; https://wiki.osdev.org/Creating_a_64-bit_kernel
; https://wiki.osdev.org/Entering_Long_Mode_Directly

KERNEL_VIRTUAL_BASE equ 0xFFFFFFFF80000000 ; 512GB
KERNEL_LMA equ 0x1000000 ; 16MB
KERNEL_STACK_SIZE equ 0x4000 ; 16KB

extern Multiboot2Entry
extern BootPageTable
extern UpdatePageTable
extern UpdatePageTable64
extern DetectCPUID
extern Detect64Bit
extern LoadGDT32
global MB2_start
extern MB2_start_c

[bits 32]

section .bootstrap.data
global MB2_HeaderMagic
MB2_HeaderMagic:
    times (0x64) dq 0

global MB2_HeaderInfo
MB2_HeaderInfo:
	times (0x64) dq 0

section .bootstrap.text
MB2_start:
	cli
	; mov word [0xb8F00], 0x072E ; .
    mov [MB2_HeaderMagic], eax
    mov [MB2_HeaderInfo], ebx

	; We need to check if the CPU supports 64-bit mode
	call DetectCPUID
	call Detect64Bit

	; mov word [0xb8F02], 0x072E ; .

	mov ecx, cr0
	and ecx, 0x7fffffff ; Clear PG
	mov cr0, ecx

	mov ecx, cr4
	or ecx, 0x10 ; Set PSE
	or ecx, 0x20 ; Set PAE
	mov cr4, ecx

	; Load the GDT and update the page table
	call LoadGDT32
	call UpdatePageTable

	; Load the new page table
	mov edi, BootPageTable
	mov cr3, edi

	; mov word [0xb8F04], 0x072E ; .

	; Enable long mode
	mov ecx, 0xC0000080 ; EFER
	rdmsr
	or eax, 0x800 | 0x100 | 0x1 ; Set LME, LMA, SCE
	wrmsr

	mov ecx, cr0
	or ecx, (0x80000000 | 0x1) ; Set PG and PE
	mov cr0, ecx


	lgdt [GDT64.Ptr]
    ; xor eax, eax
    ; sgdt [eax]
    ; test eax, eax
    ; jz .InvalidGDT

	; .InvalidGDT:
	; 	mov word [0xb8F07], 0x4 ; Red
	; 	hlt

	jmp GDT64.code:HigherHalfStart

[bits 64]
HigherHalfStart:
	cli
	; mov word [0xb8F06], 0x072E ; .
	call UpdatePageTable64

	; Load the new page table
	mov rdi, BootPageTable
	mov cr3, rdi

	mov ax, GDT64.data
	mov ds, ax
	mov es, ax
	mov fs, ax
	mov gs, ax
	mov ss, ax

	; mov word [0xb8F08], 0x072E ; .
	mov rsp, (KernelStack + KERNEL_STACK_SIZE)
	mov rbp, (KernelStack + KERNEL_STACK_SIZE)

	cld
	cli

	call Multiboot2Entry
	.Loop:
		hlt
		jmp .Loop

section .bootstrap.bss
align 16
KernelStack: 
	resb KERNEL_STACK_SIZE

; Access bits
PRESENT equ 1 << 7
NOT_SYS equ 1 << 4
EXEC equ 1 << 3
DC equ 1 << 2
RW equ 1 << 1
ACCESSED equ 1 << 0
 
; Flags bits
GRAN_4K equ 1 << 7
SZ_32 equ 1 << 6
LONG_MODE equ 1 << 5

section .bootstrap.data
GDT64:
    .null: equ $ - GDT64
        dq 0
    .code: equ $ - GDT64
        dd 0xFFFF
        db 0
        db PRESENT | NOT_SYS | EXEC | RW
        db GRAN_4K | LONG_MODE | 0xF
        db 0
    .data: equ $ - GDT64
        dd 0xFFFF
        db 0
        db PRESENT | NOT_SYS | RW
        db GRAN_4K | SZ_32 | 0xF
        db 0
    .tss: equ $ - GDT64
        dd 0x00000068
        dd 0x00CF8900
	.Ptr:
		dw $ - GDT64 - 1
		dq GDT64