# Declare constants for the multiboot header.
.set ALIGN,    1<<0             # align loaded modules on page boundaries
.set MEMINFO,  1<<1             # provide memory map
.set FLAGS,    ALIGN | MEMINFO  # this is the Multiboot 'flag' field
.set MAGIC,    0x1BADB002       # 'magic number' lets bootloader find the header
.set CHECKSUM, -(MAGIC + FLAGS) # checksum of above, to prove we are multiboot

# Declare a multiboot header that marks the program as a kernel.
.section .multiboot.data, "aw"
.align 4
.long MAGIC
.long FLAGS
.long CHECKSUM

# Allocate the initial stack.
.section .bootstrap_stack, "aw", @nobits
stack_bottom:
.skip 16384 # 16 KiB
stack_top:

# Preallocate pages used for paging. Don't hard-code addresses and assume they
# are available, as the bootloader might have loaded its multiboot structures or
# modules there. This lets the bootloader know it must avoid the addresses.
.section .bss, "aw", @nobits
	.align 4096
boot_page_directory:
	.skip 4096
boot_page_table1:
	.skip 4096
boot_page_table2:
	.skip 4096
# Further page tables may be required if the kernel grows beyond 3 MiB.

# The kernel entry point.
.section .multiboot.text, "a"
.global _start
.type _start, @function
_start:
	movl $(boot_page_table1 - 0xC0000000), %edi
	movl $0, %esi

1:
	jl 2f
	cmpl $0x00400000, %esi
	jge 3f

	movl %esi, %edx
	orl $0x003, %edx
	movl %edx, (%edi)

2:
	# Size of page is 4096 bytes.
	addl $4096, %esi
	# Size of entries in boot_page_table1 is 4 bytes.
	addl $4, %edi
	# Loop to the next entry if we haven't finished.
	loop 1b

3:
	movl $(boot_page_table2 - 0xC0000000), %edi

4:
	jl 5f
	cmpl $0x00800000, %esi
	jge 6f

	movl %esi, %edx
	orl $0x003, %edx
	movl %edx, (%edi)

5:
	# Size of page is 4096 bytes.
	addl $4096, %esi
	# Size of entries in boot_page_table1 is 4 bytes.
	addl $4, %edi
	# Loop to the next entry if we haven't finished.
	loop 4b

6:
	movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 0 * 4
	movl $(boot_page_table2 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 1 * 4
	movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 768 * 4
	movl $(boot_page_table2 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 769 * 4

	# Set cr3 to the address of the boot_page_directory.
	movl $(boot_page_directory - 0xC0000000), %ecx
	movl %ecx, %cr3

	# Enable paging and the write-protect bit.
	movl %cr0, %ecx
	orl $0x80010000, %ecx
	movl %ecx, %cr0

	# Jump to higher half with an absolute jump. 
	lea 7f, %ecx
	jmp *%ecx

.section .text

7:
	# At this point, paging is fully set up and enabled.

	# Unmap the identity mapping as it is now unnecessary. 
	movl $0, boot_page_directory + 0
	movl $0, boot_page_directory + 4

	# Reload crc3 to force a TLB flush so the changes to take effect.
	movl %cr3, %ecx
	movl %ecx, %cr3

    movl $stack_top, %esp
    call init_gdt_table
    ljmp $0x08, $code

code:
    movw $0x10, %ax
    movw %ax, %ds
    movw %ax, %es
    movw %ax, %fs
    movw %ax, %gs
    movw %ax, %ss
    cli
    call _init
    call kernel_main
    hlt