section .multiboot2
align 8
HEADER_START:
    dd 0xE85250D6
    dd 0
    dd (HEADER_END - HEADER_START)
    dd -(0xE85250D6 + (HEADER_END - HEADER_START))

; Request 640x480 framebuffer.
align 8
    dw 5
    dw 0
    dd 20
    dd 640
    dd 480
    dd 32

align 8
    dw 0
    dw 0
    dd 8
HEADER_END:

section .bss
STACK_BOTTOM: resb 16384
STACK_TOP:

align 4096
BOOT_PAGE_DIR0: resb 4096
; Tables directly next to eachother for cleanliness.
BOOT_PAGE_TBL0: resb 4096
BOOT_PAGE_TBL1: resb 4096

section .text
extern main
extern _kernel_start
extern _kernel_end

global _start:function (_start.end - _start)
_start:
    mov esp, STACK_TOP - 0xE0000000

    mov edi, BOOT_PAGE_TBL0 - 0xE0000000
    mov esi, 0
    mov ecx, 2048 - 301 ; 301 pages for the framebuffer in case they're not page aligned, which I'm not sure of yet.
.loo:
    cmp esi, _kernel_start - 0xE0000000
    jl .loopcond
    cmp esi, _kernel_end - 0xE0000000
    jge .loopend

    mov eax, esi
    or eax, 3
    mov [edi], eax
.loopcond:
    add esi, 4096
    add edi, 4
    loop .loo
.loopend:
    call _find_framebuffer - 0xE0000000
    call _page_framebuffer - 0xE0000000

    ; Identity mapping.
    ; +3 is the flags.
    mov dword [BOOT_PAGE_DIR0 - 0xE0000000 + (000 * 4)], (BOOT_PAGE_TBL0 - 0xE0000000 + 3)
    mov dword [BOOT_PAGE_DIR0 - 0xE0000000 + (001 * 4)], (BOOT_PAGE_TBL1 - 0xE0000000 + 3)

    ; Mapping to 0xE0000000.
    mov dword [BOOT_PAGE_DIR0 - 0xE0000000 + (896 * 4)], (BOOT_PAGE_TBL0 - 0xE0000000 + 3)
    mov dword [BOOT_PAGE_DIR0 - 0xE0000000 + (897 * 4)], (BOOT_PAGE_TBL1 - 0xE0000000 + 3)

    mov ecx, BOOT_PAGE_DIR0 - 0xE0000000
    mov cr3, ecx

    mov ecx, cr0
    or ecx, 0x80010000
    mov cr0, ecx

    add esp, 0xE0000000

    jmp .afterpage ; Higher-half jump.
.afterpage:
    push ebx
    call main

    cli
.hlt:
    hlt
    jmp .hlt
.end:

; Expects multiboot data to be in ebx.
_find_framebuffer:
    add ebx, 8
.loop:
    cmp dword [ebx], 0 ; End tag type.
    jne .notzero
    ret
.notzero:
    cmp dword [ebx], 8 ; Framebuffer tag type.
    je .isfb
    add ebx, [ebx + 4] ; Add size of tag to address, multiboot2 spec says it's at offset 4.
    ; Align the address, as according to
    add ebx, 7
    and ebx, ~7
    jmp .loop
.isfb:
    mov ebx, [ebx + 8] ; Get framebuffer address, at offset 8.
    ret

; Maps to the page at edi, which was set before in _start.
_page_framebuffer:
    mov ecx, 301 ; 301 in case the framebuffer address is not page aligned, to fit it perfectly. Might come up with a nicer solution later.
    mov esi, ebx ; Copy to esi for readability and to keep ebx unchanged, as we pass it later to main.
    and esi, ~4096 ; Align to page.
.loo:
    mov eax, esi
    or eax, 3
    mov [edi], eax
    loop .loo
    ret