; File: boot.asm
[org 0x7c00]        ; BIOS loads us at 0x7C00
[bits 16]           ; We start in 16-bit Real Mode

start:
    mov bp, 0x7c00
    mov sp, bp

    call clear_screen

    mov si, real_msg
    call print_string

    cli 

    lgdt [gdt_descriptor]
    
    mov eax, cr0 
    or eax, 0x1 
    mov cr0, eax

    ; Far jump into 32-bit mode. 
    jmp 0x08:init_pm

; --- Helper: Print String (BIOS INT 0x10) ---
print_string:
    pusha
    mov ah, 0x0e        ; INT 0x10 "Teletype Output" function
.loop:
    lodsb               ; Load byte at [SI] into AL, increment SI
    cmp al, 0           ; Check for null terminator
    je .done
    int 0x10            ; Call BIOS video interrupt
    jmp .loop
.done:
    popa
    ret

; --- Helper: Clear Screen ---
clear_screen:
    pusha
    mov ah, 0x00        ; Set Video Mode function
    mov al, 0x03        ; 80x25 Text Mode
    int 0x10
    popa
    ret

real_msg: db '[REAL] - Loaded successfully.', 0x0D, 0x0A, 0

gdt_start:

gdt_null: ; 8 bytes of zeros
    dd 0x0
    dd 0x0

gdt_code: ; Code Segment (0x08)
    ; Base=0, Limit=0xFFFFF, Access=0x9A, Flags=0xC
    dw 0xffff    ; Limit (bits 0-15)
    dw 0x0       ; Base (bits 0-15)
    db 0x0       ; Base (bits 16-23)
    db 10011010b ; Access Byte (0x9A)
    db 11001111b ; Flags (0xC) + Limit (bits 16-19)
    db 0x0       ; Base (bits 24-31)

gdt_data: ; Data Segment (0x10)
    ; Base=0, Limit=0xFFFFF, Access=0x92, Flags=0xC
    dw 0xffff
    dw 0x0
    db 0x0
    db 10010010b ; Access Byte (0x92)
    db 11001111b ; Flags (0xC) + Limit
    db 0x0

gdt_end:

gdt_descriptor:
    dw gdt_end - gdt_start - 1 ; Size (Limit)
    dd gdt_start               ; Start Address

; ==================================================
; 32-BIT PROTECTED MODE
; ==================================================
[bits 32]
init_pm:
    ; 5. Update Segment Registers
    ; Now that we are in 32-bit, we must point all segment registers
    ; to our new Data Segment (0x10)
    mov ax, 0x10
    mov ds, ax
    mov ss, ax
    mov es, ax
    mov fs, ax
    mov gs, ax

    mov ebp, 0x90000    ; Update stack to a safe 32-bit area
    mov esp, ebp

    ; 6. Print "SoraOS Protected" directly to Video Memory
    ; We can't use BIOS interrupts anymore! We must write to 0xB8000.
    mov ebx, 0xb8000
    mov byte [ebx], 'S'
    mov byte [ebx+1], 0x0f ; White on Black
    mov byte [ebx+2], 'O'
    mov byte [ebx+3], 0x0f
    mov byte [ebx+4], 'R'
    mov byte [ebx+5], 0x0f
    mov byte [ebx+6], 'A'
    mov byte [ebx+7], 0x0f
    mov byte [ebx+8], ' '
    mov byte [ebx+9], 0x0f
    mov byte [ebx+10], 'P'
    mov byte [ebx+11], 0x0f
    mov byte [ebx+12], 'M'
    mov byte [ebx+13], 0x0f

    jmp $

; --- The Magic Boot Sector Footer ---
; Fill the rest of the 512 bytes with zeros
times 510-($-$$) db 0
; The BIOS signature (must be at the very end)
dw 0xAA55