Computer Fundamentals and System Concepts

Multitasking is the OS capability to run multiple programs simultaneously, managing CPU time and resources among them. Multiuser refers to multiple users, not multiple programs.

Option (3) is correct.

A node is a physical element that exists at run time and represents a computational resource, generally having at least some memory and often processing capability.

A distributed system is defined as a collection of independent computers that appears to its users as a single, coherent computer system. The key characteristic is that the multiple machines work together transparently, so users perceive them as one unified system rather than separate entities.

The distributed systems are physically separated computers, which are combined together through a network so that they can communicate and share computational tasks and provide a rich set of features to the users.

A real-time system is one that must respond to inputs within strict time constraints - missing a deadline can cause system failure. Aircraft control systems are classic examples because they must process sensor data and adjust controls within milliseconds to maintain flight safety. Payroll processing and library systems have no critical timing requirements.

Overlay allows running large programs that exceed available memory by swapping program parts in and out of the same memory region. Only the currently executing portion needs to be in memory at any time. This extends effective memory without requiring physical RAM for the entire program.

Trojan horses disguise themselves as legitimate or useful software to trick users into installing them. Once executed, they provide unauthorized access or perform malicious actions. Unlike viruses, they don't self-replicate but rely on social engineering.

An access matrix is a security model that defines what subjects (users, processes) can perform what operations on objects (files, resources). Rows represent subjects, columns represent objects, and each cell contains access rights. This model forms the basis for implementing access control policies.

Computer viruses are malicious programs that replicate by inserting copies into other programs or documents. They can cause damage by modifying or destroying files, crashing systems, and disrupting normal operations. Unlike worms, viruses typically require user action to spread.

Trojan horse programs are often designed to steal sensitive information like passwords by appearing legitimate. They can capture keystrokes, read password files, or trick users into entering credentials. The stolen passwords are then transmitted to attackers for unauthorized access.

Computer worms are standalone malicious programs that replicate themselves to spread to other computers across networks. Unlike viruses, they don't need to attach to host programs - they can run independently and exploit vulnerabilities to propagate. Worms can consume bandwidth and disrupt system operations.

Protection rings were first introduced in the MULTICS operating system, developed in the 1960s. MULTICS pioneered many security concepts including ring-based protection. This hierarchical privilege model was later adopted in other systems including the x86 architecture's ring levels (0-3).