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Questions and Answers
What is the primary goal of multiprogramming?
What is the purpose of swapping in a multiprogrammed system?
What allows a system to execute processes not completely in memory?
What is the term for a logical extension of multiprogramming where CPU switches jobs frequently?
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What is the purpose of a timer in process management?
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What is the term for a program in execution?
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In a multiprogrammed system, what is stored in memory at a time?
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What is the purpose of the mode bit in dual-mode operation?
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What is the result when a system call changes mode to kernel?
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What is the primary benefit of symmetric multiprocessing?
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Study Notes
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
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Traditional computer environments have evolved, with the blurring of lines between office and home environments.
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Client-server computing involves smart PCs as clients, requesting services from servers, and has become a dominant computing model.### Multiprocessing Systems
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Advantages: increased throughput, economy of scale, and increased reliability through graceful degradation or fault tolerance.
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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.
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