mov rax,r9+rcx*2
Why is this invalid syntax? Doesn't this count as "mov r64,r/m64" ? I'm a beginner so I'm sorry for my ignorance.
It becomes valid when the expression is enclosed in square brackets mov rax,[r9+rcx*2]. Why is that?
mov rax,r9+rcx*2
This is an invalid syntax because the source operand (rightmost operand) contains registers and operations that the assembler can't perform at compile-time simply because the assembler does not know what values these registers contain. Only at runtime are these values revealed.
Using the square brackets, [r9 + rcx * 2] becomes a valid addressing mode that uses a base register (R9) and an index register (RCX) that gets scaled by a factor (2).
mov rax, [r9 + rcx * 2]
When combined with the mov instruction, the memory contents at the calculated address will be returned in the indicated register (RAX).
lea rax, [r9 + rcx * 2]
When combined with the lea instruction, the calculated address itself will be returned in the indicated register (RAX).
In both cases, the address calculation happens at runtime.