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(a)What is an interrupt?[2 marks ]
An interrupt is an asynchronous event that
suspends the CPU from its normal operation (1)and jumps to a pre-programmed
routine (1).
(b)What is Memory Cycle Time?Explain why it is necessary to reduce
this,and give one way in which a reduction can be achieved.[4 marks
]
Memory cycle time refers to t e interval
between two successive memory accesses (1). From the time a r/w request
is made,the r/w function completed and the second access can commence
(1).
Because this time is large compared to CPU execution speed,so the
CPU may have to wait for data to arrive (1).
To interpose a higher speed memory ("cache")between CPU
and Main memory (1). Allow credit for alternative valid answers.
(c)What do you understand by the following terms:
(i)OSI Reference Model [2 marks ]
(ii)Statistical Time-Division Multiplexing (STDM)[2 marks ]
Two marks from the following,or other valid
points:
Provides a framework for defining standards for interconnecting heterogeneous
computer systems (1).Defines consistent language and boundaries for
establishing protocols (1).Seven layers,ranging from physical up to
application layers (1). Two marks from the following,or other valid
points:
Dynamically allocates time slots on demand,and thus does not send
empty slots (1). Takes advantage that attached devices are not all
transmitting all of the time (1). Therefore able to use lower data
rates to support many devices (1).
(d)The registers in the CPU may be divided into two groups,according
to the functions that they serve.Explain what these two functions
are.[4 marks ]
User Visible registers: (1)enable machine/assembly
language program instructions that allow for minimizing memory references
by optimizing use of registers (1). Control & Status registers:
(1)used by the CPU to control operation of CPU and tocontrol the execution
of program instructions (1).
(e)Briefly describe why many organizations are now using distributed
systems.
[3 marks ]
Allow credit for these or other valid answers:
There is a need to process data at specific locations but need to
communicate with other locations (1).Trade-off between communication
costs and computer costs indicates the feasibility of placing computers
at locations where data are collected/needed (1).
Permits sharing of work among sites and as the ability to use different
types of equipment (1).
(f)Describe three benefits of using subroutines.[3 marks ]
Subroutine allows the same piece of code
to be used many times on different data values and thus saves on memory
space (1).
Subroutines also allow large programming tasks to be divided into
smaller,manageable components (1).
Subroutines by their very nature make debugging easier and less time
consuming (1).
(g)Given that a computer system has a 40-bit wide instruction word
and there are 12 instructions in the instruction set;4 bits for the
opcode and 36 bits remaining for the o erands.Give the number of addressable
locations for a 3-address,2-address and a 1-address machine.Show your
workings.[3 marks ]
3-address:4 bits for opcode,12 bits for the
operands.Number of addressable locations: 4096
2-address:4 bits for opcode,18 bits for operand.Number of addressable
locations: 256K
1-address:4 bits for opcode,36 bits for operand.Number of addressable
locations:64M
Award (3)if the answer is entirely correct.Award (1)or (2)if some
answers are correct and others not.If all the answers are wrong,but
these are the result of a systematic error,award (1)or (2)as appropriate,for
the method.
(h)A large company has two office blocks in one large city.Select
a transmission medium suitable for creating a company LAN in that
city between the two buildings.Explain reasons for your choice.
Would your chosen medium be suitable if the offices were in different
cities,100km apart?Explain why/why not.[7 marks ]
One possibility is terrestrial microwave
(1).
Give marks for these points,subject to maximum.2 if explained well,1
if not.Life of sight is needed (2).Distance is limited by curvature
of the earth (2).Possible interference/attenuation due to atmospheric
conditions (2),lack of security (2).
This would probably not be suitable,due to both curvature of earth,and
attenuation (1). Also allow credit for other reasonable answers,if
they contain suitable material from the course.
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