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The Organisation of a computer system can be conceived in terms of a series of levels or abstraction maintaining a level hierarchy in which each level in the hierarchy with more or less fixed tasks and responsibilities performs a specified range of operations at minimum cost, and relies on its just upper level in the hierarchy. All machine activities are usually obtained by means of the operations carried out by three basic levels, namely the processor level, the register level, and - the lowest one - the gate level. The type of components to be used in each such level is mostly standardized, but each of these three levels can be realized in many different ways that entirely depend on the specific Organisation being followed in building up a computer system. Processor-level system is absolutely well defined, and comprises CPUs and other processors, memories, I/O devices, and interconnection networks. Its performance is measured and evaluated by a non-profit Organisation named System Performance Evaluation Corporation, often abbreviated as SPEC. Register level consists of combinational devices (such as multiplexers, decoders, adders, and word gates), sequential devices (such as registers, counters etc.), and various general-purpose programmable elements (including PALs, PLAs, FPGAs, and ROM). The gate level is the lowest level in the architectural design of computing systems. This level comprises logic gates as basic functional components that operate on a well-defined theory based on Boolean algebra. Unlike combinational circuits, sequential circuits have memory, which is usually built from gates, and 1-bit storage elements (flip-flops) that store the circuit's state and are synchronized by means of clock signals.


  • 2.1 Discuss the contribution of level concept in the architectural design of the computer. Is it possible for a multilevel computer to have the device level and the digital logic level as not the lowest levels? Justify.
  • 2.2 What is the difference between translation and interpretation? In what sense are hardware and software equivalent? Not equivalent? Do you think that level concept in the design of computers is still pertinent even in the design of modern computers of today? Explain.
  • 2.3 "Introduction of microprogramming level in computer design opened a new horizon": discuss.
  • 2.4 Discuss the methodology being commonly followed in the design of computers.
  • 2.5 What are the standard combinational components and sequential components used at the register level of computer design?
  • 2.6 What is the use of a multiplexer? Construct a 16-to-l-line multiplexer with two 8-to-l-line multiplexers and one 2-to-l-line multiplexer. Use block diagrams for the three multiplexers.
  • 2.7 A digital computer has 16 registers, each with 32 bits. The registers are connected by a common bus which is constructed with multiplexers.

a. How many selection inputs are there in each multiplexer?

b. How many multiplexers are there in the bus?

c. What size of multiplexers is needed?

[Hint: b) Each multiplexer transfers one bit of the selected register. The number of multiplexers needed to construct the bus is equal to n, the number of bits in each register. Here, it is 32.

c. The size of each multiplexer must be к x 1, where к is the number of the registers, since it multiplexes к data lines, each from a register.

  • 2.8 Design a 5 x 32 decoder using four 3x8 decoders (with enable inputs) and one 2x4 decoder.
  • 2.9 State and explain the role being played by an encoder in the design of a computer. Draw the logic diagram of a 2-bit encoder, a circuit with four input lines, exactly one of which is high at any instant, and two output lines whose 2-bit binary value tells which input is high.
  • 2.10 What is the difference between serial and parallel transfer? Using a shift register with parallel load, explain how to convert serial input data to parallel output and parallel input data to serial output.
  • 2.11 The content of a 4-bit register is initially 1001. The register is shifted five times to the right with the serial input being 11011. What is the content of the register after each shift?
  • 2.12 Why counter is considered an important component at the time of building a computer? Explain with a diagram the operation of a counter.
  • 2.13 What does PLD stand for? How are the circuits reconfigured electronically in a PLD? What is actually being "programmed" in a PLD?
  • 2.14 What is the contribution of PLDs in the circuit design of modern computers? What are the different types of PLDs commonly used in computer circuits?
  • 2.15 State with diagram the different states of a tri-state buffer and explain its operation. What are the applications of tri-state buffer? Show how computer bus can be organised using tri-state buffer.

Suggested References and Websites

Armstrong, J. R. and Gray, F. G. Structured Logic Design with VHDL. Englewood Cliffs, NJ: Prentice- Hall, 1993.

Hamacher, C., Vranesic, Z. G., and Zanky, S. G. Computer Organisation, 5th ed. New York: McGraw- Hill, 2002.

Langholz, G., Francioni, J., and Kandel, A. Elements of Computer Organisation. Englewood Cliffs, NJ: Prentice-Hall, 1989.

Mano, M. Logic and Computer Design Fundamentals. Upper Saddle River, NJ: Prentice-Hall, 2004.

Siewiorek, D. P, Bell, C. G., and Newell, A. Computer Structures: Reading and Examples. New York: McGraw-Hill, 1982.

Tanenbaum, A. S. Structured Computer Organisation, 4th ed. Upper Saddle River, NJ: Prentice-Hall, 1999.

System Performance Evaluation Corporation: web page:

Standard benchmarks are used to measure and compare the performance of different computer systems.

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