xref: /kvm-unit-tests/x86/realmode.c (revision eacef4e2e65a17c9329f59ba2984af1bc44927c8)

asm(".code16gcc");

typedef unsigned char u8;
typedef unsigned short u16;
typedef unsigned u32;
typedef unsigned long long u64;

void test_function(void);

asm(
	"test_function: \n\t"
	"mov $0x1234, %eax \n\t"
	"ret"
   );

static int strlen(const char *str)
{
	int n;

	for (n = 0; *str; ++str)
		++n;
	return n;
}

static void print_serial(const char *buf)
{
	unsigned long len = strlen(buf);

	asm volatile ("cld; addr32/rep/outsb" : "+S"(buf), "+c"(len) : "d"(0xf1));
}

static void exit(int code)
{
        asm volatile("out %0, %1" : : "a"(code), "d"((short)0xf4));
}

struct regs {
	u32 eax, ebx, ecx, edx;
	u32 esi, edi, esp, ebp;
	u32 eip, eflags;
};

static u64 gdt[] = {
	0,
	0x00cf9b000000ffffull, // flat 32-bit code segment
	0x00cf93000000ffffull, // flat 32-bit data segment
};

static struct {
	u16 limit;
	void *base;
} __attribute__((packed)) gdt_descr = {
	sizeof(gdt) - 1,
	gdt,
};

static void exec_in_big_real_mode(const struct regs *inregs,
				  struct regs *outregs,
				  const u8 *insn, int insn_len)
{
	unsigned long tmp;
	static struct regs save;
	int i;
	extern u8 test_insn[], test_insn_end[];

	for (i = 0; i < insn_len; ++i)
		test_insn[i] = insn[i];
	for (; i < test_insn_end - test_insn; ++i)
		test_insn[i] = 0x90; // nop

	save = *inregs;
	asm volatile(
		"lgdtl %[gdt_descr] \n\t"
		"mov %%cr0, %[tmp] \n\t"
		"or $1, %[tmp] \n\t"
		"mov %[tmp], %%cr0 \n\t"
		"mov %[bigseg], %%gs \n\t"
		"and $-2, %[tmp] \n\t"
		"mov %[tmp], %%cr0 \n\t"

		"xchg %%eax, %[save]+0 \n\t"
		"xchg %%ebx, %[save]+4 \n\t"
		"xchg %%ecx, %[save]+8 \n\t"
		"xchg %%edx, %[save]+12 \n\t"
		"xchg %%esi, %[save]+16 \n\t"
		"xchg %%edi, %[save]+20 \n\t"
		"xchg %%esp, %[save]+24 \n\t"
		"xchg %%ebp, %[save]+28 \n\t"

		"test_insn: . = . + 32\n\t"
		"test_insn_end: \n\t"

		"xchg %%eax, %[save]+0 \n\t"
		"xchg %%ebx, %[save]+4 \n\t"
		"xchg %%ecx, %[save]+8 \n\t"
		"xchg %%edx, %[save]+12 \n\t"
		"xchg %%esi, %[save]+16 \n\t"
		"xchg %%edi, %[save]+20 \n\t"
		"xchg %%esp, %[save]+24 \n\t"
		"xchg %%ebp, %[save]+28 \n\t"

		/* Save EFLAGS in outregs*/
		"pushfl \n\t"
		"popl %[save]+36 \n\t"

		"xor %[tmp], %[tmp] \n\t"
		"mov %[tmp], %%gs \n\t"
		: [tmp]"=&r"(tmp), [save]"+m"(save)
		: [gdt_descr]"m"(gdt_descr), [bigseg]"r"((short)16)
		: "cc", "memory"
		);
	*outregs = save;
}

#define R_AX 1
#define R_BX 2
#define R_CX 4
#define R_DX 8
#define R_SI 16
#define R_DI 32
#define R_SP 64
#define R_BP 128

int regs_equal(const struct regs *r1, const struct regs *r2, int ignore)
{
	const u32 *p1 = &r1->eax, *p2 = &r2->eax;  // yuck
	int i;

	for (i = 0; i < 8; ++i)
		if (!(ignore & (1 << i)) && p1[i] != p2[i])
			return 0;
	return 1;
}

#define MK_INSN(name, str)                         \
	asm (				           \
		".text 1\n\t"                      \
		"insn_" #name ": " str " \n\t"	   \
		"insn_" #name "_end: \n\t"	   \
		".text\n\t"                        \
		);				   \
	extern u8 insn_##name[], insn_##name##_end[]

void test_xchg(void)
{
	struct regs inregs = { .eax = 0, .ebx = 1, .ecx = 2, .edx = 3, .esi = 4, .edi = 5, .ebp = 6, .esp = 7}, outregs;

	MK_INSN(xchg_test1, "xchg %eax,%eax\n\t");
	MK_INSN(xchg_test2, "xchg %eax,%ebx\n\t");
	MK_INSN(xchg_test3, "xchg %eax,%ecx\n\t");
	MK_INSN(xchg_test4, "xchg %eax,%edx\n\t");
	MK_INSN(xchg_test5, "xchg %eax,%esi\n\t");
	MK_INSN(xchg_test6, "xchg %eax,%edi\n\t");
	MK_INSN(xchg_test7, "xchg %eax,%ebp\n\t");
	MK_INSN(xchg_test8, "xchg %eax,%esp\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test1,
                              insn_xchg_test1_end - insn_xchg_test1);

	if (!regs_equal(&inregs, &outregs, 0))
		print_serial("xchg test 1: FAIL\n");
	else
		print_serial("xchg test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test2,
                              insn_xchg_test2_end - insn_xchg_test2);

	if (!regs_equal(&inregs, &outregs, R_AX | R_BX) ||
            outregs.eax != inregs.ebx ||
            outregs.ebx != inregs.eax)
		print_serial("xchg test 2: FAIL\n");
	else
		print_serial("xchg test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test3,
                              insn_xchg_test3_end - insn_xchg_test3);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX) ||
            outregs.eax != inregs.ecx ||
            outregs.ecx != inregs.eax)
		print_serial("xchg test 3: FAIL\n");
	else
		print_serial("xchg test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test4,
                              insn_xchg_test4_end - insn_xchg_test4);

	if (!regs_equal(&inregs, &outregs, R_AX | R_DX) ||
            outregs.eax != inregs.edx ||
            outregs.edx != inregs.eax)
		print_serial("xchg test 4: FAIL\n");
	else
		print_serial("xchg test 4: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test5,
                              insn_xchg_test5_end - insn_xchg_test5);

	if (!regs_equal(&inregs, &outregs, R_AX | R_SI) ||
            outregs.eax != inregs.esi ||
            outregs.esi != inregs.eax)
		print_serial("xchg test 5: FAIL\n");
	else
		print_serial("xchg test 5: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test6,
                              insn_xchg_test6_end - insn_xchg_test6);

	if (!regs_equal(&inregs, &outregs, R_AX | R_DI) ||
            outregs.eax != inregs.edi ||
            outregs.edi != inregs.eax)
		print_serial("xchg test 6: FAIL\n");
	else
		print_serial("xchg test 6: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test7,
                              insn_xchg_test7_end - insn_xchg_test7);

	if (!regs_equal(&inregs, &outregs, R_AX | R_BP) ||
            outregs.eax != inregs.ebp ||
            outregs.ebp != inregs.eax)
		print_serial("xchg test 7: FAIL\n");
	else
		print_serial("xchg test 7: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xchg_test8,
                              insn_xchg_test8_end - insn_xchg_test8);

	if (!regs_equal(&inregs, &outregs, R_AX | R_SP) ||
            outregs.eax != inregs.esp ||
            outregs.esp != inregs.eax)
		print_serial("xchg test 8: FAIL\n");
	else
		print_serial("xchg test 8: PASS\n");
}

void test_shld(void)
{
	struct regs inregs = { .eax = 0xbe, .edx = 0xef000000 }, outregs;
	MK_INSN(shld_test, "shld $8,%edx,%eax\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_shld_test,
			      insn_shld_test_end - insn_shld_test);
	if (outregs.eax != 0xbeef)
		print_serial("shld: FAIL\n");
	else
		print_serial("shld: PASS\n");
}

void test_mov_imm(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(mov_r32_imm_1, "mov $1234567890, %eax");
	MK_INSN(mov_r16_imm_1, "mov $1234, %ax");
	MK_INSN(mov_r8_imm_1, "mov $0x12, %ah");
	MK_INSN(mov_r8_imm_2, "mov $0x34, %al");
	MK_INSN(mov_r8_imm_3, "mov $0x12, %ah\n\t" "mov $0x34, %al\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mov_r16_imm_1,
			      insn_mov_r16_imm_1_end - insn_mov_r16_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 1234)
		print_serial("mov test 1: FAIL\n");
	else
		print_serial("mov test 1: PASS\n");

	/* test mov $imm, %eax */
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mov_r32_imm_1,
			      insn_mov_r32_imm_1_end - insn_mov_r32_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 1234567890)
		print_serial("mov test 2: FAIL\n");
	else
		print_serial("mov test 2: PASS\n");

	/* test mov $imm, %al/%ah */
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mov_r8_imm_1,
			      insn_mov_r8_imm_1_end - insn_mov_r8_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x1200)
		print_serial("mov test 3: FAIL\n");
	else
		print_serial("mov test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mov_r8_imm_2,
			      insn_mov_r8_imm_2_end - insn_mov_r8_imm_2);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x34)
		print_serial("mov test 4: FAIL\n");
	else
		print_serial("mov test 4: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mov_r8_imm_3,
			      insn_mov_r8_imm_3_end - insn_mov_r8_imm_3);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x1234)
		print_serial("mov test 5: FAIL\n");
	else
		print_serial("mov test 5: PASS\n");
}

void test_sub_imm(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(sub_r32_imm_1, "mov $1234567890, %eax\n\t" "sub $10, %eax\n\t");
	MK_INSN(sub_r16_imm_1, "mov $1234, %ax\n\t" "sub $10, %ax\n\t");
	MK_INSN(sub_r8_imm_1, "mov $0x12, %ah\n\t" "sub $0x10, %ah\n\t");
	MK_INSN(sub_r8_imm_2, "mov $0x34, %al\n\t" "sub $0x10, %al\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_sub_r16_imm_1,
			      insn_sub_r16_imm_1_end - insn_sub_r16_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 1224)
		print_serial("sub test 1: FAIL\n");
	else
		print_serial("sub test 1: PASS\n");

	/* test mov $imm, %eax */
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_sub_r32_imm_1,
			      insn_sub_r32_imm_1_end - insn_sub_r32_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 1234567880)
		print_serial("sub test 2: FAIL\n");
	else
		print_serial("sub test 2: PASS\n");

	/* test mov $imm, %al/%ah */
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_sub_r8_imm_1,
			      insn_sub_r8_imm_1_end - insn_sub_r8_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x0200)
		print_serial("sub test 3: FAIL\n");
	else
		print_serial("sub test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_sub_r8_imm_2,
			      insn_sub_r8_imm_2_end - insn_sub_r8_imm_2);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x24)
		print_serial("sub test 4: FAIL\n");
	else
		print_serial("sub test 4: PASS\n");
}


void test_xor_imm(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(xor_r32_imm_1, "mov $1234567890, %eax\n\t" "xor $1234567890, %eax\n\t");
	MK_INSN(xor_r16_imm_1, "mov $1234, %ax\n\t" "xor $1234, %ax\n\t");
	MK_INSN(xor_r8_imm_1, "mov $0x12, %ah\n\t" "xor $0x12, %ah\n\t");
	MK_INSN(xor_r8_imm_2, "mov $0x34, %al\n\t" "xor $0x34, %al\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xor_r16_imm_1,
			      insn_xor_r16_imm_1_end - insn_xor_r16_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0)
		print_serial("xor test 1: FAIL\n");
	else
		print_serial("xor test 1: PASS\n");

	/* test mov $imm, %eax */
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xor_r32_imm_1,
			      insn_xor_r32_imm_1_end - insn_xor_r32_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0)
		print_serial("xor test 2: FAIL\n");
	else
		print_serial("xor test 2: PASS\n");

	/* test mov $imm, %al/%ah */
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xor_r8_imm_1,
			      insn_xor_r8_imm_1_end - insn_xor_r8_imm_1);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0)
		print_serial("xor test 3: FAIL\n");
	else
		print_serial("xor test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_xor_r8_imm_2,
			      insn_xor_r8_imm_2_end - insn_xor_r8_imm_2);
	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0)
		print_serial("xor test 4: FAIL\n");
	else
		print_serial("xor test 4: PASS\n");
}

void test_cmp_imm(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(cmp_test1, "mov $0x34, %al\n\t"
			   "cmp $0x34, %al\n\t");
	MK_INSN(cmp_test2, "mov $0x34, %al\n\t"
			   "cmp $0x39, %al\n\t");
	MK_INSN(cmp_test3, "mov $0x34, %al\n\t"
			   "cmp $0x24, %al\n\t");

	/* test cmp imm8 with AL */
	/* ZF: (bit 6) Zero Flag becomes 1 if an operation results
	 * in a 0 writeback, or 0 register
	 */
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_cmp_test1,
			      insn_cmp_test1_end - insn_cmp_test1);
	if ((outregs.eflags & (1<<6)) != (1<<6))
		print_serial("cmp test 1: FAIL\n");
	else
		print_serial("cmp test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_cmp_test2,
			      insn_cmp_test2_end - insn_cmp_test2);
	if ((outregs.eflags & (1<<6)) != 0)
		print_serial("cmp test 2: FAIL\n");
	else
		print_serial("cmp test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_cmp_test3,
			      insn_cmp_test3_end - insn_cmp_test3);
	if ((outregs.eflags & (1<<6)) != 0)
		print_serial("cmp test 3: FAIL\n");
	else
		print_serial("cmp test 3: PASS\n");
}

void test_add_imm(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(add_test1, "mov $0x43211234, %eax \n\t"
			   "add $0x12344321, %eax \n\t");
	MK_INSN(add_test2, "mov $0x12, %eax \n\t"
			   "add $0x21, %al\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_add_test1,
			      insn_add_test1_end - insn_add_test1);
	if (outregs.eax != 0x55555555)
		print_serial("add test 1: FAIL\n");
	else
		print_serial("add test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_add_test2,
			      insn_add_test2_end - insn_add_test2);
	if (outregs.eax != 0x33)
		print_serial("add test 2: FAIL\n");
	else
		print_serial("add test 2: PASS\n");
}

void test_eflags_insn(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(clc, "clc");
	MK_INSN(stc, "stc");
	MK_INSN(cli, "cli");
	MK_INSN(sti, "sti");
	MK_INSN(cld, "cld");
	MK_INSN(std, "std");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_clc,
			      insn_clc_end - insn_clc);
	if (outregs.eflags & 1)
		print_serial("clc test: FAIL\n");
	else
		print_serial("clc test: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_stc,
			      insn_stc_end - insn_stc);
	if (!(outregs.eflags & 1))
		print_serial("stc test: FAIL\n");
	else
		print_serial("stc test: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_cli,
			      insn_cli_end - insn_cli);
	if (outregs.eflags & (1 << 9))
		print_serial("cli test: FAIL\n");
	else
		print_serial("cli test: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_sti,
			      insn_sti_end - insn_sti);
	if (!(outregs.eflags & (1 << 9)))
		print_serial("sti test: FAIL\n");
	else
		print_serial("sti test: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_cld,
			      insn_cld_end - insn_cld);
	if (outregs.eflags & (1 << 10))
		print_serial("cld test: FAIL\n");
	else
		print_serial("cld test: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_std,
			      insn_std_end - insn_std);
	if (!(outregs.eflags & (1 << 10)))
		print_serial("std test: FAIL\n");
	else
		print_serial("std test: PASS\n");
}

void test_io(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(io_test1, "mov $0xff, %al \n\t"
		          "out %al, $0xe0 \n\t"
		          "mov $0x00, %al \n\t"
			  "in $0xe0, %al \n\t");
	MK_INSN(io_test2, "mov $0xffff, %ax \n\t"
			  "out %ax, $0xe0 \n\t"
			  "mov $0x0000, %ax \n\t"
			  "in $0xe0, %ax \n\t");
	MK_INSN(io_test3, "mov $0xffffffff, %eax \n\t"
			  "out %eax, $0xe0 \n\t"
			  "mov $0x000000, %eax \n\t"
			  "in $0xe0, %eax \n\t");
	MK_INSN(io_test4, "mov $0xe0, %dx \n\t"
			  "mov $0xff, %al \n\t"
			  "out %al, %dx \n\t"
			  "mov $0x00, %al \n\t"
			  "in %dx, %al \n\t");
	MK_INSN(io_test5, "mov $0xe0, %dx \n\t"
			  "mov $0xffff, %ax \n\t"
			  "out %ax, %dx \n\t"
			  "mov $0x0000, %ax \n\t"
			  "in %dx, %ax \n\t");
	MK_INSN(io_test6, "mov $0xe0, %dx \n\t"
			  "mov $0xffffffff, %eax \n\t"
			  "out %eax, %dx \n\t"
			  "mov $0x00000000, %eax \n\t"
			  "in %dx, %eax \n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_io_test1,
			      insn_io_test1_end - insn_io_test1);

	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0xff)
		print_serial("I/O test 1: FAIL\n");
	else
		print_serial("I/O test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_io_test2,
			      insn_io_test2_end - insn_io_test2);

	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0xffff)
		print_serial("I/O test 2: FAIL\n");
	else
		print_serial("I/O test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_io_test3,
			      insn_io_test3_end - insn_io_test3);

	if (!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0xffffffff)
		print_serial("I/O test 3: FAIL\n");
	else
		print_serial("I/O test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_io_test4,
			      insn_io_test4_end - insn_io_test4);

	if (!regs_equal(&inregs, &outregs, R_AX|R_DX) || outregs.eax != 0xff)
		print_serial("I/O test 4: FAIL\n");
	else
		print_serial("I/O test 4: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_io_test5,
			      insn_io_test5_end - insn_io_test5);

	if (!regs_equal(&inregs, &outregs, R_AX|R_DX) || outregs.eax != 0xffff)
		print_serial("I/O test 5: FAIL\n");
	else
		print_serial("I/O test 5: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_io_test6,
			      insn_io_test6_end - insn_io_test6);

	if (!regs_equal(&inregs, &outregs, R_AX|R_DX) || outregs.eax != 0xffffffff)
		print_serial("I/O test 6: FAIL\n");
	else
		print_serial("I/O test 6: PASS\n");
}

void test_call(void)
{
	struct regs inregs = { 0 }, outregs;
	u32 esp[16];

	inregs.esp = (u32)esp;

	MK_INSN(call1, "mov $test_function, %eax \n\t"
		       "call *%eax\n\t");
	MK_INSN(call_near1, "jmp 2f\n\t"
			    "1: mov $0x1234, %eax\n\t"
			    "ret\n\t"
			    "2: call 1b\t");
	MK_INSN(call_near2, "call 1f\n\t"
			    "jmp 2f\n\t"
			    "1: mov $0x1234, %eax\n\t"
			    "ret\n\t"
			    "2:\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_call1,
			      insn_call1_end - insn_call1);
	if(!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x1234)
		print_serial("Call Test 1: FAIL\n");
	else
		print_serial("Call Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_call_near1, insn_call_near1_end - insn_call_near1);
	if(!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x1234)
		print_serial("Call near Test 1: FAIL\n");
	else
		print_serial("Call near Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_call_near2, insn_call_near2_end - insn_call_near2);
	if(!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x1234)
		print_serial("Call near Test 2: FAIL\n");
	else
		print_serial("Call near Test 2: PASS\n");
}

void test_jcc_short(void)
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(jnz_short1, "jnz 1f\n\t"
			    "mov $0x1234, %eax\n\t"
		            "1:\n\t");
	MK_INSN(jnz_short2, "1:\n\t"
			    "cmp $0x1234, %eax\n\t"
			    "mov $0x1234, %eax\n\t"
		            "jnz 1b\n\t");
	MK_INSN(jmp_short1, "jmp 1f\n\t"
		      "mov $0x1234, %eax\n\t"
		      "1:\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_jnz_short1, insn_jnz_short1_end - insn_jnz_short1);
	if(!regs_equal(&inregs, &outregs, 0))
		print_serial("JNZ short Test 1: FAIL\n");
	else
		print_serial("JNZ short Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_jnz_short2, insn_jnz_short2_end - insn_jnz_short2);
	if(!regs_equal(&inregs, &outregs, R_AX) || !(outregs.eflags & (1 << 6)))
		print_serial("JNZ short Test 2: FAIL\n");
	else
		print_serial("JNZ short Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_jmp_short1, insn_jmp_short1_end - insn_jmp_short1);
	if(!regs_equal(&inregs, &outregs, 0))
		print_serial("JMP short Test 1: FAIL\n");
	else
		print_serial("JMP short Test 1: PASS\n");
}

void test_jcc_near(void)
{
	struct regs inregs = { 0 }, outregs;
	/* encode near jmp manually. gas will not do it if offsets < 127 byte */
	MK_INSN(jnz_near1, ".byte 0x0f, 0x85, 0x06, 0x00\n\t"
		           "mov $0x1234, %eax\n\t");
	MK_INSN(jnz_near2, "cmp $0x1234, %eax\n\t"
			   "mov $0x1234, %eax\n\t"
		           ".byte 0x0f, 0x85, 0xf0, 0xff\n\t");
	MK_INSN(jmp_near1, ".byte 0xE9, 0x06, 0x00\n\t"
		           "mov $0x1234, %eax\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_jnz_near1, insn_jnz_near1_end - insn_jnz_near1);
	if(!regs_equal(&inregs, &outregs, 0))
		print_serial("JNZ near Test 1: FAIL\n");
	else
		print_serial("JNZ near Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_jnz_near2, insn_jnz_near2_end - insn_jnz_near2);
	if(!regs_equal(&inregs, &outregs, R_AX) || !(outregs.eflags & (1 << 6)))
		print_serial("JNZ near Test 2: FAIL\n");
	else
		print_serial("JNZ near Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_jmp_near1, insn_jmp_near1_end - insn_jmp_near1);
	if(!regs_equal(&inregs, &outregs, 0))
		print_serial("JMP near Test 1: FAIL\n");
	else
		print_serial("JMP near Test 1: PASS\n");
}

void test_long_jmp()
{
	struct regs inregs = { 0 }, outregs;
	u32 esp[16];

	inregs.esp = (u32)esp;
	MK_INSN(long_jmp, "call 1f\n\t"
			  "jmp 2f\n\t"
			  "1: jmp $0, $test_function\n\t"
		          "2:\n\t");
	exec_in_big_real_mode(&inregs, &outregs,
			      insn_long_jmp,
			      insn_long_jmp_end - insn_long_jmp);
	if(!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 0x1234)
		print_serial("Long JMP Test: FAIL\n");
	else
		print_serial("Long JMP Test: PASS\n");
}

void test_push_pop()
{
	struct regs inregs = { 0 }, outregs;
	MK_INSN(push32, "mov $0x12345678, %eax\n\t"
			"push %eax\n\t"
			"pop %ebx\n\t");
	MK_INSN(push16, "mov $0x1234, %ax\n\t"
			"push %ax\n\t"
			"pop %bx\n\t");

	MK_INSN(push_es, "mov $0x231, %bx\n\t" //Just write a dummy value to see if it gets overwritten
			 "mov $0x123, %ax\n\t"
			 "mov %ax, %es\n\t"
			 "push %es\n\t"
			 "pop %bx \n\t"
			 );
	MK_INSN(pop_es, "push %ax\n\t"
			"pop %es\n\t"
			"mov %es, %bx\n\t"
			);
	MK_INSN(push_pop_ss, "push %ss\n\t"
			     "pushw %ax\n\t"
			     "popw %ss\n\t"
			     "mov %ss, %bx\n\t"
			     "pop %ss\n\t"
			);
	MK_INSN(push_pop_fs, "push %fs\n\t"
			     "pushl %eax\n\t"
			     "popl %fs\n\t"
			     "mov %fs, %ebx\n\t"
			     "pop %fs\n\t"
			);

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_push32,
			      insn_push32_end - insn_push32);
	if (!regs_equal(&inregs, &outregs, R_AX|R_BX) || outregs.eax != outregs.ebx || outregs.eax != 0x12345678)
		print_serial("Push/Pop Test 1: FAIL\n");
	else
		print_serial("Push/Pop Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_push16,
			      insn_push16_end - insn_push16);

	if (!regs_equal(&inregs, &outregs, R_AX|R_BX) || outregs.eax != outregs.ebx || outregs.eax != 0x1234)
		print_serial("Push/Pop Test 2: FAIL\n");
	else
		print_serial("Push/Pop Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_push_es,
			      insn_push_es_end - insn_push_es);
	if (!regs_equal(&inregs, &outregs, R_AX|R_BX) ||  outregs.ebx != outregs.eax || outregs.eax != 0x123)
		print_serial("Push/Pop Test 3: FAIL\n");
	else
		print_serial("Push/Pop Test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_pop_es,
			      insn_pop_es_end - insn_pop_es);

	if (!regs_equal(&inregs, &outregs, R_AX|R_BX) || outregs.ebx != outregs.eax)
		print_serial("Push/Pop Test 4: FAIL\n");
	else
		print_serial("Push/Pop Test 4: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_push_pop_ss,
			      insn_push_pop_ss_end - insn_push_pop_ss);

	if (!regs_equal(&inregs, &outregs, R_AX|R_BX) || outregs.ebx != outregs.eax)
		print_serial("Push/Pop Test 5: FAIL\n");
	else
		print_serial("Push/Pop Test 5: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_push_pop_fs,
			      insn_push_pop_fs_end - insn_push_pop_fs);

	if (!regs_equal(&inregs, &outregs, R_AX|R_BX) || outregs.ebx != outregs.eax)
		print_serial("Push/Pop Test 6: FAIL\n");
	else
		print_serial("Push/Pop Test 6: PASS\n");
}

void test_null(void)
{
	struct regs inregs = { 0 }, outregs;
	exec_in_big_real_mode(&inregs, &outregs, 0, 0);
	if (!regs_equal(&inregs, &outregs, 0))
		print_serial("null test: FAIL\n");
	else
		print_serial("null test: PASS\n");
}

struct {
    char stack[500];
    char top[];
} tmp_stack;

void test_pusha_popa()
{
	struct regs inregs = { .eax = 0, .ebx = 1, .ecx = 2, .edx = 3, .esi = 4, .edi = 5, .ebp = 6, .esp = (unsigned long)&tmp_stack.top }, outregs;

	MK_INSN(pusha, "pusha\n\t"
		       "pop %edi\n\t"
		       "pop %esi\n\t"
		       "pop %ebp\n\t"
		       "add $4, %esp\n\t"
		       "pop %ebx\n\t"
		       "pop %edx\n\t"
		       "pop %ecx\n\t"
		       "pop %eax\n\t"
		       );

	MK_INSN(popa, "push %eax\n\t"
		      "push %ecx\n\t"
		      "push %edx\n\t"
		      "push %ebx\n\t"
		      "push %esp\n\t"
		      "push %ebp\n\t"
		      "push %esi\n\t"
		      "push %edi\n\t"
		      "popa\n\t"
		      );

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_pusha,
			      insn_pusha_end - insn_pusha);

	if (!regs_equal(&inregs, &outregs, 0))
		print_serial("Pusha/Popa Test1: FAIL\n");
	else
		print_serial("Pusha/Popa Test1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_popa,
			      insn_popa_end - insn_popa);
	if (!regs_equal(&inregs, &outregs, 0))
		print_serial("Pusha/Popa Test2: FAIL\n");
	else
		print_serial("Pusha/Popa Test2: PASS\n");
}

void test_iret()
{
	struct regs inregs = { 0 }, outregs;

	MK_INSN(iret32, "pushf\n\t"
			"pushl %cs\n\t"
			"call 1f\n\t" /* a near call will push eip onto the stack */
			"jmp 2f\n\t"
			"1: iret\n\t"
			"2:\n\t"
		     );

	MK_INSN(iret16, "pushfw\n\t"
			"pushw %cs\n\t"
			"callw 1f\n\t"
			"jmp 2f\n\t"
			"1: iretw\n\t"
			"2:\n\t");

	MK_INSN(iret_flags32, "pushfl\n\t"
			      "popl %eax\n\t"
			      "andl $~0x2, %eax\n\t"
			      "orl $0xffc08028, %eax\n\t"
			      "pushl %eax\n\t"
			      "pushl %cs\n\t"
			      "call 1f\n\t"
			      "jmp 2f\n\t"
			      "1: iret\n\t"
			      "2:\n\t");

	MK_INSN(iret_flags16, "pushfw\n\t"
			      "popw %ax\n\t"
			      "and $~0x2, %ax\n\t"
			      "or $0x8028, %ax\n\t"
			      "pushw %ax\n\t"
			      "pushw %cs\n\t"
			      "callw 1f\n\t"
			      "jmp 2f\n\t"
			      "1: iretw\n\t"
			      "2:\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_iret32,
			      insn_iret32_end - insn_iret32);

	if (!regs_equal(&inregs, &outregs, 0))
		print_serial("iret Test 1: FAIL\n");
	else
		print_serial("iret Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_iret16,
			      insn_iret16_end - insn_iret16);

	if (!regs_equal(&inregs, &outregs, 0))
		print_serial("iret Test 2: FAIL\n");
	else
		print_serial("iret Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_iret_flags32,
			      insn_iret_flags32_end - insn_iret_flags32);

	if (!regs_equal(&inregs, &outregs, R_AX))
		print_serial("iret Test 3: FAIL\n");
	else
		print_serial("iret Test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_iret_flags16,
			      insn_iret_flags16_end - insn_iret_flags16);

	if (!regs_equal(&inregs, &outregs, R_AX))
		print_serial("iret Test 4: FAIL\n");
	else
		print_serial("iret Test 4: PASS\n");
}

void test_int()
{
	struct regs inregs = { 0 }, outregs;

	*(u32 *)(0x11 * 4) = 0x1000; /* Store a pointer to address 0x1000 in IDT entry 0x11 */
	*(u8 *)(0x1000) = 0xcf; /* 0x1000 contains an IRET instruction */

	MK_INSN(int11, "int $0x11\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_int11,
			      insn_int11_end - insn_int11);

	if (!regs_equal(&inregs, &outregs, 0))
		print_serial("int Test 1: FAIL\n");
	else
		print_serial("int Test 1: PASS\n");
}

void test_imul()
{
	struct regs inregs = { 0 }, outregs;

	MK_INSN(imul8_1, "mov $2, %al\n\t"
			"mov $-4, %cx\n\t"
			"imul %cl\n\t");

	MK_INSN(imul16_1, "mov $2, %ax\n\t"
		      "mov $-4, %cx\n\t"
		      "imul %cx\n\t");

	MK_INSN(imul32_1, "mov $2, %eax\n\t"
		       "mov $-4, %ecx\n\t"
		       "imul %ecx\n\t");

	MK_INSN(imul8_2, "mov $0x12340002, %eax\n\t"
			"mov $4, %cx\n\t"
			"imul %cl\n\t");

	MK_INSN(imul16_2, "mov $2, %ax\n\t"
			"mov $4, %cx\n\t"
			"imul %cx\n\t");

	MK_INSN(imul32_2, "mov $2, %eax\n\t"
			"mov $4, %ecx\n\t"
			"imul %ecx\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_imul8_1,
			      insn_imul8_1_end - insn_imul8_1);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || (outregs.eax & 0xff) != (u8)-8)
		print_serial("imul Test 1: FAIL\n");
	else
		print_serial("imul Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_imul16_1,
			      insn_imul16_1_end - insn_imul16_1);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != (u16)-8)
		print_serial("imul Test 2: FAIL\n");
	else
		print_serial("imul Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_imul32_1,
			      insn_imul32_1_end - insn_imul32_1);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != (u32)-8)
		print_serial("imul Test 3: FAIL\n");
	else
		print_serial("imul Test 3: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_imul8_2,
			      insn_imul8_2_end - insn_imul8_2);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || (outregs.eax & 0xffff) != 8 ||
	     (outregs.eax & 0xffff0000) != 0x12340000)
		print_serial("imul Test 4: FAIL\n");
	else
		print_serial("imul Test 4: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_imul16_2,
			      insn_imul16_2_end - insn_imul16_2);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != 8)
		print_serial("imul Test 5: FAIL\n");
	else
		print_serial("imul Test 5: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_imul32_2,
			      insn_imul32_2_end - insn_imul32_2);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != 8)
		print_serial("imul Test 6: FAIL\n");
	else
		print_serial("imul Test 6: PASS\n");
}

void test_mul()
{
	struct regs inregs = { 0 }, outregs;

	MK_INSN(mul8, "mov $2, %al\n\t"
			"mov $4, %cx\n\t"
			"imul %cl\n\t");

	MK_INSN(mul16, "mov $2, %ax\n\t"
			"mov $4, %cx\n\t"
			"imul %cx\n\t");

	MK_INSN(mul32, "mov $2, %eax\n\t"
			"mov $4, %ecx\n\t"
			"imul %ecx\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mul8,
			      insn_mul8_end - insn_mul8);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || (outregs.eax & 0xff) != 8)
		print_serial("mul Test 1: FAIL\n");
	else
		print_serial("mul Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mul16,
			      insn_mul16_end - insn_mul16);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != 8)
		print_serial("mul Test 2: FAIL\n");
	else
		print_serial("mul Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_mul32,
			      insn_mul32_end - insn_mul32);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != 8)
		print_serial("mul Test 3: FAIL\n");
	else
		print_serial("mul Test 3: PASS\n");
}

void test_div()
{
	struct regs inregs = { 0 }, outregs;

	MK_INSN(div8, "mov $257, %ax\n\t"
			"mov $2, %cl\n\t"
			"div %cl\n\t");

	MK_INSN(div16, "mov $512, %ax\n\t"
			"mov $5, %cx\n\t"
			"div %cx\n\t");

	MK_INSN(div32, "mov $512, %eax\n\t"
			"mov $5, %ecx\n\t"
			"div %ecx\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			     insn_div8,
			     insn_div8_end - insn_div8);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != 384)
		print_serial("div Test 1: FAIL\n");
	else
		print_serial("div Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_div16,
			      insn_div16_end - insn_div16);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != 102 ||
	    outregs.edx != 2)
		print_serial("div Test 2: FAIL\n");
	else
		print_serial("div Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_div32,
			      insn_div32_end - insn_div32);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != 102 ||
	    outregs.edx != 2)
		print_serial("div Test 3: FAIL\n");
	else
		print_serial("div Test 3: PASS\n");
}

void test_idiv()
{
	struct regs inregs = { 0 }, outregs;

	MK_INSN(idiv8, "mov $256, %ax\n\t"
			"mov $-2, %cl\n\t"
			"idiv %cl\n\t");

	MK_INSN(idiv16, "mov $512, %ax\n\t"
			"mov $-2, %cx\n\t"
			"idiv %cx\n\t");

	MK_INSN(idiv32, "mov $512, %eax\n\t"
			"mov $-2, %ecx\n\t"
			"idiv %ecx\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			     insn_idiv8,
			     insn_idiv8_end - insn_idiv8);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != (u8)-128)
		print_serial("idiv Test 1: FAIL\n");
	else
		print_serial("idiv Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_idiv16,
			      insn_idiv16_end - insn_idiv16);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != (u16)-256)
		print_serial("idiv Test 2: FAIL\n");
	else
		print_serial("idiv Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_idiv32,
			      insn_idiv32_end - insn_idiv32);

	if (!regs_equal(&inregs, &outregs, R_AX | R_CX | R_DX) || outregs.eax != (u32)-256)
		print_serial("idiv Test 3: FAIL\n");
	else
		print_serial("idiv Test 3: PASS\n");
}

void test_cbw(void)
{
	struct regs inregs = { 0 }, outregs;

	MK_INSN(cbw, "mov $0xFE, %eax \n\t"
		     "cbw\n\t");
	MK_INSN(cwde, "mov $0xFFFE, %eax \n\t"
		      "cwde\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_cbw,
			      insn_cbw_end - insn_cbw);
	if (outregs.eax != 0xFFFE)
		print_serial("cbw test1: FAIL\n");
	else
		print_serial("cbw test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			      insn_cwde,
			      insn_cwde_end - insn_cwde);
	if (outregs.eax != 0xFFFFFFFE)
		print_serial("cwde test1: FAIL\n");
	else
		print_serial("cwde test 1: PASS\n");
}

void test_loopcc(void)
{
	struct regs inregs = { 0 }, outregs;

	MK_INSN(loop, "mov $10, %ecx\n\t"
		      "1: inc %eax\n\t"
		      "loop 1b\n\t");

	MK_INSN(loope, "mov $10, %ecx\n\t"
		       "mov $1, %eax\n\t"
		       "1: dec %eax\n\t"
		       "loope 1b\n\t");

	MK_INSN(loopne, "mov $10, %ecx\n\t"
		        "mov $5, %eax\n\t"
		        "1: dec %eax\n\t"
			"loopne 1b\n\t");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_loop, insn_loop_end - insn_loop);
	if(!regs_equal(&inregs, &outregs, R_AX) || outregs.eax != 10)
		print_serial("LOOPcc short Test 1: FAIL\n");
	else
		print_serial("LOOPcc short Test 1: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_loope, insn_loope_end - insn_loope);
	if(!regs_equal(&inregs, &outregs, R_AX | R_CX) ||
	   outregs.eax != -1 || outregs.ecx != 8)
		print_serial("LOOPcc short Test 2: FAIL\n");
	else
		print_serial("LOOPcc short Test 2: PASS\n");

	exec_in_big_real_mode(&inregs, &outregs,
			insn_loopne, insn_loopne_end - insn_loopne);
	if(!regs_equal(&inregs, &outregs, R_AX | R_CX) ||
	   outregs.eax != 0 || outregs.ecx != 5)
		print_serial("LOOPcc short Test 3: FAIL\n");
	else
		print_serial("LOOPcc short Test 3: PASS\n");
}

void realmode_start(void)
{
	test_null();

	test_shld();
	test_push_pop();
	test_pusha_popa();
	test_mov_imm();
	test_cmp_imm();
	test_add_imm();
	test_sub_imm();
	test_xor_imm();
	test_io();
	test_eflags_insn();
	test_jcc_short();
	test_jcc_near();
	/* test_call() uses short jump so call it after testing jcc */
	test_call();
	/* long jmp test uses call near so test it after testing call */
	test_long_jmp();
	test_xchg();
	test_iret();
	test_int();
	test_imul();
	test_mul();
	test_div();
	test_idiv();
	test_loopcc();
	test_cbw();

	exit(0);
}

unsigned long long r_gdt[] = { 0, 0x9b000000ffff, 0x93000000ffff };

struct __attribute__((packed)) {
	unsigned short limit;
	void *base;
} r_gdt_descr = { sizeof(r_gdt) - 1, &r_gdt };

asm(
	".section .init \n\t"

	".code32 \n\t"

	"mb_magic = 0x1BADB002 \n\t"
	"mb_flags = 0x0 \n\t"

	"# multiboot header \n\t"
	".long mb_magic, mb_flags, 0 - (mb_magic + mb_flags) \n\t"

	".globl start \n\t"
	".data \n\t"
	". = . + 4096 \n\t"
	"stacktop: \n\t"

	".text \n\t"
	"start: \n\t"
	"lgdt r_gdt_descr \n\t"
	"ljmp $8, $1f; 1: \n\t"
	".code16gcc \n\t"
	"mov $16, %eax \n\t"
	"mov %ax, %ds \n\t"
	"mov %ax, %es \n\t"
	"mov %ax, %fs \n\t"
	"mov %ax, %gs \n\t"
	"mov %ax, %ss \n\t"
	"mov %cr0, %eax \n\t"
	"btc $0, %eax \n\t"
	"mov %eax, %cr0 \n\t"
	"ljmp $0, $realmode_entry \n\t"

	"realmode_entry: \n\t"

	"xor %ax, %ax \n\t"
	"mov %ax, %ds \n\t"
	"mov %ax, %es \n\t"
	"mov %ax, %ss \n\t"
	"mov %ax, %fs \n\t"
	"mov %ax, %gs \n\t"
	"mov $stacktop, %esp\n\t"
	"ljmp $0, $realmode_start \n\t"

	".code16gcc \n\t"
	);