; KBOOT - Kern Bootloader
; Most of the code is taken from Limine (https://github.com/limine-bootloader/limine)
; I tried to add more documentation than the Limine code had, but it's not everything
; Any of my code is pretty bad, so i recommend using an existing bootloader

[org 0x7c00]
[bits 16]

start:
    jmp .bpb_end

    ; Some BIOSes will override the FAT BPB area,
    ; so we just fill it out to avoid pottentially
    ; overriding bootloader code.
    ; Source: https://github.com/limine-bootloader/limine/blob/trunk/stage1/hdd/bootsect.asm
.bpb:
    times 3-($-$$) db 0
    .bpb_oem_id:            db "KBOOT   "
    .bpb_sector_size        dw 512
    .bpb_sects_per_clust:   db 0
    .bpb_reserved_sects:    dw 0
    .bpb_fat_count:         db 0
    .bpb_root_dir_entries:  dw 0
    .bpb_sector_count:      dw 0
    .bpb_media_type:        db 0
    .bpb_sects_per_fat:     dw 0
    .bpb_sects_per_track:   dw 18
    .bpb_heads_count:       dw 2
    .bpb_hidden_sects:      dd 0
    .bpb_sect_count_big:    dd 0
    .bpb_drive_num:         db 0
    .bpb_reserved:          db 0
    .bpb_signature:         db 0
    .bpb_volume_id:         dd 0
    .bpb_volume_label:      db "KBOOT      "
    .bpb_filesystem_type:   times 8 db 0
.bpb_end:
    cli
    cld
    jmp 0x0000:.init_codeseg    ; Long jump to set CS = 0

; Zero out segment regs (Because segmentation is shit)
.init_codeseg: 
    xor si, si      ; We cannot change segment regs directly  
    mov ds, si      ; Data Segment
    mov es, si      ; Extra Segment
    mov ss, si      ; Stack Segment
    mov sp, 0x7c00  ; Set Stack Pointer
    sti

    ; We probably won't be booting from a floppy, so
    ; ignore if boot drive is < 0x80 and assume an error
    cmp dl, 0x80
    jl err.0
    ; Also anything above 0x8f is "dubious" so we also 
    ; just assume an error
    cmp dl, 0x8f
    jg err.1

; Check that int 13h extensions are supported
.int13chk:
    mov ah, 0x41
    mov bx, 0x55aa  ; Expects bx to be this magic value
    int 0x13        ; Will se CF if unsupported
    jc err.2
    cmp bx, 0xaa55  ; Also sets bx to this magic
    jne err.3

; Load the stage 2 bootloader code to 0x70000
.load_stage2:
    push 0x7000
    pop es      ; ES = 0x7000
    xor bx, bx  ; ES:BX = 0x70000
    mov eax, 512
    xor ebp, ebp
    mov ecx, 512
    call read_sectors
    jc err.4    ; CF was set -> failed to read from disk

.enable_protected:
    ; Load GDT Pointer
    lgdt [gdtp]

    ; Set PE (Protected mode Enable) flag
    cli
    mov eax, cr0
    or eax, 1   ; Bit 1 - PE
    mov cr0, eax

    ; Far jump to protected mode
    jmp code_seg:vector

%include "boot/disk.s"   ; Disk I/O helper functions
%include "boot/gdt.s"    ; Global Descriptor Table

; Handles an error
err:
; Error codes
.4: inc si  ; Failed to read stage2 from disk
.3: inc si  ; Int 13h extensions unsupported (BX wasn't magic)
.2: inc si  ; Int 13h extensions unsupported (CF was set)
.1: inc si  ; Invalid boot drive number (> 0x8f)
.0:         ; Invalid boot drive number (< 0x80)
    ; Common error handler
    add si, '0' | (0x4f << 8)   ; Store error code number char in SI, white on red 
    push 0xb800 ; VGA Text mode video memory
    pop es      ; ES = 0xb800
    mov word [es:0], si ; Write character

    ; Hang
    sti
    .h: hlt
    jmp .h

[bits 32]
; Protected mode entry vector
vector: 
    mov eax, 0x10
    mov ds, ax
    mov es, ax
    mov fs, ax
    mov gs, ax
    mov ss, ax

    and edx, 0xff

    ; Call stage 2 bootloader
    call 0x70000

; Padding and boot signature
times 510-($-$$) db 0
dw 0xaa55