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# 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
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