This is the fourth in a series of computer science videos is about the fundamental principles of Dynamic Random Access Memory, DRAM, and the essential concepts of DRAM operation. This video covers multiplexers and demultiplexers. It describes what these electronic circuits do and some of their applications beyond computer memory, including the conversion of parallel data into a serial data stream, and the sharing of a single transmission medium by multiple transmitters in a technique known as synchronous time division multiplexing (TDM). The role of a multiplexer for column selection in DRAM when reading data, and a demultiplexer for column selection in DRAM when writing data, are also mentioned. The video also shows how generic multiplexers and demultiplexers can be built using combinational logic (although the integrated circuits in a DRAM module are actually highly optimised devices built using CMOS technology).