All Binary Combinations to Decimal
Python, 60 bytes

lambda x,y:[n for n in range(1&lt;&lt;x+y)if bin(n).count('1')==y]


Test it on Ideone.

How it works

All positive numbers that can be represented in binary with x zeroes and y ones is clearly smaller than 2x + y, since the canonical binary representation of the latter has x + y + 1 digits.

The lambda simply iterates over the integers in [0, 2x + y) and keeps all integers n in that range that have y ones. Since n &lt; 2x + y is can be represented with x (or less) zeroes.