The GEOS operating system had built in support for the REU as a RAM disk, as did the C128's version of CP/M, and some disk copy programs used the REU to facilitate high-speed copying with a single disk drive. GEOS as well as other programs even used the REU for quick memory transfers within the host machine's main memory by storing a memory block into the REU and then fetching it back to another location. Using this method, only the actual data to be transferred needed to travel on the machine's data bus—unlike the ordinary method, which had the computer's CPU do the transfer, thus spending at least three quarters of the bus capacity on instruction fetches and only one quarter or less on payload data.