; This code runs in 16-bit Real Mode when the computer starts.
; It is responsible for loading the kernel and switching to 32-bit Protected Mode.

[org 0x7c00]          ; BIOS loads the bootloader at this memory address
KERNEL_OFFSET equ 0x1000 ; Memory address where we will load our kernel

; Save the boot drive index provided by BIOS in DL register
mov [BOOT_DRIVE], dl

; Initialize the stack pointer to a safe area in memory
mov bp, 0x9000
mov sp, bp

; Load the kernel from the disk into memory
call load_kernel

; Switch to VGA Mode 13h: 320x200 resolution with 256 colors
; This is a graphical mode where each byte in video memory is a pixel color
mov ax, 0x0013
int 0x10

; Enter 32-bit Protected Mode
call switch_to_pm

; Infinite loop in case we ever return (we shouldn't)
jmp $

; Global Descriptor Table (GDT)
; The GDT defines memory segments and their access rights for Protected Mode.
gdt_start:
	dd 0x0 ; The null descriptor is required at the beginning
	dd 0x0

gdt_code:
	; Code segment descriptor
	dw 0xffff    ; Limit (bits 0-15)
	dw 0x0       ; Base (bits 0-15)
	db 0x0       ; Base (bits 16-23)
	db 10011010b ; Access byte: Present, Privilege 0, Code, Executable, Readable
	db 11001111b ; Flags: 4KB granularity, 32-bit mode, Limit (bits 16-19)
	db 0x0       ; Base (bits 24-31)

gdt_data:
	; Data segment descriptor
	dw 0xffff    ; Limit (bits 0-15)
	dw 0x0       ; Base (bits 0-15)
	db 0x0       ; Base (bits 16-23)
	db 10010010b ; Access byte: Present, Privilege 0, Data, Writable
	db 11001111b ; Flags: 4KB granularity, 32-bit mode, Limit (bits 16-19)
	db 0x0       ; Base (bits 24-31)

gdt_end:

; The GDT descriptor is passed to the LGDT instruction
gdt_descriptor:
	dw gdt_end - gdt_start - 1 ; Size of GDT (minus 1)
	dd gdt_start               ; Starting address of GDT

; Constants for segment selectors (offsets within the GDT)
CODE_SEG equ gdt_code - gdt_start
DATA_SEG equ gdt_data - gdt_start

; Switch to 32-bit Protected Mode
[bits 16]
switch_to_pm:
	cli                     ; Disable interrupts while switching modes
	lgdt [gdt_descriptor]   ; Load the GDT descriptor
	mov eax, cr0            ; Enable Protected Mode bit in Control Register 0
	or eax, 0x1
	mov cr0, eax
	jmp CODE_SEG:init_pm    ; Far jump to flush the CPU pipeline and switch to 32-bit mode

[bits 32]
init_pm:
	; Update all segment registers to use the new Data Segment selector
	mov ax, DATA_SEG
	mov ds, ax
	mov ss, ax
	mov es, ax
	mov fs, ax
	mov gs, ax

	; Re-initialize the stack for 32-bit mode at a higher memory address
	mov ebp, 0x90000
	mov esp, ebp

	; Call the entry point that leads to our kernel
	call BEGIN_PM

; Routine to load sectors from disk using BIOS interrupts
[bits 16]
load_kernel:
	mov bx, KERNEL_OFFSET ; Destination address in memory
	mov dh, 15            ; Number of sectors to load (adjust if kernel grows)
	mov dl, [BOOT_DRIVE]  ; Drive index to read from
	call disk_load
	ret

disk_load:
	push dx
	mov ah, 0x02 ; BIOS Read Sector function
	mov al, dh   ; Number of sectors
	mov ch, 0x00 ; Cylinder 0
	mov dh, 0x00 ; Head 0
	mov cl, 0x02 ; Sector 2 (Sector 1 is the bootloader)
	int 0x13     ; Call BIOS disk interrupt
	jc disk_error ; Jump if Carry Flag is set (indicates error)
	pop dx
	cmp dh, al    ; Verify that we read the expected number of sectors
	jne disk_error
	ret

disk_error:
	; Halt execution if disk read fails
	jmp $

; 32-bit entry point to jump into the compiled C kernel
[bits 32]
BEGIN_PM:
	call KERNEL_OFFSET ; Jump to the memory location where kernel was loaded
	jmp $              ; Safety hang

; Boot sector padding and signature
BOOT_DRIVE db 0
times 510-($-$$) db 0 ; Pad the rest of the 512 bytes with zeros
dw 0xaa55             ; The standard boot signature required by BIOS