Cs 101 ( Module 51-60)

Operating System Components

• The kernel is the internal part of an operating system that performs basic functions required by the computer installation.
• The kernel contains software components such as:
  • File manager: coordinates the use of mass storage facilities, maintains records of files stored, and allows files to be grouped into directories.
  • Device drivers: communicate with controllers or peripheral devices to carry out operations, and translate generic requests into technical steps.
  • Memory manager: coordinates the machine's use of main memory, assigns memory space, and ensures that each program's actions are restricted to its allotted space.

Process of Booting

• The boot strap process (booting) is performed by a computer each time it is turned on, transferring the operating system from mass storage to main memory.
• The CPU expects to find the beginning of the program to be executed at a predetermined address, but the main memory is volatile, losing its contents when the computer is turned off.
• To resolve this, a small portion of main memory is constructed from nonvolatile memory cells (ROM), which stores a program called the boot loader.
• The boot loader directs the CPU to transfer the operating system from mass storage to main memory, and then executes a jump instruction to that area of memory.

Process and its Administration

• A program is a static set of instructions, while a process is the activity of executing a program under the control of the operating system.
• Associated with a process is its current status, called the process state, which includes the value of the program counter, values in CPU registers, and values in associated memory cells.
• The scheduler and dispatcher within the operating system's kernel coordinate the execution of processes, maintaining a record of processes, introducing new processes, and removing completed processes.
• The scheduler maintains a process table, containing information such as memory area assigned, priority, and whether the process is ready or waiting.

Multiprogramming

• In a time-sharing/multitasking system, the dispatcher oversees the execution of scheduled processes by dividing time into short segments (time slices) and switching the CPU's attention among processes.
• The procedure of changing from one process to another is called a process switch (or context switch).
• When the dispatcher awards a time slice to a process, it initiates a timer circuit that generates an interrupt signal when the slice is completed.

Handling Competition between Processes

• An important task of an operating system is the allocation of machine resources to processes, including peripheral devices and features within the machine.
• Resource allocation appears simple but can lead to malfunctions in a poorly designed system.
• Issues arise when two processes demand the same resource at the same time, or when a process is utilizing one resource and waiting for another, leading to a deadlock.

Semaphores

• Semaphores are used to control access to shared resources, such as a printer, in a time-sharing/multitasking operating system.
• A flag is used to keep track of whether the printer is allocated, with a clear flag indicating availability and a set flag indicating allocation.
• However, this simple flag system has a problem, as the task of testing and setting the flag may be interrupted, leading to multiple processes accessing the same resource.### Mutual Exclusion and Semaphores
• A semaphore is a flag that prevents multiple processes from accessing a critical region at the same time
• A critical region is a sequence of instructions that should be executed by only one process at a time
• Mutual exclusion is the requirement that only one process at a time be allowed to execute a critical region
• A semaphore is used to guard a critical region, ensuring that only one process can enter the region at a time
• A process must find the semaphore clear and set it before entering the critical region, then clear the semaphore when exiting

Deadlock

• Deadlock is a condition in which two or more processes are blocked from progressing because each is waiting for a resource that is allocated to another
• Three conditions must be satisfied for deadlock to occur:
  • Competition for non-sharable resources
  • Resources are requested on a partial basis
  • Once a resource has been allocated, it cannot be forcibly retrieved
• Deadlock can be removed by attacking any one of the three conditions
• Techniques that attack the third condition are known as deadlock detection and correction schemes
• Techniques that attack the first two conditions are known as deadlock avoidance schemes

Security

Accounts and Access Control

• An account is a record within the operating system containing user information, such as name, password, and privileges
• The operating system uses account information to control access to the system during the login procedure
• The administrator (or super user) has highly privileged access to the operating system and can perform maintenance activities

Auditing and Security

• Auditing software records and analyzes activities within the computer system to detect destructive behavior
• Auditing software can expose attempts to login using incorrect passwords and identify activities that do not conform to a user's past behavior
• Sniffing software is a type of software that records activities and reports them to a would-be intruder
• Carelessness of users is a major obstacle to computer security, including selecting easily guessed passwords and sharing passwords with others

Security Threats from Inside

• Once an intruder gains access to a computer system, they can explore and look for places to insert destructive software
• Gaining access to the administrator's account provides the highest level of access and control
• If access is gained through a general user's account, the intruder must trick the operating system into allowing access beyond the user's privileges