A Simple Computer Design: The μP 3

Chapter 9 develops a model of a simple computer. The fetch, decode, and execute cycle is introduced and a brief model of the computer is developed using VHDL. A short assembly language program can be entered in the FPGA’s internal memory and executed in the simulator.

A traditional digital computer consists of three main units, the processor or central processing unit (CPU), the memory that stores program instructions and data, and the input/output hardware that communicates to other devices. As seen in Figure 9.1, these units are connected by a collection of parallel digital signals called a bus. Typically, signals on the bus include the memory address, memory data, and bus status. Bus status signals indicate the current bus operation, memory read, memory write, or input/output operation. data inside the processor. Registers such as PC, IR, AC, MAR and MDR are built using D flip-flops for data storage. One or more arithmetic logic units (ALUs) are also contained inside the CPU. The ALU is used to perform arithmetic and logical operations on data values. Common ALU operations include add, subtract, and logical and/or operations. Register-to-bus connections are hard wired for simple point-to-point connections. When one of several registers can drive the bus, the connections are constructed using multiplexers, open collector outputs, or tri-state outputs. The control unit is a complex state machine that controls the internal operation of the processor. The primary operation performed by the processor is the execution of sequences of instructions stored in main memory. The CPU or processor reads or fetches an instruction from memory, decodes the instruction to determine what operations are required, and then executes the instruction. The control unit controls this sequence of operations in the processor.