this is my boot.s:
Code: Select all
# 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
# 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:
# Physical address of boot_page_table1.
# 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_table1 - 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 1023 pages. The 1024th will be the VGA text buffer.
movl $1023, %ecx
1:
# Only map the kernel.
cmpl $_kernel_start, %esi
jl 2f
cmpl $(_kernel_end - 0xC0000000), %esi
jge 3f
# 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 $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:
# Map VGA video memory to 0xC03FF000 as "present, writable".
movl $(0x000A0000 | 0x003), boot_page_table1 - 0xC0000000 + 1023 * 4
# 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 + 0x003), boot_page_directory - 0xC0000000 + 0
movl $(boot_page_table1 - 0xC0000000 + 0x003), boot_page_directory - 0xC0000000 + 768 * 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
.section .text
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.
call kernel_main
# Infinite loop if the system has nothing more to do.
cli
1: hlt
jmp 1b
