CISC (Von Neumann) VS RISC (Harvard)

    

          The Instruction set architechture (ISA) is plays an important role in the microprocessors. There are different instruction set architechture microprocessors available in them market, they are:

1.    Complex Programming Instruction Set Computer (CISC)

This was first ISA. In the complex instruction set computer (CISC) instruction set is complex instruction which will take long time to execute. Complex instruction may consist of opcode addressing mode operand etc. The execution speed will be slow. X86 architechture is the example.

2.    Reduced Instruction Set Computer (RISC)

In the reduced instruction set computer (RISC), the instruction set will be small and execution speed will be fast. The implementation is simple and does not require complex architechture. RISC is widely used in embedded applications. SHARC and POWERPC use RISC.

There are some differences between CISC and RISC:

CISC	RISC
Very large instruction sets reaching up to and above three hundred separate instruction.	Small set of instruction.
Performance was improved by allowing the simplification of program compilers, as the range of more advanced instructions available led to less refinements having to be made at the compilation process. However, the complexity of the processor hardware and architecture that resulted can cause such chips to be difficult to understand and program for, and also means they can be expensive to produce.	Simplified and reduced instruction set, numbering one hundred instruction or less. Because of simple instructions, RISC chips requires fewer transistors to produce processors. Also the reduced instruction set means that the processor can execute the instructions more quickly, potentially allowing forgreater speeds. However, only allowing such simple instructions means a greater burden is placed upon the software itself. Less instruction in the instruction set means a greater emphasis on the efficient writing of software with the instructions that are available.
More specialized addressing modes and registers also being implemented, with variable length instruction codes.	Addressing modes are simplified back to four or less, and the length of the codes is fixed in order to allow standardization across the instruction set.
Instruction pipelining can not be implemented easily.	Instruction pipelining can be implemented easily.
Many complex instructions can access memory, such as direct addition between data in two memory locations.	Only LOAD/STORE instructions can access memory.
Mainly used in normal PC’s, workstations and servers.	Mainly used for real time applications.
CISC systems shorten time by reducing the number of instruction per program.	RISC systems shorten execution time by reducing the clock cycles per instruction (i.e. simple instructions take less time to interpret.
Examples of CISC Processors: Intel x86.	Examples of RISC Processors: Atmel AVR, PIC, ARM.

Some differencess between Von-Neumann and Harvard:

VON -NEUMANN	HARVARD
It is named by early computer scientist John Von Neumann.	This name is oridinated by “Harvard Mark I” a relay based old computer.
The computer has single storage system for storing data as well as program to be executed.	The omputer has two separate memories for storing data and program.
Processor needs two clock cycles to complete an instructions is not possible with this architecture.	Procesor can complete an instruction in one cycle if appropriate-pipelining strategies are implemented.
In the first clock cycle the processor gets the instruction from memory and decodes it. In the next clock cycle the required data is taken from memory. For each instruction this cycle repeats and hence needs two cycles to complete an instruction.	In the first stage of pipeline the instruction to be executed can be taken from program memory. In the second stage of pipeline data is taken from the data memory using the decoded instruction or address.
This is a relatively older architecture and was replaced by Harvard architecture.	Most of the modern computing architectures are based on Harvard architecture. But the number of stages in the pipeline varies from system to system.

            There are other architecture like Very Long Instruction word (VLIW) and FUSION of RISC and CISC architecture etc.

            Microprocessor should be choosen according to the application for small application you don’t need CISC. You cannot use RISC for large application. According to the application and peripherals you are using micro processor should be taken.

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Control Unit (CU)

Control Unit (CU)

A control unit is circuitry that directs operations within the computer's processor by directing the input and output of a computer system. The processor then controls how the rest of the computer operates (giving directions to the other parts and systems). A control unit works by gathering input through a series of commands it receives from instructions in a running programs and then outputs those commands into control signals that the computer and other hardware attached to the computer carry out.

The control unit is basically circuitry inside the CPU, controlling the operations inside the CPU and "directing traffic" in a sense. The functions a control unit performs can depend on the type of CPU, since the varying degrees of architecture between all the different CPUs determine the functions of the control unit.

A CU takes its input from the instruction and status registers. Its rules of operation, or microprogram, are encoded in a programmable logic array (PLA), random logic or read-only memory (ROM).

CU functions are as follows:

• Controls sequential instruction execution
• Interprets instructions
• Guides data flow through different computer areas
• Regulates and controls processor timing
• Sends and receives control signals from other computer devices
• Handles multiple tasks, such as fetching, decoding, execution handling and storing results

CUs are designed in two ways:

• Hardwired control: Design is based on a fixed architecture. The CU is made up of flip-flops, logic gates, digital circuits and encoder and decoder circuits that are wired in a specific and fixed way. When instruction set changes are required, wiring and circuit changes must be made. This is preferred in a reduced instruction set computing (RISC) architecture, which only has a small number of instructions.
• Microprogram control: Microprograms are stored in a special control memory and are based on flowcharts. They are replaceable and ideal because of their simplicity.

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