Microprocessor and Von-Neumann Architecture

Microprocessor:A microprocessor is a multipurpose, programmable, Clock-driven, register based electronic device that reads binary instructions from a storage device called memory, accepts binary data as input and process data according to the instructions, and provide results as output. In other words, a microprocessor is the chip containing some control and logic circuits that is capable of making arithmetic and logical decisions based on input data and produces the corresponding arithmetic or logical output.A typical programmable machine can be represented with four components: Microprocessor, memory, input and output as shown in the figure below.

Figure: A programmable Machine

Figure: A programmable Machine

Application of Microprocessor: Microprocessors are used in many devices in the modern world, but the application of the microprocessors can be broadly classified into two groups i.e. reprogrammable and embedded.

  • Personal computers, Servers, notebook, smartphones etc. are reprogrammable application of microprocessors.
  • Home appliances. Machinery, automated systems, communication devices are examples of embedded application of microprocessor. The microprocessor inside the embedded system are also known as the Micro-controllers.

Evolution of Microprocessors

  • Intel developed and delivered the first commercially viable microprocessor way back in the early 1970’s: the 4004 and 4040 devices. The 4004 was not very powerful and all it could do was add and subtract with 4-bit data only at a time.
  • Intel rapidly followed their 4-bit offerings with their 8008 and 8080 eight-bit CPUs.
  • Intel started facing competition from Motorola, MOS Technology, and an upstart company formed by disgruntled Intel employees, Zilog. To compete, Intel produced the 8085 microprocessor. To the software engineer, the 8085 was essentially the same as the 8080 but had lots of hardware improvements that made it easier to design into a circuit.
  • During late 70’s Motorola’s 6800 series was easier to program, MOS Technologies’ 65xx family was also easier to program but very inexpensive, and Zilog’s Z80 chip was upward compatible with the 8080 with lots of additional instructions and other features. By 1978 most personal computers were using the 6502 or Z80 chips, not the Intel offerings.
  • The first microprocessor to make a real splash in the market was the Intel 8088, introduced in 1979 and incorporated into the IBM PC (which appeared around 1982 for the first time). If we are familiar with the PC market and its history, we know that the PC market moved from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium II to the Pentium III to the Pentium 4. Intel makes all of these microprocessors and all of them are improvements of design base of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5,000 times faster.

    Von Neumann Architecture:

    It is named after the mathematician and early computer scientist John Von Neumann. It is known as stored program architecture. The computer has single storage system (memory) for storing data as well as program to be executed. It is widely used computer architecture. The other architecture is Harvard Architecture which have separate memory for data and program respectively.

    Figure: Von-Neumann Architecture

    Figure: Von-Neumann Architecture

    The control unit (CU) has overall responsibility for the system. It controls various activities within the microprocessor.

    The arithmetic-logic unit (ALU), as its name also suggests, performs arithmetic (addition, subtraction, etc.) and logic (and, or, etc.) operations.

    The registers are small memory within the CPU. This is the only storage to which the ALU has direct access. To manipulate any values from memory they must first be loaded into registers.


    Memory is device where both program code and data reside while the program is in execution. This is the stored program concept, often attributed to John von Neumann. Memory (or Main Memory) is sometimes referred to as Primary Storage, as opposed to Secondary Storage, which usually refers to disk drives, tape drives and the like. The major difference between primary and secondary storage is access time.

    Input and Output

    Microprocessor takes in input from the input devices as well as from the data stored in the secondary storage. It gives output through output device after processing according to the instruction.

    The Memory Subsystem

    Memory is where the computer stores information and (running) programs. Memory is made up of bits: the bit is the smallest unit of storage, it can hold one of two values: 1 and 0 (zero). Bits are grouped into larger units called bytes (8 bits). Group of Bytes are called as words.

    There are two important attributes associated with every chunk of memory:

    • Its address and
    • Its contents

    Memory Operations

    Memory is capable of doing two things:

    1. Deliver the current contents of a designated memory cell to the CPU (memory read).
    2. Store new contents in a designated memory cell (memory write).

    For the first operation (known as a memory read) the CPU must pass two pieces of information to the memory subsystem:

    • The address of the memory cell to be
    • An indication that a read is

    The memory subsystem, having retrieved the information, must then forward it on to the CPU.

    In order to carry out the second operation (a memory write), the CPU must pass:

    • The address of the memory cell to be stored
    • The new contents to be stored.
    • An indication that a write is

    In order to perform these operations Processor needs following registers and control signal:The Memory Address Register (MAR): Which contains the address of the requested cell, the one we want either to read from or to write into.

    The Memory Data Register (MDR): The memory subsystem places the retrieved contents of memory (read) and the CPU places the new contents of memory (write).

    A Read/Write Line (R/W): Used to indicate whether the required operation is a read or a write.

    Input/ Output Subsystem:

    In order to communicate with anything outside the CPU/Memory system we need an I/O subsystem. This subsystem needs to be able to control many different types of device, for example: Screen, keyboard, printer, but they all have very different characteristics.

    I/O Controllers

    Compare to RAM, I/O devices are extremely slow. If the CPU had to wait for peripheral I/O to complete before carrying on then the CPU is going to spend a good deal of its time doing nothing useful.

    The solution is to use an I/O Controller, which is a special purpose processor which has a small memory buffer, and a control logic to control the I/O device (e.g. move disk arm).

    In its simplest form, the procedure for performing an I/O request (input or output, the scheme is the same) is as follows:

    1. CPU sends I/O request information to the I/O This is usually done by having the controller’s registers accessible to the CPU.
    1. The controller takes over responsibility for the I/O For example, on a disk read, the controller will be responsible for moving the arm to the appropriate cylinder. Data arriving from the disk will be buffered in the controller’s internal memory and then by the process of Direct Memory Access or DMA it is transferred to RAM.

    Basic Architecture of Microprocessor Based System

    Figure: Microprocessor based system with bus architecture

    Basic Components of Microprocessor based system are as follows:

    • Control Unit
    • Arithmetic and Logic Unit
    • Memory
    • Input/ Output
    • CPU Registers
    • System Bus

    CPU Registers:

    In computer architecture, a processor register is a very fast computer memory used to speed the execution of computer programs by providing quick access to commonly use values-typically, the values being in the midst of a calculation at a given point in time.

    These registers are the top of the memory hierarchy, and are the fastest way for the system to manipulate data. In a very simple microprocessor, it consists of a single memory location, usually called an accumulator. Registers are built from fast multi- ported memory cell. They must be able to drive its data onto an internal bus in a single clock cycle. The result of ALU operation is stored here and could be re-used in a subsequent operation or saved into memory.

    Registers are normally measured by the number of bits they can hold, for example, an “8-bit register” or a “32-bit register”. Registers are now usually implemented as a register file, but they have also been implemented using individual flip-flops, high speed core memory, thin film memory, and other ways in various machines.

    There are several other classes of registers

    • Accumulator: It is most frequently used register used to store operand before the execution of an instruction and to store result after the execution of the
    • General Purpose registers: General purpose registers are used to store data, address and intermediate results during program Its contents can be accessed through assembly programming. It can be further classified into Integer and Floating point registers.
    • Special purpose Registers: Users do not access these These are used by computer system at the time of program execution. Some types of special purpose registers are given below:
    • Memory Address Register (MAR): It stores address of data or instructions to be fetched from memory or Input/ Output
    • Memory Buffer Register (MBR): It stores instruction and data received from the memory and sent to be written in memory
    • Memory Data Register (MDR): It is similar to It stores instruction and data received from the memory and to be sent to the memory for writing.
    • Instruction Register (IR): Instructions are stored in instruction When one instruction is completed, next instruction is fetched from memory for processing.
    • Program Counter (PC): It stores the address of next instruction to be

    System Bus

    The system bus is a cable which carries data communication between the major components of the computer, including the microprocessor. Not all of the communication that uses the bus involves the CPU, although naturally the examples used in this tutorial will center on such instances.

    The system bus consists of three different groups of wiring, called the data bus, control bus and address bus. These all have separate responsibilities and characteristics, which can be outlined as follows:

    Control Bus

    The control bus carries the signals relating to the control and co-ordination of the various activities across the computer, which can be sent from the control unit within the CPU. Different architectures result in differing number of lines of wire within the control bus, as each line is used to perform a specific task. For instance, different, specific lines are used for each of read, write and reset requests.

    Data Bus

    This is used for the exchange of data between the processor, memory and peripherals, and is bi-directional so that it allows data flow in both directions along the wires. Again, the number of wires used in the data bus (sometimes known as the ‘width’) can differ. Each wire is used for the transfer of signals corresponding to a single bit of binary data. As such, a greater width allows greater amounts of data to be transferred at the same time.

    Address Bus

    The address bus contains the connections between the microprocessor and memory/IO devices that carry the signals relating to the addresses which the CPU is processing at that time, such as the locations that the CPU is reading from or writing to. The width of the address bus corresponds to the maximum addressing capacity of the bus, or the largest address within memory that the bus can work with. The addresses are transferred in binary format, with each line of the address bus carrying a single binary digit. Therefore the maximum address capacity is equal to two to the power of the number of lines present (2^n lines).

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