Publisher Summary Many of today's complex instruction set computer (CISC) processors owe their architectural philosophy to the early 8-bit microprocessors either as a foundation or as an example of how not to design a high performance microprocessor. A microprocessor is described as a data processor: information is obtained, modified according to a predetermined set of instructions and either stored or passed to other elements within the system. Each instruction follows a simple set path of fetching an instruction, decoding and acting on it, and fetching data from external memory as necessary. This chapter presents basic functional blocks within a typical 8-bit microprocessor. It also describes the stages needed to execute an instruction to load an accumulator with a data byte. The programmer has a very simple register model for this type of processor. It has two 8-bit accumulators used for storing data and performing arithmetic operations. The program counter is 16 bits in size, and two further 16 bit registers are provided for stack manipulations and address indexing.