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Computer Science CS25510: Computer Hardware

Time allowed: 2 hours
Calculators are not allowed in this examination

Answer THREE from FIVE questions

All questions carry equal marks

1. This question is based upon introductory material and covers analogue and digital systems,
   simple physical laws, semiconductor technology, and propagation delay times for digital
   computer hardware systems.

   a) The human brain is composed of a very large number of interconnected neurons and
      can be regarded as an analogue computer. Given that humans are capable of highly
      intelligent computation, why then are modern computers digital based hardware
      systems, as opposed to analogue based hardware systems? Illustrate your answer by
      describing the advantages and disadvantages of both digital and analogue hardware
      systems.                                                               [10 Marks]

   b) A logic device requires a voltage input that can be changed from logic HIGH to logic
      LOW (and vice versa) with the aid of a ‘push to make’ switch. Using the knowledge
      you have gained from practical classes, draw a simple circuit that could achieve this
      requirement. Using simple physical laws, and a circuit mechanical analogue, explain
      the function of any component(s) that you have used.                     [12 Marks]

   c) You have been asked to recommend which semiconductor technology would be the
      most appropriate to use in the building of two computer hardware systems. One
      computer is going to be used on-board a mobile robot for outdoor operations, the other
      computer is to be used as a static system within a laboratory. Which semiconductor
      technology would you recommend for each of these computers? State the reasons for
      your recommendation.                                                       [6 Marks]

   d) You are asked to design an electronic circuit that requires a number of ICs to be
      cascaded together. Timing in this circuit will be crucial, and you have been asked to
      calculate the circuit’s propagation delay time. Explain this term, and indicate why it is
      an important consideration in modern digital computer hardware.
                                                                                    [5 Marks]

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2. This question is concerned with the laws and theorems of Boolean algebra. First the
   question examines the proof of a commonly used simplification rule, and second the
   simplification of a combinational logic circuit is examined.

  a) Complex logic circuits often can be simplified using the laws and theorems of Boolean
     algebra. Below is one commonly used simplification rule:

                                     A + A.B = A + B
     Using the laws and theorems of Boolean algebra, derive a proof for this simplification
                                                                                [16 Marks]

  b) Using the laws and theorems of Boolean algebra, simplify the following logic circuit
     (Figure 1), and draw the resultant equivalent logic circuit. Show all of your working
                                                                                [17 Marks]

                                        Figure 1.

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3. This question first examines two uses of an exclusive-OR gate in computer hardware. The
   question then focuses upon flip-flops and examines the differences between the commonly
   available flip-flop types. Finally, the question examines a flip-flop application.

   a) The exclusive-OR gate can be used as part of an electronic circuit to perform many
      useful computing functions such as binary number comparison and addition. Using
      exclusive-OR gates and additional logic gates where necessary, design logic diagrams
      for both a 2-bit binary number comparator that tests for equality, and a 2-bit binary
      number full-adder.                                                       [16 Marks]

   b) Flip-flops are synchronous bistable devices that have many useful applications in
      computer hardware. Three types of edge-triggered flip-flops are the S-R, D and J-K
      types. With the aid of diagrams and truth tables, describe the differences between these
      flip-flops, and indicate why the J-K flip-flop is preferred over the S-R flip-flop.
                                                                                    [9 Marks]

   c) A computer control application requires a binary counter. With the aid of a schematic
      diagram, and using your knowledge of J-K flip-flops, describe how you could generate
      a four bit binary counter.
                                                                                    [8 Marks]

4. This question concerns microcontrollers with emphasis on the Motorola 68HC11 device.
   The peripheral capabilities of microcontrollers, a programming problem and how
   additional memory can be added are examined.

   a) Microcontrollers are used extensively to control real machines and electrical goods.
      Typically motors and sensors are present in these systems, and a microcontroller
      requires a number of peripheral capabilities. Based upon your knowledge of the
      Motorola 68HC11A8 microcontroller, describe the peripheral capabilities found in
      commercially available microcontrollers.
                                                                                    [7 Marks]

   b) Using the knowledge you have gained from programming the 68HC11 simulator,
      describe how interrupts and the output compare mechanism (OC) could be used to
      perform pulse-width modulated (PWM) DC motor speed control for a mobile robot.
      You may assume that a DC motor to microcontroller interface circuit is available that
      switches the motor on upon receipt of a logic 1, and switches the motor off, upon
      receipt of a logic 0. Your description should include either a flow-chart of your
      solution, pseudo-code, or Motorola 68HC11 assembly language code.
                                                                                  [13 Marks]
   c) Microcontrollers can be operated in multiplexed expanded mode, thus allowing more
      external RAM to be added. If additional RAM is required for a microcontroller
      application, explain the design process that is required. Use a simple example to
      illustrate this process.
                                                                                  [13 Marks]

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     5. This question examines the execution of an assembly language program at the level of a
        timing diagram, and asks what are the major differences between CISC and RISC based
        computer hardware architectures.

        a) Below is a short Motorola 6802 assembly language program.
                        LABEL     LDAB       #1
                                  STAB       OUTPUT_PORT
                                 LDAB       #0
                                 STAB       OUTPUT_PORT
                                 LDAB       $0000
                                 BEQ        LABEL
           This program outputs a continuous serial stream of bits from the least significant bit of
           an 8 bit VIA parallel output port. You may assume that the label OUTPUT_PORT has
           been initialised to the correct address for the VIA. In addition to the VIA, the Motorola
           6802 microprocessor printed circuit board has ROM and RAM ICs. By monitoring
           various pins associated with the VIA, ROM, RAM, 6802 microprocessor, and data bus,
           whilst executing the above program, the following timing diagram has been obtained
           (Figure 2).






6802 VMA


                                                  Figure 2.

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  With reference to Figure 2, and using the knowledge you have gained from the
  laboratory, explain what is occurring for each waveform in the timing diagram, and
  discuss the relationships between the waveforms.

  If you wanted to determine how the machine code instructions generated from the
  above assembly program relate to the timing diagram, together with the memory
  address for each machine code instruction, what additional measurements would you
  have to perform?
                                                                         [13 Marks]

b) The Motorola 68HC11 and 6802 both utilise a CISC hardware architecture, whereas
   SUN’s SPARC microprocessors employ a RISC hardware architecture. Describe the
   differences between CISC and RISC, and put forward an argument for which hardware
   architecture you believe to be the best.
                                                                         [20 Marks]

                                          Page 5 of 5

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