libcaca/kernel/boot/gdt.c

/*
 *'freely' inspired by http://jojo.ouvaton.org/dossiers/boot_sector/tutorial02.html
 * (actually, that's mostly copied, with minor compilation fixes)
 */
#include "kernel.h"
#include "klibc.h"

#define GDTBASE	0x800	/* Physical address of GDT */
#define GDTSIZE	0xFF	/* Maximum table size (in entries) */

/* Segment descriptor  */
struct gdtdesc {
	u16 lim0_15;	
	u16 base0_15;
	u8 base16_23;
	u8 acces;
	u8 lim16_19 : 4;
	u8 other : 4;
	u8 base24_31;
}  __attribute__ ((packed));

/* GDTR register  */
struct gdtr {
	u16 limite ;
	u32 base ;
}  __attribute__ ((packed));
struct gdtr kgdtr;


struct gdtdesc kgdt[GDTSIZE] = {{0, 0, 0, 0, 0, 0, 0}};
unsigned int kgdtptr = 1;

void init_code_desc(u32 base, u32 limite, struct gdtdesc* desc);
void init_data_desc(u32 base, u32 limite, struct gdtdesc* desc);
void add_gdt_desc(struct gdtdesc desc);
void init_gdt(void);



void init_gdt_desc(u32 base, u32 limite, u8 acces, u8 other, struct gdtdesc* desc) {
	desc->lim0_15 = (limite & 0xffff);
	desc->base0_15 = (base & 0xffff);
	desc->base16_23 = (base & 0xff0000)>>16;
	desc->acces = acces;
	desc->lim16_19 = (limite & 0xf0000)>>16;
	desc->other = (other & 0xf);
	desc->base24_31 = (base & 0xff000000)>>24;
	return;
}


void init_code_desc(u32 base, u32 limite, struct gdtdesc* desc) {
	init_gdt_desc(base, limite, 0x9B, 0x0D, desc);
}

void init_data_desc(u32 base, u32 limite, struct gdtdesc* desc) {
	init_gdt_desc(base, limite, 0x93, 0x0D, desc);
}

void add_gdt_desc(struct gdtdesc desc) {
	kgdt[kgdtptr] = desc;
	kgdtptr++;
}

void init_gdt(void) {
	struct gdtdesc code, data, stack;
    
	/* initialisation des descripteurs de segment */
	init_code_desc(0x0, 0xFFFFF, &code);
	init_data_desc(0x0, 0xFFFFF, &data);
	init_gdt_desc(0, 0x10, 0x97, 0x0D, &stack);
	add_gdt_desc(code);
	add_gdt_desc(data);
	add_gdt_desc(stack);
    
	/* initialisation de la structure pour GDTR */
	kgdtr.limite = GDTSIZE*8;
	kgdtr.base = GDTBASE;
    
	/* recopie de la GDT a son adresse */	
	memcpy((void*)kgdtr.base, kgdt, kgdtr.limite);
    
	/* chargement du registre GDTR */
	asm("lgdtl (kgdtr)");
    
	/* initialisation des segments */
	asm("	movw $0x10,%ax	\n \
		movw %ax, %ds	\n \
		movw %ax, %es	\n \
		movw %ax, %fs	\n \
		movw %ax, %gs	\n \
		movw $0x18,%ax	\n \
		movw %ax, %ss	\n \
		movl $0x1FFFF,%esp	\n \
		nop	\n \
		nop	\n \
		ljmp $0x08,$next	\n \
		next:	\n");
}