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.
There is an opcode for:
mov eax, [edx+ecx]
and there is an opcode which can be assembled from:
mov eax, [edx-CONSTANT]
because the assembler uses the regular opcode but negates the constant during assembly:
mov eax, [edx+(-CONSTANT)]
However there is not an opcode for:
mov eax, [edx-ecx]
So basically you're attempting to execute an instruction that doesn't exist.
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).
You're not allowed to subtract in the offset like that. The intent is that you can point at the base of a memory buffer and then add an offset into it. Subtraction would result in pulling you out of the specified memory buffer...
