Processor Design : Simple Processor Design

The Design of a processor is normally represents the way the components of the processor are interrelated. The design of a processor is explained by its internal registers, the timing and control structure and the set of instructions it uses. The Processor design process involves choosing an instruction set and a certain execution paradigm (e.g  RISC, CISC, VLIW). The basic block diagram of a simple Processor is given below :

The processor is made up of four building blocks which are :

Logic : Every block within the computer can be considered to be made from Boolean logic gate, however, this category refers specific, larger logic blocks e.g. adders, address decoders, instruction decoders etc.

Multiplexers : a multiplexer or mux is selects one of several Data inputs and forwards the selected input into a single line. From one point of view a computer just moves information from one point to another. Controlling the path taken by this information are multiplexers, switching junctions, allowing information to be passed between functional blocks.

Registers : Registers are the fast and short term memory. As part of the Fetch-Decode-Execute cycle a computer needs to remember its state, the instruction to be processed and any results generated.

Memory :  Memory allows data items to be read or written in it. It can be accessed randomly.

The mode of operation of any microprocessor is the execution of lists of instructions. Instructions typically include those to compute or manipulate data values using registers, change or retrieve values in read/write memory, perform relational tests between data values and to control program flow. CPU design focuses on four main areas:

  • Datapaths
  • Control Unit
  • Memory
  • Clock circuitry

1. Datapaths :   A datapath is a collection of functional units such as arithmetic logic units or multipliers, that perform data processing operations. It is the hardware that performs all the required operations, for example, ALU, registers, and internal buses. The datapath is the pathway that the data takes through the microprocessor. As the data travels to different parts of the datapath, the command signals from the control unit cause the data to be manipulated in specific ways, according to the instruction. The datapath consists of the circuitry for transforming data and for storing temporary data. It contains ALUs capable of transforming data through operations such as addition, subtraction, logical AND, OR, inverting, and shifting.

2. Control Unit :   The control unit reads the instruction, and activates the appropriate parts of the datapath.  It tells the computer's memory, arithmetic/logic unit and input and output devices how to respond to a program's instructions. It directs the operation of the other units by providing timing and control signals. Most computer resources are managed by the CU.

3. Memory :   Processor relies on memory for storing the instructions and the data used by software programs. The memory unit is responsible for communicating with the system memory. For example RAM, Cache memory. The memory unit is typically one of the slowest components of a micro-controller, because of the external interface with RAM is typically much slower than the speed of the processor.

4. Clock circuitry :   The clock circuitry in the CPU is used to synchronize information between all processor components. The CPU requires a fixed number of clock cycles, to execute each instruction. The faster the clocks cycle, the faster the CPU, or the faster it can execute instructions.

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