The AT89S8253 microcontroller has in total of 40 Special Function Registers. For the sake of the compatibility with the previous 8051 models, the core registers (22 in total) are the same for all of them, while the others were added later for the purpose of controlling upgraded functions of the microcontroller.As shown in the table above, each of these registers has its name and specific address in RAM. Unoccupied locations are intended for the future upgraded versions of the microcontroller and shouldn’t be used. As their name suggests, these registers are mostly in control of one specific circuit within the microcontroller such as timers or SPI etc. and they will be discussed later in the book. This chapter covers only those SFRs controlling more than one circuit within the microcontroller.