Aug 252015

Microprocessor architecture can be defined as the set of basic rules that define the components of a microprocessor, their organization, along with the way of functioning of those components for operation. In this post, we are describing about two different microprocessor architectures. They are:

  1. Von Neumann Architecture: This approach is also known as ‘stored program concept’ and was first adopted by John Von Neumann.vonneumann
    The main memory is used to store data and instructions. The ALU is capable of performing arithmetic and logical operations on binary data. The program control unit interprets the instructions in memory and causes them to be executed. The I/O unit gets operated from control unit.
    The Von Neumann architecture is the fundamental basis for the architecture of today’s digital computers. Thus, it is important to have an idea of internal structure of the CPU and CU of Von Neumann machine.
    The memory of Von Neumann machine consists of 1000 storage locations called words, of 40 bits each. Both the data and instructions are stored in it. The CU operates the computer by fetching instructions from the memory and executing them one at a time.
    The storage location of CU and ALU are called registers. The various registers of this model are:
  • Memory Buffer Register (MBR): It consists of words to be stored in memory or is used to receive a word from the memory.
  • Memory Address Register (MAR): It contains the address in memory of the word to be written from or read into MBR.
  • Instruction Register (IR): It contains 8 bit opcode being executed.
  • IBR (Instruction Buffer Register): It is used to temporarily hold the instructions from word in memory.
  • Program Counter (PC): It contains the address of the next instruction to be fetched from the memory.
  • The accumulator (A) and the multiplier quotient (MQ) are employed to temporarily hold the operands and the result of ALU operations.

In Von Neumann architecture, the same memory is used for storing instructions and data. Similarly, a single bus, either data bus or address buss is used for reading data and instructions from or writing data and instruction to the memory.
Later, it was revealed that this feature of Von Neumann architecture limited the processing speed of the computer. So, to improve the processing speed of computer, Harvard architecture was introduced.

  • Harvard Architecture: The Harvard architecture consists of separate memory locations for the programs and data. Each memory space has its own address and data bus. As a result of this, both the instructions, and data can be fetched from the memory concurrently. Thus, a significant processing speed is observed over Von Neumann type architecture.harvard
    From the figure, we can see that there are two data and two address buses for the program and data memory space separately. The program memory data bus and data memory bus are multiplexed to form single data bus whereas program memory address bus and data memory address bus are multiplexed to form a single address bus. Hence, there are two blocks of RAM chip, one for program memory and the other for data memory space. The data memory address arithmetic unit generates the data memory address. The data memory address bus carries the memory address of the data whereas the program memory address bus carries the memory address of the instruction. There is central arithmetic logic unit which consists of ALU, multiplier, accumulator and scaling chief register. The program counter is used to address program memory. The contents are updated following each decode operation. The control unit controls the sequence of operations to be executed. The data and control bus are bidirectional whereas the address bus is unidirectional.

Dec 062012

C (Brain) Virus:

C (Brain) virus was created by Pakistani brothers Amjad and Basit Alvi in the year 1985 A.D. at Lahore. Originally, it was created and designed to prevent unauthorized use of software sold by ‘Alvis’ Brain computer shop at Lahore, Pakistan. It was put into programs like Lotus 123 and WordStar and sold at heavy discounts to foreign tourists. Once the host used the infected disk, it became the carrier of the virus and copied itself quickly to

uninfected computers and diskettes. The virus, hiding in the boot sector replicated quickly and within months it spread throughout the world of personal computers. Later, various strains of this computer virus appeared which were fatal and malicious.

The Stone: The Stone affects the boot sector computers. It affects the Master Boot Record containing partition table, in hard disks. This virus is written in two sections – the resident portion and the installation portion.

The Vienna-648 Virus: The Vienna-648 virus is probably the simplest file infector virus ever created. It is not very efficient and can be written in far less number of instructions. It infects COM files, increasing their size by 648 bytes by appending its code at the end of the file. It doesn’t infect files having size less than 10 bytes. It is also known as the 1-in-8 virus, since on every eighth infection, it creates an instruction for rebooting the computer. Due to this, the file becomes unexecutable and the computer reboots. In case this happens with COMMAND.COM, the computer never boots because every time COMMAND.COM gets recalled and tries to reboot the computer.

The 8290 (Print Screen) Virus: This virus is a boot sector infector. This virus checks for the word, ‘8290 hex’ at a certain place in the boot record. If the word is not present, it won’t be able to affect the computer and its data and instruction. Due to this reason, it is called the 8290 virus. It moves the original boot record to the 11th sector on the floppies and 114th sector on 20 MB hard disks and then transfers its own programs as the boot record.

The Joshi Virus: The Joshi virus is one of the intelligent types of viruses which use stealth techniques in order to baffle affected hackers. This virus affects the Master Boot Record which contains the partition table in hard disks. On 5th January of every year, the virus gets activated. Due to its activation, the whole computer system gets hanged and a message is displayed as “type Happy Birthday Joshi”. The computer can be saved from hanging and it can be used again if the use types the message “Happy Birthday Joshi” in his hanged computer.

The Bloody Virus: The Bloody Virus is also taken as the reincarnation of the Stone virus. This virus affects the Master Boot Record of hard disks which contains the partition table. It was modeled after the Stone virus.

The Ping-Pong Virus: The Ping-Pong virus affects the boot sector of hard disks. The virus gets activated in every 30 minutes and displays a bouncing dot on the monitor of the computer. The dot passes through any text that is displayed on the screen. This virus has no other major effect rather than the creation of 1 KB bad sector where it places the original boot sector.

The Dark Avenger: The Dark Avenger is one of the most dangerous and infectious viruses. Once it infects a disk, it becomes too much difficult to remove it from the disk. It affects the EXE and COM files by attaching itself to the end of the file and puts a JMP instruction at the start of the file. It is a memory resident virus and becomes a resident when an infected file is run. The message “Program too big to fit in memory” is displayed on the screen and the computer gets hanged. The size of the infected files are increased in size by 1800 bytes.