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Questions and Answers
What does memory consist of?
A large array of bytes, each with its own address.
What can the CPU directly access?
Main memory and registers.
The CPU can access memory faster than registers.
False
What is the purpose of hardware memory protection?
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What does the base register hold?
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What is the purpose of the limit register?
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Who can modify the values of the base and limit registers?
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What is Address Binding?
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What are the stages of address binding?
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What is the primary role of the memory table in an operating system?
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What is a key feature of dynamic partitioning in memory management?
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Which strategy minimizes memory wastage by allocating the smallest hole that fits a requested memory size?
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What happens when a process requests 18 KB of memory using the best-fit strategy?
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When using the first-fit memory allocation strategy, what is the search method for available holes?
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What occurs when a process finishes and its memory is returned to the set of holes?
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Which statement accurately describes the worst-fit memory allocation strategy?
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What is the primary drawback of the first-fit and best-fit memory allocation strategies?
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What is a challenge commonly associated with dynamic storage allocation?
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What happens in the worst-fit strategy for memory allocation?
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According to the 50-percent rule, what can be expected in terms of fragmentation?
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Why is the first-fit strategy considered fast and simple to implement?
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What is a consequence of using dynamic partitioning in memory allocation?
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In the example partition list, which value represents the smallest hole for a 18 KB request when using best-fit?
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Which of the following statements regarding fragmentation is correct?
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What is a common effect when using allocation strategies like first-fit and best-fit?
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What is internal fragmentation in memory allocation?
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What is one solution to the problem of external fragmentation?
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Which memory management technique allows for noncontiguous allocation of physical memory?
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In the first-fit allocation strategy, what is the primary criterion for selecting a block for a process?
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What should be considered when using the best-fit memory allocation strategy?
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What is one of the drawbacks of dynamic partitioning in memory management?
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Which of the following best describes the term 'memory wastage' in memory allocation?
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What is the goal of memory compaction?
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Study Notes
Memory
- Memory consists of a large array of bytes, each with its own address.
- The CPU fetches instructions from memory according to the value of the program counter.
- The memory unit sees only a stream of memory addresses, it doesn't know how they are generated or what they are for.
Direct Access to Memory and Registers
- The CPU can directly access only two types of storage: main memory and registers built into the processing cores.
- Any data or instruction being used must be in one of these locations.
- Registers are extremely fast (accessible in one clock cycle), but memory access is slower as it requires communication over the memory bus.
- Cache is used to improve performance.
Hardware Memory Protection
- The operating system must be protected from unauthorised access by user processes and to prevent user processes from accessing each other's memory.
- This protection is enforced by hardware using mechanisms like base and limit registers.
- Each process has a separate memory space with a range of legal addresses that the process may access.
Hardware Memory Protection (continued)
- The base register holds the smallest legal memory address for a process.
- The limit register specifies the range of legal addresses.
- The CPU compares every address generated in user mode against these registers.
- Any illegal access results in a trap to the operating system, which treats the attempt as a fatal error.
Hardware Memory Protection (continued)
- Base Address Register: The starting memory address of a process in main memory, it defines the lowest legal memory address that the process can access.
- Limit Register: Specifies the size of the process or the range of addresses the process is allowed to access, the value in the limit register is typically the length of the addressable memory region for that process, starting from the base address.
- The base address tells where the process begins in memory.
- The limit tells how much memory (in terms of size) the process can access, defining the upper boundary of accessible memory as base + limit.
User and Kernel Mode
- The base and limit registers can be loaded only by the operating system, which uses a special privileged instruction.
- Protection mechanisms ensure that user processes cannot access kernel memory or manipulate hardware directly.
- Only the operating system can modify the value of base and limit registers, as this operation can only be performed in kernel mode.
- User programs cannot change these register values.
User and Kernel Mode (continued)
- The operating system, executing in kernel mode, is given unrestricted access to both operating-system memory and users' memory.
Address Binding - Process Execution
- Programs are stored as binary executable files on disk and need to be loaded into memory to run.
- The process accesses instructions and data from memory during execution.
- When a process terminates, its memory is reclaimed for use by other processes.
Address Binding
- Address binding refers to the process of mapping logical (virtual) addresses to physical addresses in memory.
- This occurs in different stages during a program's lifecycle: compile-time, load-time, and execution-time.
- the memory addresses assigned to a process remain valid and accessible, allowing programs to execute properly within the constraints of allocated memory spaces.
Dynamic Storage Allocation Problem
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Dynamic storage allocation involves managing memory by satisfying varying-sized memory block requests from a list of free blocks (holes).
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A hole is a contiguous unoccupied block of memory ready for allocation.
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To satisfy a request, three strategies are commonly used:
- First-Fit: Allocate the first hole large enough.
- Best-Fit: Allocate the smallest large enough hole.
- Worst-Fit: Allocate the largest hole.
First-Fit
- Involves allocating the first available hole that is big enough.
- Searching can start at the beginning of the hole set or where the previous first-fit search ended.
- Example: Given memory partitions: | 10 KB | 20 KB | 19 KB | 25 KB | 30 KB |
- A process requesting 18 KB of memory would allocate the 20 KB partition, leaving 2 KB as free memory.
Best-Fit
- Involves allocating the smallest hole that is big enough.
- Requires searching the entire hole list unless it is ordered by size.
- Example: Given memory partitions: | 10 KB | 20 KB | 19 KB | 25 KB | 30 KB |
- A process requesting 18 KB of memory would allocate the 19 KB partition.
Worst-Fit
- Involves allocating the largest hole.
- Requires searching the entire list unless it is sorted by size.
- Example: Given memory partitions: | 10 KB | 20 KB | 19 KB | 25 KB | 30 KB |
- A process requesting 18 KB of memory would allocate the 30 KB partition, leaving 12 KB as free memory.
Fragmentation
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First-Fit: Simple, but suffers from external fragmentation.
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Best-Fit: Also prone to external fragmentation, but potentially less severe than first-fit.
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External Fragmentation: Enough total memory to fulfill a request, but available space is not contiguous.
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50-Percent Rule: With First-Fit, statistical analysis suggests that with N allocated blocks, approximately 0.5N blocks are lost to fragmentation, making one-third of memory unusable.
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The operating system tracks memory availability using a memory table.
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Internal Fragmentation: Occurs when the allocated memory for a process is slightly larger than requested. This is the unused memory within a partition.
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Compaction: A solution to external fragmentation, where memory contents are shuffled to consolidate free memory into a single large block.
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Non-contiguous Address Space: Allows processes to be allocated physical memory wherever available, addressing external fragmentation. This is the strategy used in paging.
Memory Location - Dynamic Partitioning
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Initially, all memory is treated as a single large "hole" that can be divided into smaller holes as needed.
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When a process arrives, the system searches for a large enough hole to fit it. If necessary, the hole is split into two.
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When a process finishes, its memory is returned to the set of holes, which can then merge with adjacent holes to create larger ones.
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The operating system must keep track of available memory using a memory table.
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Description
This quiz covers key concepts of memory management in computer architecture, including the structure of memory, direct access to memory and registers, and hardware protection mechanisms. Test your understanding of how the CPU interacts with memory and the importance of memory protection in operating systems.
