Q:

assembly pointer arithmetic LEA instruction

Does the LEA instruction support negative displacement?
mov rax, 1
lea rsi, [rsp - rax]

When I use the above code in my asm file I got the error:
$ nasm -f macho64 test.asm
$ error: invalid effective address

I Know that we can do something like this in C:
void foo(char *a, size_t b) {
    *(a - b) = 1;
}

then I assume that:
lea rsi, [rsp - rax]    

will work.
And I also try to see what the GCC compiler do by using:
$ gcc -S foo.c // foo.c has the function foo(above) in it

but my asm knowleage is not enough for me the understand the asm output
from the GCC compiler.
Can anyone explain why:
lea rsi, [rsp - rax]    ;; invalid effective address

does not work. And I'm using these to achieve the samething:
;; assume rax has some positive number
neg rax    
lea rsi, [rsp + rax]
neg rax

or
sub rsp, rax
mov rsi, rsp
add rsp, rax

What is a more standard way of doing it?
I'm using NASM version 2.11.08 compiled on Nov 26 2015 on MAC OSX 10.11
Thank you in advance for your help!

A:

The lea instruction doesn't care about the sign of the displacement. But you do need to always add the components together.
mov rax, -1
lea rsi, [rsp + rax]

Remember subtracting 1 is the same as adding -1.

