August 1997
CS202: COMPUTER ARCHITECTURE AND DATA COMMUNICATION

QUESTION 1 (Compulsory)

Total Marks: 20 Marks

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SUGGESTED SOLUTIONS
Solutions and allocated marks are indicated in green.
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1. (a) What is meant by the following terms?
(i) Register addressing [2]
When machine instruction has operand field which contains a reference to a register, [1]
the contents of which are used as an effective address of the actual data. [1]
(ii) Implied addressing [2]
When machine instruction has no operand field; [1]
the data is implied in the opcode. [1]
(iii) Fetch cycle [2]
When a machine instruction is retrieved from memory [1]
into the instruction register. [1]
(iv) Execute cycle [2]
When contents of instructions register is interpreted by control circuitry to determine operation [1]
and operands. [1]
(v) I/O direct memory access (DMA) [2]
When I/O processor is able to transfer data directly to/from memory [1]
without involvement of CPU. [1]
(b) (i) Explain, with the aid of diagram, the role of a multiplexer in data communications. [3]
Multiplexer converts several low-speed signals from different devices [1]
and transmits the combined signal over a high-speed line, which is cheaper than transmitting over several low-speed lines. [1]
Another multiplexer (demultiplexer) redistributes the signals. [1]
A diagram such as that on Page 10-2 of the study guide is an acceptable way of presenting this answer; one mark should be allocated to each point stated above, whether it is illustrated through an annotation or label in the diagram or explained purely in words.
(ii) What are the differences between time-division multiplexing (TDM) and frequency-division multiplexing (FDM)? What are the disadvantages of each? [5]
In FDM the channel is divided into narrow bands of different frequencies (sub-channels) each for separate transmission. [1]
In TDM the channel is subdivided by allocating time intervals for the different signals. [1]
Max 3 marks from the following or similar: TDM is suitable only for digital signals. [1]
FDM has a fixed number of bands - once they are used up, no further sub-channels can be added. [1]
TDM requires a modem; FDM does not. [1]
In TDM if a terminal is not transmitting, there is still a time slot assigned to it on the high-speed channel. [1]
(iii) How does statistical time-division multiplexing (STDM) improve on TDM? What additional resource does STDM have? [2]
In STDM the time-slots are allocated on demand, and so empty slots are not sent. [1]
It requires address information and buffers to store data temporarily during peak loads. [1]