The two players can also play online while using split screen. This game mode is normally exclusive to the Xbox 360 version of Left 4 Dead and Left 4 Dead 2, but may be accessed in the PC versions if you have a controller for the second player and modify some files.
Serial memory processing - Wikipedia. Serial memory processing is the act of attending to and processing one item at a time. This is usually contrasted against parallel memory processing, which is the act of attending to and processing all items simultaneously. https://en.wikipedia.org/wiki/Serial_memory_processing
Serial Processing in Action. Programs written for serial processing only use one core at a time and process tasks in sequential order. A serial processor works a lot like having a dozen open checkout lanes at grocery store with one cashier running between the different lanes, checking out everyone at the same time.