Computer System Structure and Operating Systems

Computer System Structure

• A computer system consists of hardware, an operating system, application programs, and users.
• Hardware provides basic computing resources such as the CPU, memory, and I/O devices.
• The operating system controls and coordinates the use of hardware among applications and users.
• Application programs define how system resources are used to solve user problems.
• Users can be people, machines, or even other computers.

What is an Operating System?

• An operating system manages the computer's hardware.
• It provides a foundation for application programs and acts as an intermediary between the user and the hardware.
• Operating system goals include executing user programs, facilitating problem-solving, making the system user-friendly, and efficiently using hardware.

What Operating Systems Do?

• User View: Users desire convenience, ease of use, and good performance. They are less concerned with resource utilization.
• System View: The operating system is the program closest to the hardware.
• The operating system is a resource allocator, managing resources like CPU time, memory space, file storage, I/O devices, and making decisions on conflicting requests for efficient and fair use.
• The operating system is a control program, controlling program execution to prevent errors and misuse of the computer.
• The core program running at all times on the computer is called the kernel.

Computer System Organization

• A modern general-purpose computer system comprises one or more CPUs and multiple device controllers interconnected through a common bus that provides access to shared memory.
• The bootstrap program is loaded at power-up or reboot, usually stored in ROM or EPROM (firmware). It initializes the system and loads the operating system kernel for execution.
• Events are signaled through interrupts, either from hardware or software.
• Hardware interrupts can occur at any time, while software interrupts (system calls) are executed by the program.
• Interrupts transfer control to interrupt service routines, usually via an interrupt vector that contains the addresses of all service routines.
• Traps or exceptions are software-generated interrupts caused by errors or user requests.
• Operating systems are interrupt-driven.

Storage Structure

• The basic unit of computer storage is the bit, representing either 0 or 1.
• A byte comprises 8 bits and is the smallest unit of storage on most computers.
• A word is the native unit of data in a computer architecture, composed of one or more bytes.
• Computer storage is typically measured and manipulated in bytes and collections of bytes (KB, MB, GB, TB, PB).
• Main memory, also known as RAM, is the only large storage media directly accessible by the CPU. It is volatile and provides random access.
• Secondary storage, such as hard disks and SSDs, extends main memory by providing a larger, non-volatile storage capacity.
• Hard disks consist of magnetic recording material on rigid platters, logically divided into tracks and sectors.
• SSDs are solid-state drives that offer faster speeds, non-volatility, and various technologies, making them increasingly popular.
• Storage systems are organized in a hierarchy based on speed, cost, and volatility.

Multiprocessing Architecture

• Multiprocessor systems, also called parallel systems, are growing in use and importance.
• Benefits include increased throughput, economy of scale, and increased reliability (graceful degradation or fault tolerance).
• Two types of multiprocessing systems: Asymmetric multiprocessing assigns specific tasks to each processor, while Symmetric multiprocessing allows each processor to perform all tasks.

Operating-System Operations

• Operating systems are interrupt-driven.
• Hardware interrupts are generated by devices, while software interrupts arise from software errors, requests for system services, or process problems like infinite loops or incorrect modifications.
• Dual-mode operation (user mode and kernel mode) is essential for system protection.
• User mode allows system and application software to run, while kernel mode (supervisor mode) provides privileged access for system operations.
• A mode bit distinguishes between kernel (0) and user (1) modes, ensuring system protection.
• System calls change to kernel mode, and the return from the call resets to user mode.

Protection and Security

• Operating systems must have tools for security and protection.
• Protection refers to mechanisms controlling process or user access to system resources.
• Security focuses on defending the system against internal and external attacks, including denial-of-service, worms, and viruses.

Computing Environments

• There is a trend toward providing more ways to access computing environments.
• Companies establish portals for web-based access to internal servers.
• Network computers (thin clients) are used in place of traditional workstations for situations requiring enhanced security or easier maintenance.
• Mobile computers interconnect using wireless networks.