TABLE OF CONTENTS The central processing unit (CPU) is the brains of a computer system. The CPU consists of the arithmetic logic unit (ALU) and the control unit, as indicated previously. The ALU can come in a variety of configurations from a single simple unit, shown in Figure 2.1, that performs simple adds, subtracts, increments, decrements, load, and store, up to extremely complex units that perform operations such as multiply, divide, exponentiation, sine, cosine, and so on. The primary operation of the ALU is to take zero or more operands and perform the function called for in the instruction. In addition to the ALU, the CPU consists of a set of registers to store operands and intermediate results and to maintain information used by the CPU to determine the state of its computations. There are registers for the status of the ALU's operation, for keeping count of the instruction to be performed next, to keep data flowing in from memory or out to memory, to maintain the instruction being executed, and for the location of operands being operated on by the CPU.
Each of these registers has a unique function within the CPU, and each is necessary for various classes of computer architectures. A typical minimal architecture for a CPU and its registers is shown in Figure 2.3. This architecture consists of a primary memory connected to the CPU via buses that use a memory address register and memory data register to address a location in memory and transfer the contents...
