We tried making a RAM register with air pistons.
We managed to make one single register, but we used all of post-grad air pistons kit in our pneumatics lab. Just for the heck of it. Ours could retain the ram memory even without power, obviously. It would have become even bigger if we tried to do anything larger.
But hey, kudos, that was mighty impressive. I guess that's what would have happened if we didn't have single-die processors, or in a post-apocalyptic steampunk future.