; File: boot.asm
global _start
extern kmain

section .multiboot
    ; Multiboot header
    dd 0x1BADB002           ; magic number
    dd 0x00000003           ; flags
    dd -(0x1BADB002 + 0x00000003) ; checksum


section .text
[bits 32]
_start:
    mov esp, kernel_stack_top 
    cli                     ; Clear interrupts

    ; Load the gdt_ptr
    lgdt [gdt_ptr]

    mov eax, pml4_table
    mov cr3, eax            ; Load the PML4 table into CR3

    ; Link PML4 -> PDPT
    mov dword [pml4_table], pdpt_table + 0x3
    mov dword [pml4_table + 4], 0x0

    ; Link PDPT -> PD
    mov dword [pdpt_table], pd_table + 0x3
    mov dword [pdpt_table + 4], 0x0

    ; Link PD -> 2MB Physical Page 0
    mov dword [pd_table], 0x83
    mov dword [pd_table + 4], 0x0

    mov eax, cr4
    or eax, 0x20            ; Enable PAE
    mov cr4, eax

    mov ecx, 0xC0000080   ; IA32_EFER MSR
    rdmsr
    or eax, 0x00000100     ; Set LME (Long Mode Enable) bit
    wrmsr

    ; Enable long mode
    mov eax, cr0
    or eax, 0x80000000      ; Set the Long Mode Enable (LME) bit
    mov cr0, eax

    ; Jump to 64-bit code Segment
    jmp 0x08:long_mode_start

section .bss
; We define the stack size and labels in the uninitialized data section (.bss)
kernel_stack_bottom:
    ; Reserve 16 KB for the stack
    resb 16 * 1024
kernel_stack_top:


pml4_table:
    ; Define a simple PML4 table here (identity mapping for simplicity)
    ; In a real kernel, you would set up proper paging structures
    resb 4096            ; Reserve 4 KB for the PML4 table 
pml4_end:

pdpt_table:
    resb 4096            ; Reserve 4 KB for the PDPT table 
pdpt_end:

pd_table:
    resb 4096            ; Reserve 4 KB for the PD table 
pd_end:

section .data
gdt_ptr:
    dw (gdt_end - gdt_start - 1)                 ; Limit (set by linker/code later)
    dq gdt_start                 ; Base Address (set by linker/code later)
gdt_start:               ; Start of the actual GDT entries
    ; Entry 0: Null Descriptor
    dq 0x0000000000000000
    dq 0x00AF9A000000FFFF
    dq 0x00AF92000000FFFF
gdt_end:

section .text
[bits 64] ; Tell NASM to assemble in 64-bit mode

long_mode_start:
    ; 1. Pass GRUB's Multiboot info (in ebx) as the first C argument (in rdi)
    ;    We must use the 64-bit register RDI, but the data is in the 32-bit EBX.
    ;    Moving ebx to rdi automatically clears the upper 32 bits of rdi.
    mov rdi, rbx 

    ; 2. Call the C kernel's main function
    call kmain

    ; 3. If kmain ever returns, halt the CPU
    hlt