Operating System Concepts: Computer Startup

Computer System Organization

• A computer system consists of one or more CPUs, device controllers, and a common bus that provides access to shared memory.
• The system operates by executing CPUs and devices concurrently, competing for memory cycles.

Bootstrap Program

• The bootstrap program is loaded at power-up or reboot and is typically stored in ROM or EPROM (firmware).
• It initializes all aspects of the system and loads the operating system kernel, starting its execution.

Interrupt Handling

• Interrupts transfer control to the interrupt service routine through the interrupt vector, which contains the addresses of all service routines.
• The interrupt architecture must save the address of the interrupted instruction to prevent lost interrupts.
• An operating system is interrupt-driven, and traps are software-generated interrupts caused by errors or user requests.

I/O Structure

• After I/O starts, control returns to the user program only upon I/O completion or without waiting for I/O completion.
• The system uses a wait instruction to idle the CPU until the next interrupt or a system call to request the operating system to allow the user to wait for I/O completion.

Direct Memory Access (DMA) Structure

• DMA is used for high-speed I/O devices that can transmit information at close to memory speeds.
• The device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention.

Storage Hierarchy

• The storage system is organized in a hierarchy based on speed, cost, and volatility.
• Caching is the process of copying information into a faster storage system, and main memory can be viewed as a cache for secondary storage.

Process Management

• A process is an active entity that needs resources to accomplish its task, and it can be single-threaded or multi-threaded.
• Process management involves creating, deleting, suspending, and resuming processes, as well as providing mechanisms for process synchronization, communication, and deadlock handling.

Memory Management

• Memory management determines what is in memory when, optimizing CPU utilization and computer response to users.
• It involves keeping track of which parts of memory are currently being used and by whom, and deciding which processes and data to move into and out of memory.

Storage Management

• Storage management provides a uniform, logical view of information storage, abstracting physical properties to logical storage units (files).
• File-system management involves creating and deleting files and directories, primitives to manipulate files and directories, mapping files onto secondary storage, and backing up files onto stable storage media.

Mass-Storage Management

• Mass-storage management involves free-space management, storage allocation, and disk scheduling, and is crucial for the speed of computer operation.
• Tertiary storage includes optical storage and magnetic tape, which must also be managed.

Performance of Various Levels of Storage

• The movement of data between levels of the storage hierarchy can be explicit or implicit, and multitasking environments must be careful to use the most recent value.

I/O Subsystem

• The I/O subsystem is responsible for memory management of I/O, including buffering, caching, and spooling, as well as providing a general device-driver interface and drivers for specific hardware devices.

Protection and Security

• Protection involves controlling access of processes or users to resources defined by the OS, and security involves defending the system against internal and external attacks.
• Systems distinguish among users, using user IDs, group IDs, and privilege escalation to determine access control.

Computing Environments

• Traditional computer environments have evolved, with the blurring of lines between office and home environments.

• Client-server computing involves smart PCs as clients, requesting services from servers, and has become a dominant computing model.### Multiprocessing Systems

• Advantages: increased throughput, economy of scale, and increased reliability through graceful degradation or fault tolerance.

• Two types: asymmetric multiprocessing and symmetric multiprocessing.

Symmetric Multiprocessing Architecture

• A dual-core design allows multiple processors to share the same memory and bus.

Clustered Systems

• Like multiprocessor systems, but multiple systems working together.
• Usually sharing storage via a storage-area network (SAN).
• Provides a high-availability service which survives failures.
• Two types: asymmetric clustering (one machine in hot-standby mode) and symmetric clustering (multiple nodes running applications, monitoring each other).
• Some clusters are for high-performance computing (HPC), requiring applications to be written to use parallelization.

Operating System Structure

• Multiprogramming needed for efficiency, as a single user cannot keep CPU and I/O devices busy at all times.
• Multiprogramming organizes jobs (code and data) so CPU always has one to execute.
• A subset of total jobs in system is kept in memory.
• One job selected and run via job scheduling.
• When it has to wait (for I/O for example), OS switches to another job.
• Timesharing (multitasking) is a logical extension, allowing users to interact with each job while it is running, creating interactive computing.
• Response time should be less than 1 second.
• Each user has at least one program executing in memory (process).
• CPU scheduling is used when several jobs are ready to run at the same time.
• Swapping moves processes in and out of memory to run.
• Virtual memory allows execution of processes not completely in memory.

Operating-System Operations

• Interrupt driven by hardware.
• Software error or request creates exception or trap (e.g. division by zero, request for operating system service).
• Other process problems include infinite loop, processes modifying each other or the operating system.
• Dual-mode operation allows OS to protect itself and other system components.
• User mode and kernel mode are distinguished using a mode bit provided by hardware.
• Some instructions are designated as privileged, only executable in kernel mode.
• System call changes mode to kernel, and return from call resets it to user.

Transition from User to Kernel Mode

• Timer is used to prevent infinite loop or process hogging resources.
• Set interrupt after a specific period, and operating system decrements counter.
• When counter is zero, generate an interrupt, and set up before scheduling process to regain control or terminate program that exceeds allotted time.

Process Management

• A process is a program in execution.