os/kernel/cpu/i386/boot.s

.extern kmain

# Declare a multiboot header that marks the program as a kernel.
.section .multiboot
 header_start:
     .long 0xe85250d6                # magic number (multiboot 2)
     .long 0                         # architecture 0 (protected mode i386)
     .long header_end - header_start # header length
     # checksum
     .long 0x100000000 - (0xe85250d6 + 0 + (header_end - header_start))

     # info request tag
     info_tag_start:
     .word 1    # type
     .word 0    # flags
     .long info_tag_end-info_tag_start #size
     .long 4
     .long 6
     info_tag_end:

     # entry addr tag
     .word 3 # type
     .word 0 # flags
     .long 12 # size
     .long _start- 0xC0000000 # entry_addr
     .long 0 #alignment

		 # page align tag
		 .word 6 # type
		 .word 0 # flags
		 .long 8 # size

     # required end tag
     .word 0    # type
     .word 0    # flags
     .long 8    # size
 header_end:


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

.global int_stack_top

.section .interrupt_stack, "aw", @nobits
int_stack_bottom:
.skip 16384 # 16 KiB
int_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_tables:
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 .text
.global _start
.type _start, @function
_start:
	cmp $0x36d76289, %eax
	jnz no_multiboot
	# Physical address of boot_page_tables.
	# TODO: I recall seeing some assembly that used a macro to do the
	#       conversions to and from physical. Maybe this should be done in this
	#       code as well?
	movl $(boot_page_tables - 0xC0000000), %edi
	# First address to map is address 0.
	# TODO: Start at the first kernel page instead. Alternatively map the first
	#       1 MiB as it can be generally useful, and there's no need to
	#       specially map the VGA buffer.
	movl $0, %esi
	# Map 2048 pages.
	movl $2048, %ecx

1:
	# Map physical address as "present, writable". Note that this maps
	# .text and .rodata as writable. Mind security and map them as non-writable.
	movl %esi, %edx
	orl $0x007, %edx
	movl %edx, (%edi)

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

	# The page table is used at both page directory entry 0 (virtually from 0x0
	# to 0x3FFFFF) (thus identity mapping the kernel) and page directory entry
	# 768 (virtually from 0xC0000000 to 0xC03FFFFF) (thus mapping it in the
	# higher half). The kernel is identity mapped because enabling paging does
	# not change the next instruction, which continues to be physical. The CPU
	# would instead page fault if there was no identity mapping.

	# Map the page table to both virtual addresses 0x00000000 and 0xC0000000.
	movl $(boot_page_table1 - 0xC0000000 + 0x007), boot_page_directory - 0xC0000000 + 0
	movl $(boot_page_table1 - 0xC0000000 + 0x007), boot_page_directory - 0xC0000000 + 768 * 4
	movl $(boot_page_table2 - 0xC0000000 + 0x007), 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

  #Enable PSE (Page Size Extension, allows for 4MiB pages)
  movl %cr4, %ecx
  orl $0x00000010, %ecx
  movl %ecx, %cr4
  
	# Jump to higher half with an absolute jump.
	lea 4f, %ecx
	jmp *%ecx

4:
	# 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

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

	# Set up the stack.
	mov $stack_top, %esp

	# Enter the high-level kernel.
	add $0xC0000000, %ebx
	push %ebx
	call kmain

	# Infinite loop if the system has nothing more to do.
	no_multiboot:
	loop: jmp loop