KOK may be correct, just check the owner's manual and make sure the check valve is there.
If there is not a check valve mentioned in the book you will need to go to the HD or L or a plumbing supplier and ask for a dishwasher air gap. It will be an aparatus about a foot long with two different size places to connect to the bottom and will have a chrome hood on top. If your sink has a hole cover on it you can remove it and mount the air gap. If there is no hole you will need a 1 1/2" hole to mount it. The small part connects to the dishwasher drain and the larger one to the disposall. It will not interfere with the draining but when the pump shuts off the disposall side drains down and can't siphon back into the dishwasher. I sure don't want the contents of my disposall running in my dishwasher.