bootloader.asm

; bootloader.asm
; A simple 16-bit bootloader that loads a kernel from the next sector.

org 0x7C00               ; BIOS loads boot sector at this address

bits 16                  ; We are in 16-bit real mode

; --- Initialize segment registers ---
; The BIOS initializes CS:IP to 0x07C0:0x0000.
; Set DS, ES, SS to 0x07C0 as well for consistent addressing of bootloader data.
; Set SP to 0x7C00 (or slightly before) so the stack grows downwards from within our bootloader's segment.
xor ax, ax               ; AX = 0
mov ds, ax               ; DS = 0 (for accessing global data if needed)
mov es, ax               ; ES = 0 (for extra segment access)
mov ss, ax               ; SS = 0 (stack segment)
mov sp, 0x7C00           ; SP points to the end of our 512-byte boot sector,
                         ; so the stack grows downwards into memory *before* the boot sector.

; --- Print "Loading LuckyOS Kernel..." message ---
mov si, msg_loading      ; Load address of message into SI
call print_string        ; Call our string printing routine

; --- Load Kernel ---
; INT 13h, AH=0x02: Read Sectors From Drive
mov ah, 0x02             ; Function: Read Sectors
mov al, 20               ; Number of sectors to read (Increased to 20 for your kernel)
mov ch, 0                ; Cylinder (track) number = 0
mov cl, 2                ; Starting sector number = 2 (sector 1 is 0-indexed, so 2nd physical sector)
                         ; Bootloader is sector 1 (0-indexed). Kernel starts right after.
mov dh, 0                ; Head number = 0
mov dl, 0x00             ; Drive number (0x00 for floppy A:, 0x80 for first hard drive)
                         ; Changed to 0x00 for common floppy drive usage with -fda
mov bx, 0x1000           ; Offset into ES (which is currently 0) for target address
                         ; Kernel will be loaded to 0x0000:0x1000 (linear address 0x1000)

int 0x13                 ; Call BIOS disk service

jc disk_error            ; If carry flag set (error), jump to error handler

; --- Jump to Kernel ---
; Jump to the loaded kernel's entry point
jmp 0x0000:0x1000        ; Far jump to segment 0x0000, offset 0x1000

; --- String Printing Routine (used by bootloader) ---
print_string:
    mov ah, 0x0E         ; Teletype output (BIOS INT 10h function)
.repeat:
    lodsb                ; Load byte from [DS:SI] into AL, increment SI
    cmp al, 0            ; Check for null terminator
    je .done
    int 0x10             ; Print character
    jmp .repeat
.done:
    ret

; --- Error Handler ---
disk_error:
    mov si, msg_error    ; Load address of error message
    call print_string
    jmp $                ; Infinite loop to halt

; --- Data ---
msg_loading db 'Loading LuckyOS Kernel...', 0
msg_error   db 'Disk Error! Failed to load the kernel.', 0

; --- Boot Sector Padding and Signature ---
times 510 - ($ - $$) db 0 ; Pad with zeros to 510 bytes
dw 0xAA55                ; Boot signature (0x55AA)