Convert digit 9 to binary.

First, you need to expand 9 in powers of two (because the base of binary CC is 2). Alternatively, you can divide by a column “all the way”.
We get the following expression:
9 = 8 + 1
Taking into account the fact that in any CC (including binary) the digits grow from right to left, then
9 = 1001
In binary SS, the first bit on the left corresponds to 2 ^ 0, the second to 2 ^ 1, the third to 2 ^ 2, and so on. In this variant, there are 2 ^ 3 (therefore, in the place of this bit there is 1) and 2 ^ 0 (similarly).