Introduction to Operating Systems

Question 1

What are the two main functions of an OS?

Answer 1

• Interact with the hardware
• Interact with the user

Question 2

Why are instructions related to I/O devices typically privileged instructions? (can be executed in kernel mode but not in user mode)

Answer 2

Restriction is in place for security and protection purposes

Question 3

What is the difference between Kernel and User Mode?

Answer 3

• Kernel mode, the CPU has unrestricted access to system resources and can execute privileged instructions
• User mode, access is restricted and certain operations are prohibited

Question 4

How does having two distinct modes aid in designing an OS?

Answer 4

• Preventing user programs from interfering with system-level operations
• Accidentally or maliciously causing system crashes or other errors)

Question 5

What is the purpose of system call in an OS?

Answer 5

System call provides the services of the operating system to the user programs via the Application Program Interface (API)

Question 6

What is an Open System Call, its Syntax and Conditions that cause them to fail

Answer 6

• Initialises access to a file in a file system, also specifies the mode in which the file should be opened (flags) (e.g., read, write, append)
• May fail due to various factors, file not found, permission denied, invalid pathname
• int open(const char *pathname, int flags, mode_t mode);

Question 7

What is an Close System Call, its Syntax and Conditions that cause them to fail

Answer 7

• Close an open file in an operating system
• May fail due to invalid file descriptor, interrupted by a signal, I/O error
• int close(int fd);

Question 8

What is an Read System Call, its Syntax and Conditions that cause them to fail

Answer 8

• Read data from a file or other input source into a buffer in user memory
• May fail due to invalid file descriptor, permission denied, invalid buffer
• ssize_t read(int fd, void *buf, size_t count);

Question 9

What is an Lseek System Call, its Syntax and Conditions that cause them to fail

Answer 9

• lseek is used to reposition the read/write file offset of an open file descriptor
• May fail due to invalid file descriptor, unsupported device, invalid whence parameter, out-of-bounds offset
• off_t lseek(int fd, off_t offset, int whence);

Question 10

Virtual Machine features

Answer 10

• Isolation, operates independently
• Resource allocation and virtualisation, share the physical hardware of the host
• Platform independence, can emulate a different operating system than the host machine
• Security, provide an isolated environment that can be used to run potentially unsafe applications

Question 11

Virtual Machine downsides

Answer 11

• Performance overhead, introduce additional layers between the hardware and the operating system
• Resource contention, share the physical resources of the host machine, degrade performance
• Limited hardware access, require direct hardware access, may face limitations or degraded performance
• Compatibility issues can limit usability for certain use cases