146 lines
4.1 KiB
ArmAsm
146 lines
4.1 KiB
ArmAsm
.extern kmain
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# Declare a multiboot header that marks the program as a kernel.
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.section .multiboot
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header_start:
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.long 0xe85250d6 # magic number (multiboot 2)
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.long 0 # architecture 0 (protected mode i386)
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.long header_end - header_start # header length
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# checksum
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.long 0x100000000 - (0xe85250d6 + 0 + (header_end - header_start))
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# info request tag
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info_tag_start:
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.word 1 # type
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.word 0 # flags
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.long info_tag_end-info_tag_start #size
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.long 4
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.long 6
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info_tag_end:
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# entry addr tag
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.word 3 # type
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.word 0 # flags
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.long 12 # size
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.long _start- 0xC0000000 # entry_addr
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.long 0 #alignment
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# page align tag
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.word 6 # type
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.word 0 # flags
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.long 8 # size
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# required end tag
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.word 0 # type
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.word 0 # flags
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.long 8 # size
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header_end:
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# Allocate the initial stack.
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.section .bootstrap_stack, "aw", @nobits
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stack_bottom:
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.skip 16384 # 16 KiB
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stack_top:
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.global int_stack_top
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.section .interrupt_stack, "aw", @nobits
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int_stack_bottom:
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.skip 16384 # 16 KiB
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int_stack_top:
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# Preallocate pages used for paging. Don't hard-code addresses and assume they
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# are available, as the bootloader might have loaded its multiboot structures or
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# modules there. This lets the bootloader know it must avoid the addresses.
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.section .bss, "aw", @nobits
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.align 4096
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boot_page_directory:
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.skip 4096
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boot_page_tables:
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boot_page_table1:
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.skip 4096
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boot_page_table2:
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.skip 4096
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# Further page tables may be required if the kernel grows beyond 3 MiB.
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# The kernel entry point.
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.section .text
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.global _start
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.type _start, @function
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_start:
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cmp $0x36d76289, %eax
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jnz no_multiboot
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# Physical address of boot_page_tables.
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# TODO: I recall seeing some assembly that used a macro to do the
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# conversions to and from physical. Maybe this should be done in this
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# code as well?
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movl $(boot_page_tables - 0xC0000000), %edi
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# First address to map is address 0.
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# TODO: Start at the first kernel page instead. Alternatively map the first
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# 1 MiB as it can be generally useful, and there's no need to
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# specially map the VGA buffer.
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movl $0, %esi
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# Map 2048 pages.
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movl $2048, %ecx
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1:
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# Map physical address as "present, writable". Note that this maps
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# .text and .rodata as writable. Mind security and map them as non-writable.
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movl %esi, %edx
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orl $0x007, %edx
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movl %edx, (%edi)
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# Size of page is 4096 bytes.
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addl $4096, %esi
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# Size of entries in boot_page_tables is 4 bytes.
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addl $4, %edi
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# Loop to the next entry if we haven't finished.
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loop 1b
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# The page table is used at both page directory entry 0 (virtually from 0x0
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# to 0x3FFFFF) (thus identity mapping the kernel) and page directory entry
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# 768 (virtually from 0xC0000000 to 0xC03FFFFF) (thus mapping it in the
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# higher half). The kernel is identity mapped because enabling paging does
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# not change the next instruction, which continues to be physical. The CPU
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# would instead page fault if there was no identity mapping.
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# Map the page table to both virtual addresses 0x00000000 and 0xC0000000.
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movl $(boot_page_table1 - 0xC0000000 + 0x007), boot_page_directory - 0xC0000000 + 0
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movl $(boot_page_table1 - 0xC0000000 + 0x007), boot_page_directory - 0xC0000000 + 768 * 4
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movl $(boot_page_table2 - 0xC0000000 + 0x007), boot_page_directory - 0xC0000000 + 769 * 4
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# Set cr3 to the address of the boot_page_directory.
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movl $(boot_page_directory - 0xC0000000), %ecx
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movl %ecx, %cr3
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# Enable paging and the write-protect bit.
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movl %cr0, %ecx
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orl $0x80010000, %ecx
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movl %ecx, %cr0
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# Jump to higher half with an absolute jump.
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lea 4f, %ecx
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jmp *%ecx
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4:
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# At this point, paging is fully set up and enabled.
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# Unmap the identity mapping as it is now unnecessary.
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movl $0, boot_page_directory + 0
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# Reload crc3 to force a TLB flush so the changes to take effect.
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movl %cr3, %ecx
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movl %ecx, %cr3
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# Set up the stack.
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mov $stack_top, %esp
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# Enter the high-level kernel.
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add $0xC0000000, %ebx
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push %ebx
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call kmain
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# Infinite loop if the system has nothing more to do.
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no_multiboot:
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loop: jmp loop
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