error: parser: instruction expected

Question

NASM is not parsing command ' mull byte [bp+10] ' in the printstr subroutine of my program. i have rechecked the code with the lecture instructions and cannot phathom why it is doing so. also, it does not prin anything on screen or clear screen when executed either directly (32-bit win7) or via dosbox.

    [org  0x100]
jmp start

message:        db  'hello world'
length:         dw  11

clr:        push es
            push ax
            push di

            mov ax, 0xb800
            mov es, ax
            mov di, 0           
   nxt:     mov word[es:di], 0x0720
            add di,2
            cmp di,4000
            jne nxt
            pop di
            pop ax
            pop es
            ret

printstr:           push bp
            mov bp,sp
            push es
            push ax
            push cx
            push si
            push di

            mov ax, 0xb800
            mov es, ax
            mov al, 80
            mull byte [bp+10]
            add ax, [bp+12]
            shl ax, 1
            mov di, al
            mov si, [bp+6]
            mov cx, [bp+4]
            mov ah, [bp+8]

nextchar:           mov al, [si]
            mov [es:di], ax
            add di, 2
            add si, 1
            loop nextchar

            pop di
            pop si
            pop cx
            pop ax
            pop es
            pop bp
            ret 10

start:      call clr
            mov ax, 30
            push ax
            mov ax, 20
            push ax
            mov ax, 1
            push ax
            mov ax, message
            push ax
            push word [length]
            call printstr

            mov ax, 0x4c00
            int 0x21

Answer 1

According to the reference it's mul, not mull.