# 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 .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: .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 boot_page_table3: .skip 4096 boot_page_table4: .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 $0x2BADB002, %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 4096 pages. movl $4096, %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 movl $(boot_page_table3 - 0xC0000000 + 0x007), boot_page_directory - 0xC0000000 + 770 * 4 movl $(boot_page_table4 - 0xC0000000 + 0x007), boot_page_directory - 0xC0000000 + 771 * 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 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