In contrast, the BBC Micro employs one or two sets of 8 16-kilobit devices, with the RAM running at twice the speed (4 MHz) of the CPU (2 MHz), allowing the video system (screen refresh) and CPU memory accesses to be interleaved, with each accessor able to transfer bytes at 2 MHz. The RAM access limitations imposed by the Electron's ULA therefore reduce the effective CPU speed by as much as a factor of four relative to the BBC Micro in the more demanding display modes, and as much as a factor of two otherwise. Byte transfers from ROM occur at 2 MHz, however.