DayOfWeek DayOfMonth Month 2XXX XX.XX am/pm – XX.XX am/pm
(Duration: X hours XX minutes)
EXAMINATION FOR
MSc & POSTGRADUATE DIPLOMA IN INFORMATION TECHNOLOGY or
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DEGREES OF MSci, MEng, BEng, BSc, MA and MA (Social Sciences)
System and Networks
Answer ALL Questions
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INSTRUCTIONS TO INVIGILATORS
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Express the following in 8-bit two’s complement code, giving your answers in hexadecimal.
(b) Give the range of values that can be represented by a 16-bit two’s complement code (you may express the range using powers of 2). Given an 8-bit two’s complement codeword, how would you convert it into the 16-bit two’s complement codeword representing the same number.
(c) Find two 8-bit codewords, X and Y which represent numbers such that X>Y if a two’s complement code is in use but X
Rd:=Ra logic shifted left Rb places Rd:=Ra logic shifted right Rb places If Rd=0 then PC:=x+Ra
If Rd<>0 then PC:=x+Ra
Rd:= pc, pc: =x +Ra
PC:= interrupt handler
PC:= x +Ra
CONTINUED OVERLEAF
What is meant by the memory hierarchy in a computer? Explain what cache memory is for, how it works and how it fits into such a hierarchy.
(b) The following Sigma 16 code is intended to take a 6-element array (only first element is shown) and double all the elements. However, although the code will assemble, it contains several errors. Identify these errors and explain how you would correct them. Write out the corrected program.
LOAD R1,1[R0] ADD R2,R0,R0 LOAD R3,n[R0]
;Set R1 to constant 1 ;i:=0
;Set R3 to n.
;if yes, exit ;load x[i] ;x[i]:=2*x[i] ;i:=i+1 ;store x[i] ;loop
FORLOOP CMPGT
JUMPF R5,OUT[R0]
LOAD R1,X[R0]
ADD R1,R1,R1 ADD R2,R2,R1 STORE R3,X[R2] JUMP FORLOOP[R0]
; Data Area
n DATA X DATA
(c) Estimate how many cycles the corrected program would take to run
(d) In the corrected program estimate the advantage a system with a cache memory would gain if a primary memory cycle takes 10ns and a cache cycle 1ns.
CONTINUED OVERLEAF Page 3
Explain what is meant by packet switching? Give two reasons why most packet implementations limit packet size?
(b) Describe the Internet Protocol, IP, and explain how IP routers forward arriving packets.
(c) Routing is said to be adaptive if the same route can take different forwarding decisions can be taken at different times, for packets going to the same destination. Suggest why might this be a desirable feature in IP networks and how it could be implemented. Give one possible disadvantage of an adaptive routing system.
(d) IP packets crossing the Internet can experience bit errors. However, IP itself can only detect errors in its headers. Why are header errors considered more serious by IP? How are errors in the data carried by IP packets detected and rectified in the Internet communication system?
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