The information presented in this chapter is collected from the Microcontroller Architectural Overview, AT89C51, AT89LV51, AT89C52, AT89LV52, AT89C2051, and AT89C1051 data sheets of this book. The material has been selected and rearranged to form a quick and convenient reference for the programmers of Atmel’s microcontroller family of devices. This guide pertains specifically to the AT89C51, AT89LV51, AT89C52, and AT89LV52.