banan-os/kernel/arch/i386/boot.s

115 lines
2.9 KiB
ArmAsm

# Declare constants for the multiboot header
.set ALIGN, 1<<0 # align loaded modules on page boundaries
.set MEMINFO, 1<<1 # provide memory map
.set MB_FLAGS, ALIGN | MEMINFO # this is the Multiboot 'flag' field
.set MB_MAGIC, 0x1BADB002 # 'magic number' lets bootloader find the header
.set MB_CHECKSUM, -(MB_MAGIC + MB_FLAGS) #checksum of above, to prove we are multiboot
# Multiboot header
.section .multiboot.data, "aw"
.align 4
.long MB_MAGIC
.long MB_FLAGS
.long MB_CHECKSUM
# Create stack
.section .bootstrap_stack, "aw", @nobits
stack_bottom:
.skip 16384 # 16 KiB
stack_top:
# Preallocate pages
.section .bss, "aw", @nobits
.align 4096
boot_page_directory:
.skip 4096
boot_page_table1:
.skip 4096
# Kernel entrypoint
.section .multiboot.text, "a"
.global _start
.type _start, @function
_start:
# Physical address of boot_page_table1
movl $(boot_page_table1 - 0xC0000000), %edi
# First address to map is 0
movl $0, %esi
# Map 1023 pages, 1024th will be VGA memory
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 entry in boot_page_table1 is 4 bytes
addl $4, %edi
# Loop to next entry if we haven't finished
loop 1b
3:
# Map VGA memory to 0xC03FF000 as "present, writable"
movl $(0x000B8000 | 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 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:
# Now paging is fully set up and enabled
# Unmap the identity mapping since it is not unnecessary
movl $0, boot_page_directory + 0
# Reload crc3 to force a TLB flush for the changes to take effect
movl %cr3, %ecx
movl %ecx, %cr3
# Setup stack
mov $stack_top, %esp
# Call into C code
call kernel_main
# Hang if kernel_main returns
cli
1: hlt
jmp 1b