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Computer Components, Security, Binary Arithmetic, and Process Scheduling

## Understanding Key Components of a Computer

Knowledge and Understanding

A1 describe the key components that make up a computer, and outline how some of them work;

A2 discuss the issues involved in sending data from one computer to another;

A3 discuss issues of computer security and apply different forms of security attack using very simple encryption algorithms;

Transferable Skills and other Attributes

B4 - express and manipulate numbers using a variety of bases and representations, particularly binary;

B5 - design and describe simple logic circuits.

1. For all parts of this question, you must show FULL working out in binary.

Choose a number between 50 and 90. This will be numberA. Choose another number between 30 and 45. This will be numberB. Using 2’s complement 8-bit binary arithmetic, calculate numberA – numberB giving your answer in binary and decimal. (5 marks)

Choose one number from the set { 9, 10, 11, 12, 13, 14, 15, 16 }  this is numberC. Choose a number from { 5, 9, 12 } this is numberD. Using 8-bit binary arithmetic, calculate numberC × numberD giving your answer in binary and decimal. (5 marks)

Using numberC and numberD from (b), convert the numbersNnumberC.125 numberD.5 into IEEE-754 format. Use binary arithmetic to calculate numberC.125 + numberD.5 giving your answer in binary and decimal. (5 marks)

Choose a number between 150 and 200 – this is numberE. Convert numberE into hexadecimal. Choose a number between 110 and 160 – this is numberF. Convert numberF into octal. Using 8-bit binary, calculate numberE AND number Give your answer in binary, octal, decimal and hexadecimal.

2. Choose a short phrase of between 20-25 characters. You could choose a website headline, song title, etc. Convert this phrase using Caesar shift encryption and a key of 5 to create your ciphertext.

Carry out a brute force attack to try to work out the decryption key and plaintext. (10 marks)

(Make sure you show all of your working out and state any assumptions you make. All of the marks are awarded for demonstrating an understanding of the general process of carrying out a brute force attack on Caesar encryption. This means that no marks will be awarded if your security attack is not brute force, even if you get the correct key and the correct plaintext – as you already know them.)

• Computer A sends 6 packets of data to computer B using Sliding Windows Flow Control
• The transmission time (time to put on the network) for a packet of data is 1 'time unit'
• Transmission time for an acknowledgement is 0 ‘time units’ (they are very small)
• The propagation time (time to travel through network) for any transmission is random (between 2 and 4 ‘time units’, you choose a random time for each packet and acknowledgement sent).
• B's packet processing time is 1. B cannot process multiple packets simultaneously.
• The initial window size is 3

Draw a diagram to show how flow will be controlled while the data is being sent.(10 marks)

3. A small computer using paged memory has enough RAM to store four1K pages. A process is using 5K of memory, and accesses the pages in the sequence:

After this there are 10 more page accesses, of the pages. You decide how the pages are accessed but make sure that each page is accessed twice, and the same page is not accessed consecutively. Remember there are 5 pages in total and each needs to be accessed twice, after the initial 4 page accesses. Calculate how many page faults will be generated. (10 marks if you use the Clock Algorithm) (5 marks if you use the LRU (Least Recently Used) Algorithm)

(For either algorithm, you must show your working out)

There are 6 processes A-F. Each of these has a runtime. For each of the processes choose a runtime from between 2 and 8 seconds. No two processes should have the same runtime.

A small single-processor computer is executing the set of processes:

Process A starts at time 0 seconds

Process B starts at time 1 second

Process C starts at time 3 seconds

Process D starts at time 5 seconds

Process E starts at time 7 seconds

Process F starts at time 8 seconds

Show the sequence of events if these processes are scheduled using the pre-emptive SRT (Shortest Remaining Time) algorithm.