.set ALIGN,		   1 << 0
.set MEMINFO,	   1 << 1
.set VIDMODE,	   1 << 2
.set FLAGS,		   ALIGN | MEMINFO
.set MAGIC,		   0x1BADB002
.set CHECKSUM,	   -(MAGIC + FLAGS)

/* the amount of memory to allocate for bootstrap page directories. */
#define BOOTSTRAP_PDIR_SIZE 0x1000

.extern start_64	# defined in start_64.S

.code32

.section .boot.rodata, "a", @progbits
no_long_mode_err:
    .asciz "This system is not 64-bit. Press any key to reboot."

unsupported_cpu_err:
    .asciz "Your CPU is not supported by Magenta. Press any key to reboot."

.section .boot.bss, "aw", @nobits
bootstrap_pml4t:		# a single PML4T
	.skip 0x1000
bootstrap_pdpt:		# a single PDPT
	.skip 0x1000
bootstrap_pdir_buf:	# a buffer of page directories. each pdir entry can be used to map 2MB of virtual memory.
	.skip BOOTSTRAP_PDIR_SIZE
bootstrap_stack_bottom:
	.skip 0x1000
bootstrap_stack_top:

.section .boot.text, "ax", @progbits
.align 4
.long MAGIC
.long FLAGS
.long CHECKSUM

# Print an error message and wait for RETURN if the system's CPU doesn't support long mode.
no_long_mode:
	# print error message
	mov $no_long_mode_err, %esi

	mov $0xb8000, %edi
1:
	movsb
	movb $12, (%edi)
	incl %edi
	cmpb $0, (%esi)
	jne 1b

2:
	inb $0x64, %al
	test $0x01, %al
	jz 3f
	inb $0x60, %al
	jmp 2b

3:
	inb $0x64, %al
	test $0x01, %al
	jz 3b

	int $0xff
	cli
	hlt

# Print an error message and wait for RETURN if the system's CPU isn't supported.
unsupported_cpu:
	# print error message
	mov $unsupported_cpu_err, %esi

	mov $0xb8000, %edi
1:
	movsb
	movb $12, (%edi)
	incl %edi
	cmpb $0, (%esi)
	jne 1b

2:
	inb $0x64, %al
	test $0x01, %al
	jz 3f
	inb $0x60, %al
	jmp 2b

3:
	inb $0x64, %al
	test $0x01, %al
	jz 3b

	int $0xff
	cli
	hlt

init_page_tables:
	/* STEP 1: add an entry in the PML4T pointing to the PDPT.
	   NOTE that we actually add two entries in the PML4T that both point to the same PDPT.
	   This allows us to map the kernel at both vaddr 0x0 AND vaddr 0xffffffff80000000 using the same PDPT */

	mov $bootstrap_pml4t, %edi
	mov $bootstrap_pdpt, %esi

	# in %ESI, build a PML4T entry containing the base address of the PDPT
	xor $0xFFF, %esi
	or $0x3, %esi

	# add the PML4T entries at PML4T[0] and PML4T[511]
	mov %esi, (%edi)
	add $0xFF8, %edi
	mov %esi, (%edi)

	/* STEP 2: initialise the PDPT.
	   We need to create an entry in the PDPT for every page directory we intend to use. */

	# BOOTSTRAP_PDIR_SIZE contains the total size of the page directory buffer.
	# use it to calculate the number of page directories we're using.
	mov $0, %edx
	mov $BOOTSTRAP_PDIR_SIZE, %eax
	mov $0x1000, %ebx
	idiv %ebx

	mov $bootstrap_pdpt, %edi
	mov $bootstrap_pdir_buf, %esi
	/* %ESI is a pointer into the pagedir buffer.
	   %EDI is a pointer into the PDPT.
	   %EAX is the total number of page directories.
	   %EBX is where we build the PDPT entry. */
1:
	mov %esi, %ebx
	xor $0xFFF, %ebx
	or $0x3, %ebx

	# skip the first 4 bytes of the 8-byte entry, leaving them zero.
	add $0x4, %edi
	mov %ebx, (%edi)

	add $0x1000, %esi
	add $0x4, %edi
	sub $0x1, %eax

	cmp $0x0, %eax
	jne 1b

	/* STEP 3: initialise the page directories.
	   Each page directory can be used to map 2MiB of virtual memory.
	   The exact number of page directories in use is determined by BOOTSTRAP_PDIR_SIZE */

	/* the total number of page directory entries is stored in %EAX */
	mov $0, %edx
	mov $BOOTSTRAP_PDIR_SIZE, %eax
	mov $0x8, %ebx
	idiv %ebx

	mov $0x0, %esi
	mov $bootstrap_pdir_buf, %edi
	/* %EDI is a pointer into the pagedir buffer.
	   %ESI is the physical address being mapped. must be a multiple of 2MiB.
	   %EAX is the total number of page directories.
	   %EBX is where we build the pagedir entry. */

2:
	mov %esi, %ebx
	xor $0xFFF, %ebx
	or $0x3, %ebx

	# skip the first 4 bytes of the 8-byte entry, leaving them zero.
	add $0x4, %edi
	mov %ebx, (%edi)

	add $0x200000, %esi
	add $0x4, %edi
	dec %eax
	jnz 2b

	ret

long_mode_switch:
	ret

.global _start
.type _start, @function

_start:
	mov $bootstrap_stack_top, %esp

	push %ebx								# store the pointer to the multiboot info block.

	# check if long mode is supported
	movl $0x80000000, %eax					# Extended-function 80000000h.
	cpuid									# Is largest extended function
	cmpl $0x80000000, %eax					# any function > 80000000h?
	jbe no_long_mode						# If not, no long mode.
	movl $0x80000001, %eax					# Extended-function 80000001h.
	cpuid									# Now EDX = extended-features flags.
	btl $29, %edx							# Test if long mode is supported.
	jnc no_long_mode						# Exit if not supported.

	movl $1, %eax
	cpuid
	btl $3, %edx

	mov $0xb8000, %eax
	movb $76, (%eax)
	inc %eax
	movb $0xF, (%eax)

	call init_page_tables
	call long_mode_switch					# calls start_64, does not return

	cli
	hlt