ORG 0x7C00
BITS 16

jmp short main
nop

bdb_oem:                    db 'MSWIN4.1'           
bdb_bytes_per_sector:       dw 512
bdb_sectors_per_cluster:    db 1
bdb_reserved_sectors:       dw 1
bdb_fat_count:              db 2
bdb_dir_entries_count:      dw 0E0h
bdb_total_sectors:          dw 2880                 
bdb_media_descriptor_type:  db 0F0h                 
bdb_sectors_per_fat:        dw 9                    
bdb_sectors_per_track:      dw 18
bdb_heads:                  dw 2
bdb_hidden_sectors:         dd 0
bdb_large_sector_count:     dd 0

; extended boot record
ebr_drive_number:           db 0                    
                            db 0                    
ebr_signature:              db 29h
ebr_volume_id:              db 12h, 34h, 56h, 78h   
ebr_volume_label:           db '  JAZZ OS  '        
ebr_system_id:              db 'FAT12   '           

main:
    MOV ax,0
    MOV ds,ax
    MOV es,ax
    MOV ss,ax

    MOV sp,0x7C00

    MOV si,os_boot_msg
    CALL print

    ;4 segments
    ;reserved segment: 1 sector
    ;FAT: 9 * 2 = 18 sectors
    ;Root directory: 
    ;Data

    MOV ax, [bdb_sectors_per_fat]
    MOV bl, [bdb_fat_count]
    XOR bh,bh
    MUL bx
    ADD ax, [bdb_reserved_sectors] ;LBA of the root directory
    PUSH ax

    MOV ax, [bdb_dir_entries_count]
    SHL ax,5 ;ax *= 32
    XOR dx,dx
    DIV word [bdb_bytes_per_sector] ;(32*num of entries)/bytes per sector

    TEST dx,dx
    JZ rootDirAfter
    INC ax

rootDirAfter:
    MOV cl,al
    POP ax
    MOV dl, [ebr_drive_number]
    MOV bx, buffer
    CALL disk_read

    XOR bx,bx
    MOV di,buffer

searchKernel:
    MOV si, file_kernel_bin
    MOV cx,11
    PUSH di
    REPE CMPSB
    POP di
    JE foundKernel

    ADD di, 32
    INC bx 
    CMP bx, [bdb_dir_entries_count]
    JL searchKernel

    JMP kernelNotFound

kernelNotFound:
    MOV si, msg_kernel_not_found
    CALL print

    HLT
    JMP halt

foundKernel:
    MOV ax, [di+26]
    MOV [kernel_cluster], ax

    MOV ax, [bdb_reserved_sectors]
    MOV bx, buffer 
    MOV cl, [bdb_sectors_per_fat]
    MOV dl, [ebr_drive_number]

    CALL disk_read

    MOV bx, kernel_load_segment
    MOV es,bx
    MOV bx, kernel_load_offset

loadKernelLoop:
    MOV ax, [kernel_cluster]
    ADD ax, 31
    MOV cl, 1
    MOV dl, [ebr_drive_number]

    CALL disk_read

    ADD bx, [bdb_bytes_per_sector]

    MOV ax, [kernel_cluster] ;(kernel cluster * 3)/2
    MOV cx, 3
    MUL cx
    MOV cx, 2
    DIV cx

    MOV si, buffer
    ADD si, ax
    MOV ax, [ds:si]

    OR dx,dx
    JZ even

odd:
    SHR ax,4
    JMP nextClusterAfter
even:
    AND ax, 0x0FFF

nextClusterAfter:
    CMP ax, 0x0FF8
    JAE readFinish

    MOV [kernel_cluster], ax
    JMP loadKernelLoop

readFinish:
    MOV dl, [ebr_drive_number]
    MOV ax, kernel_load_segment
    MOV ds,ax
    MOV es,ax

    JMP kernel_load_segment:kernel_load_offset

    HLT

halt:
    JMP halt

;input: LBA index in ax
;cx [bits 0-5]: sector number
;cx [bits 6-15]: cylinder
;dh: head
lba_to_chs:
    PUSH ax
    PUSH dx

    XOR dx,dx
    DIV word [bdb_sectors_per_track] ;(LBA % sectors per track) + 1 <- sector
    INC dx  ;Sector
    MOV cx,dx

    XOR dx,dx
    DIV word [bdb_heads]

    MOV dh,dl ;head
    MOV ch,al
    SHL ah, 6
    OR CL, AH   ;cylinder

    POP ax
    MOV dl,al
    POP ax

    RET


disk_read:
    PUSH ax
    PUSH bx
    PUSH cx
    PUSH dx
    PUSH di

    call lba_to_chs

    MOV ah, 02h
    MOV di, 3   ;counter

retry:
    STC
    INT 13h
    jnc doneRead

    call diskReset

    DEC di
    TEST di,di
    JNZ retry

failDiskRead:
    MOV si, read_failure
    CALL print
    HLT
    JMP halt

diskReset:
    pusha
    MOV ah,0
    STC
    INT 13h
    JC failDiskRead
    POPA
    RET

doneRead:
    pop di
    pop dx
    pop cx
    pop bx
    pop ax

    ret


print:
    PUSH si
    PUSH ax
    PUSH bx

print_loop:
    LODSB
    OR al,al
    JZ done_print

    MOV ah, 0x0E
    MOV bh, 0
    INT 0x10

    JMP print_loop

done_print:
    POP bx
    POP ax
    POP si
    RET

os_boot_msg: DB 'Loading...', 0x0D, 0x0A, 0
read_failure DB 'Failed to read disk!', 0x0D, 0x0A, 0
file_kernel_bin DB 'KERNEL  BIN'
msg_kernel_not_found DB 'KERNEL.BIN not found!'
kernel_cluster DW 0

kernel_load_segment EQU 0x2000
kernel_load_offset EQU 0

TIMES 510-($-$$) DB 0
DW 0AA55h

buffer: