Disassembly of hardware dongle code; compilable in MASM

dongle.asm

.model tiny
.code

start:
    push ds                 ; save DS register
    xor cx,cx               ; clear CX
    mov ds,cx               ; set DS to 0 to access BIOS data area

; Detect parallel port address from BIOS data area (0x0408)
    mov bx,408h
    mov cl,4                ; check up to 4 parallel port addresses
@@:
    mov dx,[bx]             ; load parallel port address
    or dx,dx
    jne parallel_port_found ; if address found, proceed
    inc bx
    inc bx
    loop @B

; No parallel port found, exit
    pop ds
    retf

parallel_port_found:
    pop ds
    cli                     ; disable interrupts

; Save current data and status register values
    in al,dx                ; read data register
    mov ah,al
    inc dx
    in al,dx                ; read status register
    push ax
    dec dx

; Initialize communication sequence
    mov al,82h
    call write_and_wait

    mov cx,40000
    call wait_loop

    mov al,80h
    call write_and_wait

    mov al,82h
    call write_and_wait

    mov cx,3880h
    call wait_loop

; Main data collection loop
    xor bx,bx               ; BX = 0 (will store results)
    mov cx,78h              ; loop counter = 0x78

@@:
    mov al,0A2h
    call write_and_wait

    inc dx
    in al,dx                ; read status register
    and al,40h              ; test bit 6

    jne check_counter_zero

    or bl,bl                ; if BL is zero
    jne store_current_counter

    mov bl,cl               ; store current counter in BL
    jmp store_current_counter

check_counter_zero:
    cmp cl,76h              ; if counter equals 0x76
    jne store_current_counter

    mov bh,cl               ; store 0x76 in BH

store_current_counter:
    mov al,82h
    dec dx
    call write_and_wait
    loop @B

; Cleanup and restore
    mov al,0
    call write_and_wait

    pop ax
    xchg ah,al
    out dx,al               ; restore data register
    inc dx
    mov al,ah
    out dx,al               ; restore status register

    sti                     ; enable interrupts
    retf

write_and_wait:
    ; Output value to port and wait
    ; DX = port address, AL = value to output
    out dx,al
    push cx
    mov cx,40000
    call wait_loop
    pop cx
    ret

wait_loop:
    ; Delay loop: decrement CX until zero
    push cx
    mov cx,40000
    pop cx
    or cx,cx
    je @F
    jne @F
@@:
    loop wait_loop
    ret

end