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

Question

The error NASM gives (despite my working OS) is "invalid effective address".

Now I've seen many examples of how to use LEA and I think I got 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 knowledgeable who could explain me WHY my assembler doesn't let me use lea the way I supposed it should be used?

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.

If your code will run on 386 or later in 16bit mode, you can use lea cx, [ecx + 9] (address-size prefix but still 16bit operand-size).

See also this Q&A on x86 addressing modes (mostly discussing 32/64bit addressing modes, and the x86 tag wiki.

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.