Invalid segment override in writing to video memory with assembly language

Question

I have this assembly code for writing to video memory an pixel:

mov ax, 0x0013
int 0x10

x db 1
y db 1
videomem dd 0xa000

mov bx, y
mov ax, 0x0140 ;320
mul bx

add ax, x

mov cx, videomem
mov [cx:ax], 0x09;

jmp $
times 510-($-$$) db 0
dw 0xaa55

The problem is that NASM returns this error:

boot.asm:15: error: invalid segment override

What should I change in line 15 so that it would work? Also, I do not know what cx and ax are so that it would return this error, and so is there a compile time debugger in NASM?