#include "gdt.h"

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

struct gdt_ptr
{
	unsigned short limit;
	unsigned int base;
} __attribute__((packed));

struct gdt_entry gdt[3];
struct gdt_ptr gp;

extern void gdt_flush();

void gdt_set_gate(int num, unsigned long base, unsigned long limit, unsigned char access, unsigned char gran)
{
	gdt[num].base_low = (base & 0xFFFF);
	gdt[num].base_middle = (base >> 16) & 0xFF;
	gdt[num].base_high = (base >> 24) & 0xFF;

	gdt[num].limit_low = (limit & 0xFFFF);
	gdt[num].granularity = ((limit >> 16) & 0x0F);

	gdt[num].granularity |= (gran & 0xF0);
	gdt[num].access = access;
}

void gdt_install()
{
	gp.limit = (sizeof(struct gdt_entry)*3) - 1;
	gp.base = (unsigned int)&gdt;

	gdt_set_gate(0, 0, 0, 0, 0);
	
	// Ring 0 code + data
	gdt_set_gate(1, 0, 0xFFFFFFFF, 0x9A, 0xCF);
	gdt_set_gate(2, 0, 0xFFFFFFFF, 0x92, 0xCF);

	// Ring 3
	gdt_set_gate(3, 0, 0xFFFFFFFF, 0xFA, 0xCF);
	gdt_set_gate(4, 0, 0xFFFFFFFF, 0xF2, 0xCF);

	gdt_flush();
}