Let's take some old games like 4 or 5 year old games that were only single core threaded.?

lets say we have a old game that is only single core threaded and the os was also single core threaded when we run it on a single core processor it uses a 100% of it just as it should now if we give it a dual core cpu it would use 50% of the dual core processor wouldn't it cause 100 percent of 1 core in a dual core cpu is only 50 percent meaning only half the processing power is in utilization am i right please give me a correct answer cause i need it for my project and don't say that the os or game could still use the other core no! th game and the os and all the programs on the computer are all single core threaded thanks and i'll give 10 points for the best answer.