SO78992609 - first_stage.asm

[BITS 16]
[ORG 0x7C00]

start:
    ; Set up segments and stack
    xor ax, ax
    mov ds, ax
    mov es, ax
    mov ss, ax
    mov sp, 0x7000      ; Move stack below bootloader

    ; Store boot drive number
    mov [bootDrive], dl

    ; Load Stage 2 in chunks (3 sectors at a time, total 32 sectors)
    mov cx, 0x0002      ; Start at sector 2 (CH=0, CL=2)
    mov bx, 0x7E00      ; Start loading at memory address 0x8000
    mov dl, [bootDrive] ; Load the boot drive number

load_loop:
    mov ah, 0x02        ; BIOS read sector function
    mov al, 3           ; Read 3 sectors at a time
    mov ch, 0           ; Cylinder number
    mov dh, 0           ; Head number
    int 0x13            ; BIOS interrupt to read disk
    jc disk_error       ; Jump if error

    ; Advance to next set of sectors
    add cx, 3           ; Increment sector number
    add bx, 1536        ; Move memory address (3 * 512 = 1536 bytes)

    cmp cx, 34          ; Stop when sector >= 34 (32 sectors total from 2)
    jl load_loop        ; Continue loading until all sectors are read

    ; Print success message
    mov si, successMsg
    call print_string

    ; Jump to stage 2 at 0x8000
    jmp 0x0000:0x7E00

disk_error:
    ; Display error and halt
    mov si, diskErrorMsg
    call print_string
    mov al, ah  ; Error code in AH
    call print_hex
    cli         ; Disable interrupts
.halt:
    hlt         ; Halt CPU
    jmp .halt   ; Loop forever

print_string:
    lodsb
    or al, al
    jz done
    mov ah, 0x0E
    int 0x10
    jmp print_string
done:
    ret

print_hex:
    push ax
    push bx
    mov bx, ax
    shr al, 4
    call print_hex_digit
    mov al, bl
    and al, 0x0F
    call print_hex_digit
    pop bx
    pop ax
    ret

print_hex_digit:
    and al, 0x0F
    add al, '0'
    cmp al, '9'
    jle .print
    add al, 7
.print:
    mov ah, 0x0E
    int 0x10
    ret

diskErrorMsg db 'Disk Error! Code: ', 0
successMsg db 'Stage 2 loaded successfully!', 13, 10, 0
bootDrive db 0

times 510 - ($ - $$) db 0
dw 0xAA55   ; Boot signature