Operating Systems Quiz
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Questions and Answers

What does memory management in an operating system determine?

  • Which instructions to execute in memory
  • What data is in memory and when (correct)
  • Optimizing programming language usage
  • How to access disk storage

    • What is the purpose of base and limit registers in memory management?

    To define the range of legal addresses that a process may access and ensure that the process can only access those legal addresses.

    Logical addresses and physical addresses are always the same in the execution-time address-binding scheme.

    False

    In the simplest dynamic relocation scheme, the relocation register adds a value to every ______ generated by a user process.

    <p>address</p> Signup and view all the answers

    Where can the operating system be placed in contiguous memory allocation?

    <p>Low memory</p> Signup and view all the answers

    What does the relocation-register scheme help the operating system do?

    <p>Change its size dynamically</p> Signup and view all the answers

    What are the memory allocation strategies discussed in the text?

    <p>All of the above</p> Signup and view all the answers

    What is external fragmentation in memory management?

    <p>Existence of non-contiguous free memory spaces</p> Signup and view all the answers

    How is the problem of internal fragmentation avoided in memory allocation?

    <p>By allocating memory in fixed-sized blocks</p> Signup and view all the answers

    What is the purpose of the high-order m-n bits in a logical address in the paging model of memory management?

    <p>Designate the page number</p> Signup and view all the answers

    Explain what the n low-order bits in a logical address designate in the paging model.

    <p>Designate the page offset</p> Signup and view all the answers

    What is the purpose of a translation look-aside buffer (TLB) in memory management?

    <p>To speed up address translation</p> Signup and view all the answers

    Paging eliminates external fragmentation.

    <p>True</p> Signup and view all the answers

    In a memory management scheme, if a TLB miss occurs, a memory reference to the ____________ must be made.

    <p>page table</p> Signup and view all the answers

    What happens when an attempt is made to write to a read-only page in memory?

    <p>Memory-protection violation occurs</p> Signup and view all the answers

    What additional bit is generally attached to each entry in the page table?

    <p>Valid-invalid bit</p> Signup and view all the answers

    Inverted Page Tables store one entry for each real page of memory.

    <p>True</p> Signup and view all the answers

    Shared memory is usually implemented using multiple virtual addresses mapped to one ________ address.

    <p>physical</p> Signup and view all the answers

    What is dynamic loading?

    <p>Dynamic loading is a technique in computer programming where a routine is not loaded until it is called.</p> Signup and view all the answers

    What is the advantage of dynamic loading?

    <p>Improves memory utilization</p> Signup and view all the answers

    What is a stub in dynamic linking?

    <p>A stub is a small piece of code that indicates how to locate the appropriate memory-resident library routine or how to load the library if the routine is not already present.</p> Signup and view all the answers

    Dynamic linking generally requires help from the operating system. True or False?

    <p>True</p> Signup and view all the answers

    What is roll out roll in in the context of swapping?

    <p>Roll out roll in is a swapping policy used for priority-based scheduling algorithms where a higher-priority process is swapped in while a lower-priority process is swapped out, and then swapped back in when needed.</p> Signup and view all the answers

    What is demand paging?

    <p>Demand paging is a memory management scheme where a process starts execution with no pages in memory and pages are only brought into memory when they are required.</p> Signup and view all the answers

    What are the components of hardware support for demand paging?

    <p>All of the above</p> Signup and view all the answers

    Effective access time is only affected by memory access time and is not influenced by page fault time.

    <p>False</p> Signup and view all the answers

    In demand paging, if a page fault occurs during instruction fetch, the instruction is fetched ____.

    <p>again</p> Signup and view all the answers

    What is segmentation in memory management?

    <p>Segmentation is a memory-management scheme that supports the user's view of memory by dividing programs into logical units called segments.</p> Signup and view all the answers

    What does a segment in memory management represent?

    <p>All of the above</p> Signup and view all the answers

    Segmentation architecture uses a segment table to map two-dimensional physical addresses.

    <p>True</p> Signup and view all the answers

    The segment table entry contains a base that specifies the starting physical address where the segment resides and a limit that specifies the ________ of the segment.

    <p>length</p> Signup and view all the answers

    Match the following memory management components to their descriptions:

    <p>Segmentation = Divides programs into logical units Paging = Divides address spaces into fixed-size pages for memory allocation</p> Signup and view all the answers

    What does LRU stand for in the context of page replacement algorithms?

    <p>Least Recently Used</p> Signup and view all the answers

    What is the key distinction between the FIFO and OPT algorithms in page replacement?

    <p>Both a and b</p> Signup and view all the answers

    LRU replacement algorithm looks forward in time for page replacements.

    <p>False</p> Signup and view all the answers

    LRU replacement algorithm associates with each page the time of that page’s last ___.

    <p>use</p> Signup and view all the answers

    What technique is typically used by operating systems to allocate pages?

    <p>Zero-fill-on-demand</p> Signup and view all the answers

    Vfork() suspends the child process while the parent process continues execution.

    <p>False</p> Signup and view all the answers

    What is an extremely efficient method of process creation where no copying of pages takes place?

    <p>vfork()</p> Signup and view all the answers

    Page replacement involves finding a ____ frame to use.

    <p>free</p> Signup and view all the answers

    Match the page replacement algorithm with its characteristics:

    <p>FIFO = Chooses the oldest page for replacement Optimal = Replaces the page that will not be used for the longest period of time</p> Signup and view all the answers

    Study Notes

    Operating Systems

    Memory Management

    • Memory management determines what is in memory and when, to optimize CPU utilization and computer response to users.
    • It involves:
      • Keeping track of which parts of memory are currently used and by whom.
      • Deciding which processes or parts thereof and data to move into and out of memory.
      • Allocating and deallocating memory space as needed.

    Memory Management Strategies

    • Protection of memory space is accomplished by having the CPU hardware compare every address generated in user mode with the base and limit registers.
    • Any attempt to access operating system memory or other users' memory results in a trap to the operating system.

    Address Binding

    • Addresses can be bound to memory addresses at:
      • Compile time: Absolute code is generated, and the process is bound to a specific memory location.
      • Load time: Relocatable code is generated, and binding is delayed until load time.
      • Execution time: Binding is delayed until run time, and special hardware is required to work.

    Logical and Physical Address Spaces

    • Logical addresses are generated by the CPU, and physical addresses are seen by the memory unit.
    • In execution-time address binding, logical and physical addresses differ.
    • The memory management unit (MMU) maps virtual addresses to physical addresses.

    Dynamic Relocation and Dynamic Loading

    • Dynamic relocation uses a relocation register to add a base value to every address generated by a user process.
    • Dynamic loading loads a routine only when it is called, to optimize memory usage.

    Dynamic Linking and Shared Libraries

    • Dynamic linking postpones linking until execution time, allowing multiple programs to share the same library code.
    • A stub is included in the program image, which checks if the needed routine is in memory, and loads it if necessary.

    Swapping

    • Swapping temporarily swaps a process out of memory to a backing store and brings it back into memory for continued execution.
    • Swapping is used in multiprogramming environments to optimize memory usage and CPU utilization.

    Swapping Implementation

    • Swapping requires a backing store, typically a fast disk.
    • The system maintains a ready queue of processes, and the dispatcher checks if the next process is in memory, swapping out a process if necessary.
    • Context-switch time in a swapping system is fairly high.### Memory Management
    • Context switching time is the time it takes to switch between processes, which includes swapping the process out of memory and swapping a new process in.
    • The swap time is affected by the transfer time, which is directly proportional to the amount of memory swapped.

    Swapping

    • Swapping is a method of memory management that involves temporarily transferring a process from main memory to a secondary storage (backing store) to free up space.
    • Swapping is a time-consuming process and can be a major bottleneck in system performance.
    • Swapping is necessary when the main memory is full and a new process needs to be loaded.

    Factors Affecting Swapping

    • Pending I/O operations can prevent a process from being swapped out.
    • I/O operations may be asynchronously accessing the user memory for I/O buffers, which can cause problems if the process is swapped out.
    • Solutions to this problem include never swapping a process with pending I/O or executing I/O operations only into operating-system buffers.

    Modified Swapping

    • Modified versions of swapping are used in many systems, including some versions of UNIX.
    • In these systems, swapping is normally disabled but will start if many processes are running and are using a threshold amount of memory.
    • Swapping is halted when the load on the system is reduced.

    Contiguous Memory Allocation

    • In contiguous memory allocation, each process is contained in a single contiguous section of memory.
    • The main memory is divided into two partitions: one for the resident operating system and one for the user processes.
    • The operating system is usually placed in low memory, and the interrupt vector is also located in low memory.

    Memory Mapping and Protection

    • The relocation-register scheme helps the operating system by providing an effective way to allow the operating system's size to change dynamically.
    • The operating system's size may change dynamically due to the addition or removal of code and buffer space for device drivers.

    Memory Allocation Strategies

    • Simplest method: Divide memory into several fixed-sized partitions, each of which may contain exactly one process.
    • Variable partition scheme: The operating system keeps a table indicating which parts of memory are available and which are occupied.
    • Strategies for selecting a free hole: first fit, best fit, and worst fit.

    Fragmentation

    • External fragmentation: The problem of having enough total memory space to satisfy a request but the available spaces are not contiguous.
    • Internal fragmentation: The problem of having a block of memory allocated to a process that is larger than the requested memory.
    • Compaction: A solution to external fragmentation that involves shuffling the memory contents to place all free memory together in one large block.

    Paging

    • Paging is a method of memory management that allows the physical address space of a process to be noncontiguous.
    • Physical memory is divided into fixed-sized blocks called frames, and logical memory is divided into blocks of the same size called pages.
    • The operating system keeps track of all free frames and sets up a page table to translate logical addresses to physical addresses.

    Address Translation Scheme

    • The address generated by the CPU is divided into a page number and a page offset.
    • The page number is used as an index into a page table, which contains the base address of each page in physical memory.
    • The page offset is combined with the base address to define the physical memory address.

    Internal Fragmentation

    • Internal fragmentation occurs when a process is allocated a frame that is larger than the requested memory.

    • The average internal fragmentation is one-half page per process.### Operating System Memory Management

    • The operating system must be aware of user processes operating in user space and map logical addresses to produce physical addresses.

    Memory Management - Paging

    • Paging increases context-switch time because the operating system maintains a copy of the page table for each process.
    • The page table is used to translate logical addresses to physical addresses.
    • The hardware implementation of the page table can be done in several ways, such as using dedicated registers or a page table base register (PTBR).

    Page Table Implementation

    • The page table can be stored in fast registers or in main memory, with a PTBR pointing to the page table.
    • The page table can be very large, and the use of fast registers is not feasible for large page tables.

    Translation Look-aside Buffer (TLB)

    • The TLB is a small, fast, associative memory cache that stores recently used page-table entries.
    • Each TLB entry consists of a key (or tag) and a value.
    • When a logical address is generated, its page number is presented to the TLB, and if the page number is found, its frame number is immediately available.
    • The TLB is used to speed up the page-table lookup process.

    TLB Usage

    • If the page number is found in the TLB, the frame number is used to access memory.
    • If the page number is not in the TLB (a TLB miss), a memory reference to the page table must be made.
    • When the frame number is obtained, the page number and frame number are added to the TLB.

    Memory Protection

    • Memory protection in a paged environment is accomplished by protection bits associated with each frame.
    • The protection bits are kept in the page table and are checked to verify that no writes are being made to a read-only page.
    • An attempt to write to a read-only page causes a hardware trap to the operating system.

    Valid-Invalid Bit

    • A valid-invalid bit is attached to each entry in the page table to indicate whether the associated page is in the process's logical address space.
    • Illegal addresses are trapped using the valid-invalid bit.

    Shared Pages

    • An advantage of paging is the possibility of sharing common code.
    • Shared code is non-self-modifying code that can be executed by multiple processes at the same time.
    • The operating system should enforce the read-only nature of shared code.

    Hierarchical Paging

    • Hierarchical paging is a solution to the problem of large page tables.
    • The page table is divided into smaller pieces, using a two-level paging algorithm.
    • The page number is further divided into a page number and a page offset.

    Hashed Page Tables

    • Hashed page tables are used to handle address spaces larger than 32 bits.
    • The virtual page number is hashed into the hash table.
    • Each entry in the hash table contains a linked list of elements that hash to the same location.

    Inverted Page Tables

    • Inverted page tables have one entry for each real page (or frame) of memory.
    • Each entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns the page.
    • The inverted page table requires an address-space identifier to be stored in each entry.

    Inverted Page Table Operation

    • When a memory reference occurs, the virtual address is presented to the memory subsystem.
    • The inverted page table is searched for a match, and if found, the physical address is generated.
    • If no match is found, an illegal address access has been attempted.

    Drawbacks of Inverted Page Tables

    • Inverted page tables have difficulty implementing shared memory.
    • The standard method of implementing shared memory cannot be used with inverted page tables.
    • A simple technique for addressing this issue is to allow the page table to contain only one mapping of a virtual address to the shared physical address.

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    Description

    This quiz assesses knowledge of operating systems concepts, covering topics from textbooks by Abraham Silberschatz and William Stallings. It's ideal for computer science students.

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