Publisher Summary This chapter focuses on the operations of the microcontroller. The microcontroller contains a program execution section and a register processing sections. The program execution section contains the program memory, instruction register and control logic, which store, decode, and execute the program. The register processing section has a block of random access memory (RAM), which starts with the special function registers (SRFs) that are used to control the processor operations, including the port registers, which are used for input and output. Microcontrollers used for prototyping and short production runs on flash memory to store the program. The program can be downloaded while the chip is in the application circuit (in-circuit programming). Alternatively, the chip is placed in a programming unit attached to the host computer for program downloading, before fitting it in the application board. The program is a list of binary machine code instructions stored in the flash memory. The program counter (PC) is a register that keeps track of the program sequence, by storing the address of the instruction currently being executed.