COA-Basic Computer Architecture

Digital Computer Architecture(Basic Computer Architecture):

There are basically two types of Digital Computer Architectures, where,

1. The first one is Von Neumann Architecture, and later is
2. HarvardArchitecture

These Architecture was adopted for designing of digital computer. Now, we will explore to each one by one in detail.

1.Von Neumann Architecture:

    Von Neumann Architecture is the first Digital Computer Architecture given by Hungarian                           mathematician and early computer scientist John Von Neumann to remove the drawback of first               electronic computer ENIAC.

    ENIAC (Electronic Numerical Integrator And Computer) was constructed and integrated under the           direction of Eckert and Mauchly at the Moore School of Engineering(University of Pennsylvania).It           was made up of more than 18000 Vacuum tubes and 1500 relays. ENIAC’s primary function was to-       compute "ballistic trajectories". It was able to perform nearly 5000 additions or subtractions per               second. The ENIAC was a decimal rather than a binary machine i.e., all numbers in an ENIAC were       represented in decimal form and arithmetic was performed on the decimal system.Its data memory         consists of 20 “accumulators”, each capable of storing a ten digit decimal number.And Each digit is         represented by 10 Vacuum tubes and only one vacuum tube was in the ON state represent one of ten     digits.

    The major drawback of ENIAC is that it was wired in for specific computation.And for modification or       replacement of programs manual setting of switches and plugging and unplugging of cables are               necessary.Thus it was very time consuming process.And also, the task of entering and editing                 programs for ENIAC was extremely tedious.

    Thus, to make programming process easier i.e., entering and editing of programming easier, the             ENIAC designers most probably the mathematician John Von Neumann who was consultant of               ENIAC project gave idea that-

    “If a program could be represented in a form suitable for storing in memory alongside the data,                 then a computer could gets its instruction by reading them from memory and a program could be             set or altered by setting the values of a portion of memory then problem can be resolved”.

    In short, John Von Neumann suggested that data and program should be stored in memory.

    This idea, known as “Stored Program Concept”, and this concept is now called as “Von Neumann           Architecture”

    Now, to convert its architectural concept into reality,  in 1946, Von Neumann and his Collogeous               at the "Institute for Advanced Studies(IAS)", in "Princeton" try to make a first stored program                     computer.They came up with  a  machine (computer), referred to as IAS, or Von Neumann or                   Princeton Machine.And since it was the first stored program computer according to John Von                   Neumann Architecture, so, John Von Neumann architecture is also sometimes known as IAS                   architecture or Princeton architecture.

    Figure shown below shows the general structure for Von Neumann Machine.

    Here we see that it can consists of five basic units whose function can be summarised as follows:

1. A Memory Unit stores both data and instruction(Program).
2. An Arithmetic  and Logical Unit perform arithmetic and logical operations on binary data.
3. The Input Unit Transmit data and Instructions from Outside World to Machine.It is operated by controlled Unit.
4. The Output Unit transmit final result and message to outside world.It is also operated by Controlled Unit.
5. The Control Unit fetches and interprets the instruction in memory and causes them to be executed.

    It is now become the usual frame of reference for many modern computer.And it is important to               note that, in the original IAS machine(Von Neumann Machine), memory unit consists of 4096                   storage locations(2¹²=4096) of 40 bits each, referred to as "words".These memory locations are             used to store data as well as instructions.

    Disadvantages of Von Neumann Architecture:

    Now , we know that, Von Neumann Architecture is a very wonderful concept and gave so many               advantages to us.But there is also some drawbacks present in Von Neumann Architecture which is         described below:

• In Von Neumann Architecture, computer has single storage space (memory) for instructions(program) and data with one data bus and one address bus between processor and memory, as shown in fig. below:

           Therefore, in it, processor need two clock cycles to complete an instruction.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 by processor.Thus, in it processor needs two cycles to                       complete(or process) one instruction, which is time consuming process.

• Because of this, pipelining of instructions is not possible with this architecture.

    So, to remove the disadvantage of it, Harvard Architecture come into picture.

2.Harvard Architecture:

    The term “Harvard” originated from the Harvard Mark I relay- based computers.It can remove the             disadvantage of Von Neumann Architecture.

    As, in Harvard Architecture, there are many physically seprate storage space in memory for both             programs and data, and buses are also seprate.That is, in Harvard Architecture memory spaces is           divided into seprate two spaces one for program and one for data.And the processor is connected to       two different memory spaces via two sets of Buses.

    Means, there is seprate address bus and data bus for program and seprate data bus and address bus      for memory data.

    The address bus and data bus in it are not seprate by program and data unlike Princeton                         Architecture

    It is depicted below:

    Thus, through this scheme, CPU can read both an instruction and data from the respective memory         spaces at the same time.This means that, in it processor can complete an instruction in one cycle,           by take instruction from one set of bus and data from other set of bus.

    This increases the speed of processing, and appropriate pipelining strategies are also implemented         via it.

    And because of this concept, today, most of the modern computer architecture are based on                   Harvard Architecture.

    Note:

    Microcontrollers with Harvard Architecture are also called RISC (Reduced Instruction Set                         Architecture) Architectures.And microprocessor with Von-Neumann’s Architecture are called                     Complex Instruction Set (CISC) Architecture.