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What is the primary function of paging in operating systems?
Why must the page size be equal to the frame size in a paging system?
What happens to unused pages in a paging system?
How is the main memory organized in a paging system?
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How many frames will be available in a main memory of size 16KB with a frame size of 1KB?
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What occurs when a process is in a waiting state in the context of paging?
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Which statement correctly describes frames in a paging system?
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What size are initial pages of processes that are 4KB each when divided in a memory with a frame size of 1KB?
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What is one primary advantage of paging in memory management?
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Which statement accurately describes segmentation?
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What is stored in the first column of the segment table?
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What does a valid logical address consist of in a segmented memory model?
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Which memory management technique assumes that processes can be stored in different noncontiguous memory areas?
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Which of the following statements about segment tables is true?
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Which of the following is a disadvantage of using paging?
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What happens if the offset in a logical address exceeds the limit in segmentation?
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What is the main characteristic of a fixed-size partition scheme in memory allocation?
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Which type of fragmentation occurs when a partition is larger than the process being loaded?
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What is external fragmentation in memory allocation?
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Which of the following is an advantage of fixed-size partitioning?
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What limitation is associated with the size of processes in a fixed-size partitioning system?
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Which of the following describes an impact on multiprogramming due to fixed-size partitioning?
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What does dynamic partitioning aim to reduce in memory allocation schemes?
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Which scenario illustrates external fragmentation?
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What is a significant advantage of the dynamic partitioning scheme?
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Which of the following is a disadvantage of the dynamic partitioning scheme?
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In dynamic partitioning, what happens when a process is completed and space is left?
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What is the primary difference between paging and dynamic partitioning?
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What aspect of dynamic partitioning makes its implementation challenging?
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Which of the following is true about non-contiguous memory allocation?
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Why does dynamic partitioning lead to a higher degree of multiprogramming?
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What is one of the primary characteristics of paging in memory management?
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What is a significant advantage of segmentation compared to paging?
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What is the primary role of the memory management unit (MMU) in virtual memory systems?
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Which of the following is a disadvantage of segmentation?
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When a page fault occurs, what is the first step in the demand paging process?
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What is a characteristic of external fragmentation in segmentation?
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What does the OS do with a process when a page fault occurs?
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In the context of virtual memory, what are page files or swap files used for?
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How is the page table affected during the demand paging process?
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Study Notes
Paging
- A memory management technique where processes are divided into fixed-size units called pages.
- Main memory is also divided into fixed-size units called frames.
- One page is stored in one frame.
- Pages are only loaded into memory when needed, otherwise they reside in secondary storage.
- Frame sizes are defined by the operating system.
- Page size must be equal to frame size for efficient mapping.
- Example:
- System with 16KB memory, frame size of 1KB, resulting in 16 frames.
- Four 4KB processes (P1, P2, P3, P4) each divided into 1KB pages.
- Initially, all frames are empty.
- Processes P2 and P4 are moved to the waiting state, freeing 8 frames.
- A new process P5 (8KB, 8 pages) waiting in the ready queue can be loaded into the available frames.
Advantages of Paging
- Reduces external fragmentation.
- Simple to implement.
- Efficient due to the equal page and frame sizes.
- Makes swapping processes easier.
Disadvantages of Paging
- Requires extra memory for the page table.
- Not suitable for systems with small RAM.
Segmentation
- Memory management technique where memory is divided into variable-sized units called segments.
- Segments can be allocated to processes, and do not need to be contiguous in memory.
- Segmentation doesn't create internal fragmentation but may create external fragmentation.
- Example:
- Process divided into code, data, and stack segments of different sizes.
- Each segment can be stored anywhere in memory.
Segment Characteristics
- Variable sized partitioning scheme.
- Secondary memory and main memory are divided into partitions of unequal sizes.
- Partition sizes depend on the length of modules.
- Partitions in secondary memory are called segments.
Segment Table
- Stores information about each segment of a process.
- Two columns:
- Size or length of the segment.
- Base address or starting address of the segment in main memory.
- Stored as a separate segment in main memory.
- The Segment Table Base Register (STBR) stores the base address of the segment table.
- Limit indicates the length or size of the segment.
- Base indicates the base address or starting address of the segment in main memory.
Translating Logical Address into Physical Address
- CPU-generated logical addresses have two parts: segment number and offset.
- Segment number is mapped to the segment table.
- Offset is compared to the limit address.
- If offset is less than the limit, the address is valid.
- Valid address: base address of the segment + offset = physical address.
Fixed-Size Partitioning
- Divides memory into fixed-size partitions.
- Partitions may or may not be the same size, but the size of each partition is fixed.
- Partitions are allocated to processes as they arrive.
- Example:
- 15KB memory divided into fixed-size partitions.
Advantages of Fixed-Size Partitioning
- Simple and easy to implement.
- Supports multiprogramming as multiple processes can be stored in main memory.
Disadvantages of Fixed-Size Partitioning
- Internal Fragmentation: occurs when a process is smaller than the allocated partition, wasting memory within the partition.
- Limitation on process size: a process larger than the maximum partition size cannot be loaded.
- External Fragmentation: occurs when there are enough free spaces in memory but they are not contiguous, preventing the allocation of a larger process.
- Degree of multiprogramming is limited and fixed.
Variable-size Partitioning (Dynamic Partitioning)
- Overcomes the drawbacks of static partitioning, particularly internal fragmentation.
- Partition sizes are not declared initially.
- Partitions are created dynamically when a process arrives, the size of the partition is equal to the size of the process.
- No internal fragmentation as there is no wasted space within partitions.
Advantages of Variable-size Partitioning
- No internal fragmentation.
- Dynamic degree of multiprogramming.
- No limitations on process size, partitions are allocated dynamically to accommodate any process size.
Disadvantages of Variable-size Partitioning
- External Fragmentation: occurs when free spaces in memory are not contiguous, preventing the allocation of a larger process. This is a major issue.
- Difficult to implement: requires dynamic memory allocation at runtime, making it more complex.
Non-Contiguous Memory Allocation
- Allows parts of a process to be stored in non-contiguous memory locations.
Advantages of Segmentation
- No internal fragmentation
- Average segment size is larger than the actual page size, potentially leading to less fragmentation.
- Less overhead, as the segment table is smaller than a page table.
- Easier to relocate segments than entire address spaces.
- Segment table is smaller than a page table in paging.
Disadvantages of Segmentation
- Can have external fragmentation.
- Difficult to allocate contiguous memory to variable-sized partitions.
- Costly memory management algorithms.
Virtual Memory
- Allows a computer to use secondary storage as if it were part of main memory.
- Uses hardware (MMU – Memory Management Unit) and software to operate.
- When an application is in use, data from that program is stored in RAM.
- The MMU maps logical addresses to physical addresses and automatically translates addresses.
- If RAM space is needed for something else (e.g. loading a page), data is swapped from RAM to virtual memory.
- The memory manager keeps track of the shifts between physical and virtual memory.
- When data is needed again, the MMU uses a context switch to resume execution.
Demand Paging
- Loads a page into memory only when it's actually needed (on demand).
- Triggered by a page fault (when the CPU tries to access a page not currently in memory).
- Example:
- CPU tries to access a page not in memory.
- An interrupt is generated, indicating a memory access fault.
- The OS puts the interrupted process in a blocking state.
- The OS finds the required page in the logical address space.
- The required page is brought from logical address space to physical address space (page replacement algorithms are used to decide which existing page to replace).
- Page table is updated accordingly.
- Process resumes execution.
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Description
This quiz explores the concept of paging, a memory management technique that divides processes and main memory into fixed-size units known as pages and frames. It covers the advantages and disadvantages of paging, including its impact on fragmentation and process swapping. Test your understanding of how paging works and its implementation in operating systems.