Can't subtract registers in an addressing mode?

Question

I finished reading PC Assembly Language and I was working on an implementation of RC4 encryption in Assembly. What I can't comprehend is why

mov eax, [edx+ecx]

works but

mov eax, [edx-ecx]

doesn't. The inline assembler gives me this error message,

non-constant expression in 'second operand'

What does that mean? Thanks in advance.

Answer 1

The following gives a good summary of x86 addressing modes. Note that there is no "register minus register" form: Wikipedia.

As a workaround, you could negate the contents of ecx then use [edx+ecx] (you may have to negate it back if you need the original value afterwards).