80x86 16-bit asm: lea cx, [cx*8+cx] causes error on NASM (compiling .com file)

Question

Title says it all. The error NASM gives (dispite my working OS) is "invalid effective address".

Now i've seen many examples of how to use LEA and i think i gots it right but yet my NASM dislikes it. I tried "lea cx, [cx+9]" and it worked; "lea cx, [bx+cx]" didn't.

Now if i extended my registers to 32-bits (i.e. "lea ecx, [ecx*8+ecx]") everything would be well but i am restricted to use 16- and 8-bit registers only.

Is here anyone so knoweledgeable who could explain me WHY my assembler doesn't let me use lea the way i supposed it should be used?

Thanks.

Edit

Thank you for the link. How ever the 0x67 prefix confused me: It allows me to use 32-bit addresses thus overriding real mode's 1MB memory limitation??? I it still doesn't enable all possible register conbinations for e.g. LEA...

-Larz

You can override real mode's 1MB memory limitation even with 16 bytes, for instance with this sequence: mov ax, 0xffff; mov ds, ax; mov bx, ds:[ax]. Now ds:[ax] = 10FFEF > 0xfffff, so you've accessed more than 1MB. – Nathan Fellman

Answer 1

This is because [bx+cx] isn't valid in any addressing mode on 16-bit x86, see this site for more info.

lea cx, [bx+di] or lea cx, [bx+si] should work.

Answer 2

lea cx,[cx*8+cx] doesn't work because "scale-index-base" addressing is only available with 32-bit registers. It's not a limitation of the assembler--it's a limitation of the processor.