What does double dollar sign mean in x86 assembly (NASM)

Question

The expression is:

times 512-($-$$) db 0

I know what the dollar sign means but I don't know what it means when it's double.

What does the double dollar sign mean?

Answer 1

When all else fails, Read The Fine Manual (section 3.5 "Expressions"):

NASM supports two special tokens in expressions, allowing calculations to involve the current assembly position: the $ and $$ tokens. $ evaluates to the assembly position at the beginning of the line containing the expression; so you can code an infinite loop using JMP $. $$ evaluates to the beginning of the current section; so you can tell how far into the section you are by using ($−$$).

So the example in your question is just padding the current section to a size of 512 with zeroes.