the whole GDT in assembly. However, i'm getting the same result with both of my methods.

In my code, i have it output "Installing GDT...\n", call gdt_install(), and then output "GDT Successfully Installed!\n"
However, it only prints "Installing GDT...\n", and (i assume) never returns from the gdt_install() function.

Could someone give me a poke in the right direction here? I'm new to OS development, so it may be an amateurish mistake. The relevant source code is posted below.

Note:
Compiled with a gcc cross compiler on Debian linux
Run in VirtualBox on windows 7, x64

kernel.c - entry point function

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#include "./kernel/gdt.h"

void kernel_main()
{
	k_clearscreen(TEXT_ATTRIB);
	
	char *tst = "Kernel is loaded.\n\n";
	k_putstring(tst, TEXT_ATTRIB);
	
	k_putstring("Installing GDT...\n", TEXT_ATTRIB);
	
	//init_gdt_entries();
	gdt_install();
	
	k_putstring("GDT installed successfully!\n\n", TEXT_ATTRIB);
	
}

gdt.h

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#if !defined(__cplusplus)
#include <stdbool.h> /* C doesn't have booleans by default. */
#endif
#include <stddef.h>
#include <stdint.h>

#define GDT_ENTRY_COUNT 3

struct GDT_ENTRY {
	
	unsigned short limit_low;
	unsigned short base_low;
	unsigned char base_mid;
	unsigned char access;
	unsigned char granularity;
	unsigned char base_high;
	
} __attribute__((packed));

struct GDT_PTR {
	
	unsigned short limit;
	unsigned long base;
	
} __attribute__((packed));

struct GDT_ENTRY gdt_entries[GDT_ENTRY_COUNT];
struct GDT_PTR gdt_ptr;

void init_gdt_entries();
extern void gdt_install();

gdt.c

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#include "gdt.h"

void init_gdt_entries()
{
	// null descriptor
	gdt_entries[0].limit_low = 0;
	gdt_entries[0].base_low = 0;
	gdt_entries[0].base_mid = 0;
	gdt_entries[0].base_high = 0;
	gdt_entries[0].access = 0;
	gdt_entries[0].granularity = 0;
	
	// code descriptor
	gdt_entries[0].limit_low = 0xFFFF;
	gdt_entries[0].base_low = 0x0000;
	gdt_entries[0].base_mid = 0x00;
	gdt_entries[0].base_high = 0x00;
	gdt_entries[0].access = 0x9A;
	gdt_entries[0].granularity = 0xCF;
	
	// data descriptor
	gdt_entries[0].limit_low = 0xFFFF;
	gdt_entries[0].base_low = 0x0000;
	gdt_entries[0].base_mid = 0x00;
	gdt_entries[0].base_high = 0x00;
	gdt_entries[0].access = 0x92;
	gdt_entries[0].granularity = 0xCF;
	
	gdt_ptr.base = (unsigned long) &gdt_entries;
	gdt_ptr.limit = (sizeof(struct GDT_ENTRY) * GDT_ENTRY_COUNT) - 1;
	
}

install_gdt.asm

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;extern gdt_ptr
global gdt_install

gdt_install:
	lgdt [m_gdt]
	mov ax, 0x10
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax
	jmp 0x08:gdt_return
	
gdt_return:
	ret


gdt_start:
	
	; null descriptor
	
	dd 0
	dd 0
	
	;code descriptor
	
	dw 0xFFFF ; limit low
	dw 0x0000 ; base low
	db 0x00	  ; base middle
	db 0x9A	  ; access
	db 0xCF	  ; granularity
	db 0x00   ; base high
	
	;data descriptor
	
	dw 0xFFFF ; limit low
	dw 0x0000 ; base low
	db 0x00	  ; base middle
	db 0x92	  ; access
	db 0xCF	  ; granularity
	db 0x00   ; base high
	
gdt_end:

m_gdt:
	dw gdt_end - gdt_start - 1;
	dd gdt_start

Posted: **Thu Jun 06, 2013 12:02 pm**

GDT base should be 0.

Posted: **Thu Jun 06, 2013 12:30 pm**

I don't understand what you mean.

I thought LGDT expected a base address for the GDT and a size of the GDT?

Edit: I totally derped... i have set the base_high, base_middle, and base_low values for the code and data descriptors to 0 now. However, i'm still having the same issue. I'll update the source code above to reflect my changes.

Posted: **Thu Jun 06, 2013 2:19 pm**

My guess is that you mix-up 16-bit and 32-bit code. In the boot part, your code segment has 16-bit default operand size, but after loading the CS from the GDT you end up with 32-bit default operand size, which leads to faults.

Posted: **Fri Jun 07, 2013 2:32 pm**

I'm doing from the assembly stub that is loaded by grub. I followed the tutorial on here for a barebones c kernel. Putting the code in there worked, for some reason. :/

Before i moved the code there, it was in a separate assembly file that was linked in, and then the function was called from my k_main function.

Posted: **Sun Jun 16, 2013 3:28 am**

Gogeta70 wrote:
gdt.c

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#include "gdt.h"

void init_gdt_entries()
{
	// null descriptor
	gdt_entries[0].limit_low = 0;
	gdt_entries[0].base_low = 0;
	gdt_entries[0].base_mid = 0;
	gdt_entries[0].base_high = 0;
	gdt_entries[0].access = 0;
	gdt_entries[0].granularity = 0;
	
	// code descriptor
	gdt_entries[0].limit_low = 0xFFFF;
	gdt_entries[0].base_low = 0x0000;
	gdt_entries[0].base_mid = 0x00;
	gdt_entries[0].base_high = 0x00;
	gdt_entries[0].access = 0x9A;
	gdt_entries[0].granularity = 0xCF;
	
	// data descriptor
	gdt_entries[0].limit_low = 0xFFFF;
	gdt_entries[0].base_low = 0x0000;
	gdt_entries[0].base_mid = 0x00;
	gdt_entries[0].base_high = 0x00;
	gdt_entries[0].access = 0x92;
	gdt_entries[0].granularity = 0xCF;
	
	gdt_ptr.base = (unsigned long) &gdt_entries;
	gdt_ptr.limit = (sizeof(struct GDT_ENTRY) * GDT_ENTRY_COUNT) - 1;
	
}

Did you mean to put all of your GDT entries at offset 0?

Gogeta70 wrote: install_gdt.asm

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;extern gdt_ptr
global gdt_install

gdt_install:
	lgdt [m_gdt]
	mov ax, 0x10
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax
	jmp 0x08:gdt_return
	
gdt_return:
	ret


gdt_start:
	
	; null descriptor
	
	dd 0
	dd 0
	
	;code descriptor
	
	dw 0xFFFF ; limit low
	dw 0x0000 ; base low
	db 0x00	  ; base middle
	db 0x9A	  ; access
	db 0xCF	  ; granularity
	db 0x00   ; base high
	
	;data descriptor
	
	dw 0xFFFF ; limit low
	dw 0x0000 ; base low
	db 0x00	  ; base middle
	db 0x92	  ; access
	db 0xCF	  ; granularity
	db 0x00   ; base high
	
gdt_end:

m_gdt:
	dw gdt_end - gdt_start - 1;
	dd gdt_start

Try this...

Code: Select all

gdt_install:
	lgdt [m_gdt]
	mov ax, 0x10
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax
	cli
	mov eax,cr0
	or al,00000001b
	mov cr0,eax ;enable protected mode
	jmp 0x08:gdt_return

bits 32
	
gdt_return:
	ret

You have to enable protected mode before trying to use GDT selectors e.g. jmp 0x08:gdt_return, otherwise it would just interpret the 0x08:gdt_return as a segment:offset address. I guess that was why it wasn't returning from the gdt_install function. And since we are in protected mode when we do the far jump, we must set the assembly to 32 bits.

More info on segmentation/real mode/protected mode here: http://wiki.osdev.org/Segmentation

I'm not sure why Bran left it out of his tutorial...

Posted: **Mon Jun 17, 2013 2:07 am**

BMW wrote: You have to enable protected mode before trying to use GDT selectors
...
I'm not sure why Bran left it out of his tutorial...

Because the kernel is booted by GRUB which does the switch to PMode already. The only reason for reloading the GDT is that the multiboot documentation states that you cannot rely on the temporary GDT put in place by a multiboot compliant loader.

Cheers,
Adam

Posted: **Mon Jun 17, 2013 2:47 am**

AJ wrote: Because the kernel is booted by GRUB which does the switch to PMode already. The only reason for reloading the GDT is that the multiboot documentation states that you cannot rely on the temporary GDT put in place by a multiboot compliant loader.

Ahh, my bad.

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