Explain Lexical Analysis
- Breaks code into tokens: numbers, variables, names, strings, delimiters etc.
- Tokens are classified by type into classes and comments are removed
- Production of non-terminals can be described by BNF grammar, but in lexing we use it to form tokens without syntactical meaning e.g. , ,
What is a challenge faced by a parser during parsing? And how can this be solved?
A challenge is choosing which grammar rules to apply, which one to use first, and which set of leaves to apply them to. This can be solved by backing up the point where the last choice was made and trying a different choice instead.
What is recursion?
A non-terminal on the left reappears in the definition part on the right
::= blah|blah
What elements would you find in a semantic record?
- Data type of the of the non-terminal and the name of the variable
- Data type of the non-terminal symbol the record relates to
What is the role of code optimisation during the compilation process?
Tries to turn code into code that will run faster and removes unnecessary instructions at machine level
What does ‘rwx’ command mean?
Read, write, execute
What is a ‘chmod’ command?
Changes the access permission of files and folders.
The first number sets permission for the user, the second number sets permission for the group and the third number sets permission for everybody else
What does ‘chmod 777’ do?
Gives everyone full access to the file
What does ‘chmod 770’ do?
The user and group members only have full access to the file
What does ‘chmod 755’ do?
The user has full access, group members and everybody else can read and execute the file, no modifications
What is a deadlock?
When two threads needs two different resources and each of them has the lock of the resource that the other need, it is a deadlock
What are the features of multi-programmed operating system?
- Each process needs its own memory space which need to be separate to avoid programs writing into the memory space of another
- Manages program access to external devices and resources e.g. keyboard, mouse, files, networks and printers
What are the features of multi-user operating system?
- A superuser(root) account for administration
- User authentication
What is cloud storage?
In a cloud system, many network drives are connected transparently across many physical locations world-wide, and a user’s data may exist in more than one location, getting synchronized to other locations if it changes in one.
What are potential problems of cloud storage?
- Legitimate access to cloud data by law enforcement
- Privacy of data stored under different legislation
Explain geostationary satellites
- High latency
- Orbits at the same angular velocity as the earth’s spin so it appears to be in a fixed spot as seen from ground stations
- Low signals at ground stations, need big antennas
Explain Medium Earth Orbit satellites (MEO)
- Need tracking antennas (movable), but can be much smaller than geostationary
- Need a minimum of 2 antennas per ground station
- Need a minimum of 4 satellites
- Lower latency and higher bit rates
Explain Low Earth Orbit satellites (LEO)
• No need for tracking antennas
- Need dozens to hundreds or even thousands of small satellites
- Even lower latencies and much higher bit rates
What is phishing?
A “social engineering” approach, typically via email designed to cause you to reveal login data or allow a malicious party to install software (malware) on your machine for further malicious activity. The installation could be by persuading you to actively install the software or by clicking on a link that will perform a “drive by download’’ of the malware
What is a virus?
Executable files that will create copies of themselves in legitimate programs and wait for these to be copied to other machines.
What are worms?
Use network functionality e.g. email or faulty network applications to spread to other machines
What are trojans?
Programs that perform a seemingly useful task but also open a back door for malicious hackers, in the form of a TCP port that can be accessed by a command and control server to issue commands to the program. Gives access to someone’s computer.
What are the features of RSA?
- The first part of the public key is the product of two large numbers
- Messages encrypted with the public key can be used to send confidential information to the owner
- Messages encrypted with the private key can be used to authenticate the owner as the sender
- Encrypting with plaintext with public keys requires using modulo arithmetic
What are the features of DES?
- Complex cipher that uses a simple algorithm repeatedly
- Uses permutation (puts bits into different locations), splitting the data, exclusive OR (XOR) operations, substitution and a secret key that is permuted and partially used at each stage to perform encryption.
- The decryption performs all stages in reverse
- Easily implemented and fast to compute
- Can be cracked using brute force
