127 lines
4.0 KiB
ArmAsm
127 lines
4.0 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 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
|