BrokenThorn Entertainment

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void gdt_flush() {
	#ifdef _MSC_VER
		_asm lgdt[gp]
	#endif
}

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// gdt entry structure
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;
};

// pointer to gdt (needed for lgdt instruction)
struct gdt_ptr
{
    unsigned short limit;
    unsigned int base;
};

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// define three entries (NULL entry, code descriptor, data descriptor)
struct gdt_entry gdt[3];
struct gdt_ptr gp;

// this will load our gdt
void gdt_flush() {
	#ifdef _MSC_VER
		_asm lgdt[gp]
	#endif
}

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() {
	unsigned short csval, dsval, ssval, esval, fsval, gsval;
	gp.limit = (sizeof(struct gdt_entry) * 3) - 1;
	gp.base = gdt;
	 
	gdt_set_gate(0, 0, 0, 0, 0);
	// access 0x1A = 00011010 (bit 47-40)
	// granularity 0xCF = 11001111 (bit 55-48)
	gdt_set_gate(1, 0, 0xFFFFF, 0x1A, 0xCF);
	// access 0x12 = 00010010 (bit 47-40)
	// granularity 0xCF = 11001111 (bit 55-48)
	gdt_set_gate(2, 0, 0xFFFFF, 0x12, 0xCF);

	// load our GDT
	 
	gdt_flush();
}