What does a double-percent sign (%%) do in gcc inline assembly?

Question

I came across a code that looks like this:

asm volatile (
    # [...]
    \"movl $1200, %%ecx;\"
    # [...]
);

I know what movl $120

Answer 1

This helps GCC to distinguish between the operands and registers. operands have a single % as prefix. '%%' is always used with registers.