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Introduction to Virtual Memory
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Today, we'll explore virtual memory, which allows us to use memory beyond the physical limits via address translation and paging. Can anyone tell me the main purpose of virtual memory?
Is it to allow multitasking?
Exactly! Virtual memory facilitates multitasking, memory protection, and dynamic memory usage. Student_2, can you describe what a virtual address is?
It's the address used by the programs, right?
Correct! And the physical address is where it's actually stored in the hardware. Think of virtual addresses as a postal address and physical addresses as the actual houseβthe location.
What about the Page Table?
Great question! The Page Table maps virtual addresses to their corresponding physical pages, a crucial part of this system.
What's the role of the MMU?
The MMU stands for Memory Management Unit, which is responsible for address translation. It works together with a cache named TLBβTranslation Lookaside Bufferβto speed up this process. Remember, MMU = Mapping!
In summary, virtual memory allows for effective memory management and protection, crucial for multitasking and secure operations.
Benefits and Limitations of Virtual Memory
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Now, let's discuss the benefits of virtual memory in embedded systems. Student_1, what do you think one of these benefits might be?
Could it be memory protection?
Exactly! Memory protection is crucial to prevent task interference. It also allows process isolation; a fault in one task doesn't crash the whole system. Student_2, can you think of another benefit?
Maybe dynamic memory management?
Yes! Dynamic memory management supports flexible heap and stack allocation, as well as code and data sharing. But, are there any limitations that come with using virtual memory?
Like unpredictable latency?
That's right! Page faults can lead to unpredictable latency and might violate real-time deadlines. Student_4, do you have any thoughts on why it's generally not suitable for low-end MCUs?
They donβt have MMUs.
Exactly! The higher overhead and increased complexity make it unfit for those systems. To sum up, while virtual memory enhances flexibility and protection, it introduces latency and complexity, especially in real-time systems.
Virtual Memory Mechanisms
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Now letβs look at the mechanisms of virtual memory! Student_2, do you recall what paging is?
It divides memory into fixed-size pages?
Right! Paging makes memory allocation simpler and offers protection. Can anyone tell me a typical size for these pages?
Usually 4 KB?
Correct again! Now, Student_3, what about segmentation?
That's when memory is divided into variable-sized segments, like code or data.
Yes! Less common today but still vital in some applications. And how about memory mapping, Student_4?
It maps files or devices into memory space for direct access.
Exactly! It's especially useful in embedded Linux environments. In a nutshell, paging, segmentation, and memory mapping are essential virtual memory mechanisms that contribute to system efficiency and flexibility.
MMU and MPU Roles
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Let's dive into the roles of MMUs and MPUs in embedded systems. Student_1, can you tell me what the MMU does?
It supports full virtual memory and protection.
Correct! The MMU is crucial for systems that require full virtual memory, paging, and protection. What does the MPU do, Student_2?
It doesnβt do paging but protects memory regions.
Exactly! The MPU is typically used in low-end RTOS systems for memory region protection without translation overhead. So, MMU = Management, and MPU = Protection. Student_3, can you summarize the difference between them?
MMUs handle virtual memory and address translation, while MPUs protect memory regions without translating addresses.
Perfect! The distinction is important for understanding real-time safety in embedded systems. Essentially, MMUs and MPUs are key components for achieving safety and efficiency in memory management.
Real-Time Considerations
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As we discuss real-time considerations, let's revisit some challenges presented by virtual memory. Student_4, why might page faults be a concern in real-time systems?
They can cause task blocking and violate deadlines.
Exactly! Page faults can disrupt real-time operations. Student_1, do you remember the problem with swapping in hard real-time systems?
It's not feasible since it can lead to unpredictable behavior.
Right! Swapping may introduce unacceptable delays. What about TLB misses, Student_2?
They cause delays due to the need to re-lookup page tables.
Correct! So, we must use virtual memory cautiously. Can anyone summarize how we should approach it in real-time systems?
We should limit its use to soft real-time systems.
Precisely! Itβs important to balance performance with safety in these systems.
Introduction & Overview
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Quick Overview
Standard
Virtual memory provides abstraction in memory management but is often rare in real-time and embedded systems. This section outlines the functionality, benefits, and limitations of virtual memory in these environments, emphasizing the intricacies of address translation, paging, and the specific hardware components involved, such as MMUs and MPUs.
Detailed
Virtual Memory in Real-Time and Embedded Applications
Virtual memory allows systems to utilize more memory than is physically present by employing address translation and paging. While pivotal in general-purpose systems for multitasking and protection, its application in real-time and embedded systems is selective due to unique requirements. This section elucidates the concept of virtual memory, its mechanisms, benefits, and drawbacks when applied in real-time environments.
Key Concepts Covered:
- Virtual and Physical Addresses: Virtual addresses are used by programs, while physical addresses denote actual memory locations in hardware. An essential component is the Page Table, which maps virtual to physical addresses, and the MMU (Memory Management Unit), responsible for address translation, utilizing TLB (Translation Lookaside Buffer) for efficiency.
- Benefits and Limitations: Virtual memory in embedded and real-time systems can provide memory protection, process isolation, dynamic memory management, and code sharing. However, it may cause unpredictable latency, higher overhead, and is unsuitable for low-end microcontrollers lacking MMUs.
- Virtual Memory Mechanisms: Three main techniques are discussedβPaging simplifies memory allocation by dividing memory into fixed-size pages, Segmentation organizes memory into variable-sized segments, and Memory Mapping facilitates direct file/device memory access, crucial in embedded systems.
- MMU and MPU Roles: The MMU extends full virtual memory capabilities with protection, while the MPU offers region protection without paging, often employed in low-end RTOS systems.
- Real-Time Considerations: The section emphasizes the challenges of page faults leading to potential deadline violations, the infeasibility of swapping in hard real-time systems, and TLB misses causing delays. A cautious approach to virtual memory is advisable in soft real-time systems only.
- Embedded Use Cases: Examples span embedded Linux systems, secure bootloaders, multimedia devices, and POSIX RT applications using memory-locking techniques to enhance realtime performance.
- Hybrid Approaches: Many systems use a combination of MPUs for region protection and static allocation methods to maintain deterministic performance for real-time tasks that avoid full virtual memory.
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Introduction to Virtual Memory
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Virtual memory allows systems to use more memory than physically available by abstracting physical memory through address translation and paging.
β In general-purpose systems, virtual memory enables multitasking, memory protection, and dynamic memory usage.
β In real-time and embedded systems, virtual memory is rare but selectively used in high-end or Linux-based embedded devices where memory isolation and protection are required.
Detailed Explanation
Virtual memory is a technology that allows a computer to use hard disk space as additional RAM, making it seem like the system has more memory available than it physically does. It works by translating virtual addresses used by programs into physical addresses in the computer's memory through a process called paging. In typical general-purpose systems, this is beneficial because it allows multiple programs to run simultaneously (multitasking), protects memory so one program doesn't interfere with another, and provides flexibility in memory usage. However, in real-time and embedded systems, which often require immediate, predictable responses, virtual memory usage is limited. It is mainly found in more advanced devices that use Linux, where it can ensure memory protection and isolation when those features are essential.
Examples & Analogies
Think of virtual memory like a magician pulling rabbits out of a hat. The magician (the computer) appears to have an endless supply of rabbits (memory) even when the reality is that there are only a few in the hat (physical RAM). When the magician needs more rabbits, they can 'create' more by having an assistant (the hard drive) off-stage bring more in, without the audience realizing how limited the actual supply is.
Concepts of Virtual Memory
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Term Description
Virtual Address Address used by programs
Physical Address Actual location in hardware memory
Page Table Maps virtual addresses to physical pages
MMU (Memory Management Unit) Hardware component for address translation
TLB (Translation Lookaside Buffer) Cache that speeds up address translation
Detailed Explanation
There are several key concepts related to virtual memory. A virtual address is what programs use to identify memory locations, whereas the physical address is where that data is actually stored in the hardware memory. The page table is crucial because it maintains the mapping between those virtual addresses and their corresponding physical pages, allowing the system to keep track of where everything is stored. The Memory Management Unit (MMU) is the hardware responsible for translating virtual addresses into physical addresses in real-time, and the Translation Lookaside Buffer (TLB) helps speed this process up by storing recent translations, so it doesnβt have to look them up from the page table each time.
Examples & Analogies
Think of the virtual address like an apartment number (virtual address) in a building (physical memory). The building has an address (physical address), but for each unit within it, there's a unique apartment number. The property manager (page table) keeps a ledger that says which apartment number corresponds to which resident (data in physical memory). The MMU is like a receptionist translating everyoneβs requests from apartment numbers to physical building addresses. In a busy building, the TLB acts as a quick reference guide, listing frequently used apartments so the receptionist can immediately provide directions without looking them up each time.
Benefits and Limitations of Virtual Memory
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β
For Embedded and RT Systems (When Used):
β Memory Protection: Prevents task interference
β Process Isolation: Fault in one task doesnβt crash the whole system
β Dynamic Memory Management: Allows flexible heap/stack allocation
β Code/Data Sharing: Multiple processes can share code sections (e.g., libraries)
β Limitations for Real-Time:
β Unpredictable Latency: Page faults can violate deadlines
β Higher Overhead: MMU and page table management increase complexity
β Not suitable for low-end MCUs without MMU
Detailed Explanation
When used in embedded and real-time systems, virtual memory can provide significant benefits. It offers memory protection to ensure that one task does not interfere with another, and it allows for process isolation so that a failure in one task does not crash the entire system. Additionally, it enables dynamic memory management, which allows programs to adjust their memory requirements as they run, and facilitates sharing code and data between processes to use memory more efficiently. However, the use of virtual memory also comes with limitations in real-time applications. The unpredictable latency caused by page faults can lead to missed deadlines, and managing the MMU and page tables adds complexity to the system, which may not be feasible for low-end microcontrollers that lack an MMU.
Examples & Analogies
Imagine a restaurant kitchen. When everything is running smoothly (memory protection, process isolation), chefs (processes) can share ingredients and space efficiently (code/data sharing). But if one chef burns a dish (fault in a task), it can cause chaos, and customers (users) get delayed (unpredictable latency). For smaller restaurants (low-end MCUs) without proper kitchen staff (MMU), trying to manage multiple chefs can lead to confusion and mistakes.
Virtual Memory Mechanisms
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-
Paging
β Divides memory into fixed-size pages (typically 4 KB)
β Simplifies memory allocation and protection
β Used in Embedded Linux and ARM Cortex-A with MMU -
Segmentation
β Divides memory into variable-sized logical segments (e.g., code, data, stack)
β Less common in embedded systems today -
Memory Mapping
β Maps files or devices directly into memory space
β Useful in embedded Linux for peripheral access or file-backed buffers
Detailed Explanation
Virtual memory utilizes different mechanisms to manage how memory is accessed. Paging is the most common, whereby memory is split into small, fixed-size segments (typically 4KB), easing memory management and protection. This is frequently seen in systems using Embedded Linux and ARM Cortex-A processors with MMUs. Segmentation, while still a concept, is less utilized today in embedded systems. It divides memory into segments of varying sizes that correspond to logical units like code or data. Memory mapping, on the other hand, allows files or hardware devices to be directly mapped into the memory space, which is particularly useful in embedded systems for interaction with peripherals or managing buffers.
Examples & Analogies
Imagine a library. Paging is like having a consistent-sized section for each book (a fixed size) which makes finding and storing them easier. Segmentation is akin to categorizing books into different genres and shelves of varying lengths, which is not always practical for every library. Memory mapping is like allowing certain books to be stored directly on the reading table so readers can access them without retrieving them from the stacks, facilitating quicker access for specific titles or materials.
MMU and MPU Roles in Embedded Systems
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Unit Role
MMU (Memory Management Unit) Supports full virtual memory, paging, and protection
MPU (Memory Protection Unit) No paging, only memory region protection; used in low-end RTOS systems (e.g., ARM Cortex-M)
β MMU β Virtual Memory + Isolation
β MPU β Real-time safety + No translation overhead
Detailed Explanation
In embedded systems, two componentsβ the Memory Management Unit (MMU) and the Memory Protection Unit (MPU)β play crucial roles. The MMU supports full virtual memory capabilities, including paging and memory protection, which is essential for complex, high-end systems requiring advanced memory management. In contrast, the MPU does not facilitate paging; rather, it provides protection across different memory regions for real-time operating systems, especially useful in low-end microcontrollers. The MMU primarily offers virtual memory along with isolation, while the MPU focuses on ensuring real-time safety without the overhead of address translation.
Examples & Analogies
Think of the MMU like a complex airport security system where multiple gates allow passengers to board different flights (handling virtual memory and paging). The MPU is like a simpler setup at a small airport where security is focused on ensuring no unauthorized boarding across different sections without the need for complex systems to manage many gates (providing memory region protection without paging).
Real-Time Considerations
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Concern Description
Page Faults Cause task blocking, violate deadlines
Swapping Not feasible in hard real-time systems
TLB Misses Introduce delay due to re-lookup of page tables
Determinism Harder to guarantee in systems with virtual memory
Solution: Use virtual memory cautiously in soft real-time systems only.
Detailed Explanation
In real-time systems, particularly hard real-time ones, the use of virtual memory presents several challenges that can compromise performance. Page faults, which occur when requested data is not in memory, can block a task and cause it to miss critical deadlines. Swapping, the process of transferring memory pages between RAM and disk, is generally impractical for hard real-time systems that need consistent timing. TLB misses refer to delays caused when requested information is not cached, necessitating a new lookup in the page table, adding latency. Overall, managing virtual memory complicates the ability to guarantee deterministic system behavior, which is crucial in real-time applications. Therefore, caution is advised when using virtual memory, and it may only be suitable for soft real-time systems where some timing flexibility exists.
Examples & Analogies
Imagine a fire response team (real-time system) needing to respond promptly to emergencies. If they hit a traffic jam (page faults), they may not reach the fire in time (violate deadlines). In extreme cases (hard real-time), rerouting (swapping) can simply be unachievable due to strict timelines. If the maps they depend on aren't updated (TLB misses), they could waste time finding a new route. Hence, the team should only use alternative routes when needed (cautious use of virtual memory) for scenarios where timing is less strict (soft real-time).
Embedded Use Cases for Virtual Memory
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Application Purpose
Embedded Linux Full virtual memory, multitasking (e.g., routers, smart TVs)
Devices
Secure Bootloaders Map memory with read-only or executable permissions
Multimedia Devices Map large buffers (e.g., video frames) using mmap()
POSIX RT Applications Use mmap() or mlock() for memory mapping and locking
Detailed Explanation
Virtual memory finds practical application in several embedded use cases. Embedded Linux devices, such as routers and smart TVs, utilize full virtual memory and multitasking capabilities to efficiently run multiple applications. Secure bootloaders incorporate technology to map memory with specific permissions to enhance security by preventing unauthorized access or writing of certain memory areas. Multimedia devices often rely on virtual memory to manage large data buffers, such as video frames, using techniques like mmap() to quickly access data in memory. Similarly, POSIX real-time applications can utilize memory mapping and locking functions to ensure that critical data remains resident in RAM, reducing the risks associated with page faults.
Examples & Analogies
Consider a smart TV (embedded Linux device). It runs multiple applications at once, like streaming, browsing, and gaming, all requiring virtual memory to handle these tasks simultaneously without slowing down (full virtual memory and multitasking). For a secure vault (secure bootloaders), mapping access permissions ensures only authorized personnel can access valuable assets (mapping memory securely). Large libraries of movie files in a multimedia platform (multimedia devices) need quick access to various frames that are efficiently managed through virtual memory techniques. A rapid-response team managing various emergency scenarios (POSIX RT Applications) can use memory mapping to keep crucial details readily accessible for immediate decisions.
Memory Locking for Real-Time Performance
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To avoid page faults, critical real-time tasks can lock their memory:
include
mlockall(MCL_CURRENT | MCL_FUTURE);
β Ensures memory is resident in RAM, not swapped
β Used in real-time POSIX applications (RT-PREEMPT Linux)
Detailed Explanation
In scenarios where avoiding page faults is crucial, real-time tasks can implement memory locking. By using specific functions from the system's memory management library, such as mlockall, applications can lock their required memory pages so that they remain in RAM and are not subject to being swapped out to disk. This is particularly important in real-time environments where any latency introduced by page faults is unacceptable. Memory locking is commonly utilized in real-time applications operating under the RT-PREEMPT feature of Linux, ensuring that these applications can perform efficiently without interruptions caused by paging.
Examples & Analogies
Think of a firefighter needing a constant supply of water to extinguish flames without interruption. By locking in the water source (memory locking), the firefighter ensures a steady flow (consistent access) without risk of running out due to changes in demand. Just as this guarantees the firefighter focuses on the task at hand without disruptions, memory locking helps real-time tasks ensure their data is available exactly when needed without delays from page faults.
Hybrid Approaches in Embedded Systems
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Many embedded systems avoid virtual memory but still use:
β MPUs for region protection
β Flat memory models with manual memory management
β Static allocation for hard real-time tasks
β User-space memory isolation in high-end RTOS like QNX or RTEMS
Detailed Explanation
Although many embedded systems choose to operate without virtual memory due to its complexityβespecially in hard real-time environmentsβvarious hybrid approaches allow them to still benefit from certain memory management techniques. Memory Protection Units (MPUs) enable fixed memory region protections to prevent unauthorized access. Some systems adopt flat memory models paired with manual memory management, where developers manage memory allocation explicitly. For critical tasks requiring guaranteed performance (hard real-time tasks), static allocation can be used. In advanced real-time operating systems like QNX or RTEMS, user-space memory isolation can provide safety and organization without full virtualization.
Examples & Analogies
Think of an apartment building (embedded system) that avoids overly complex access systems (virtual memory). Instead, it uses sturdy exterior doors with strong locks (MPUs) for region protection. Management assigns specific apartments for permanent residents (flat memory models with manual memory management) and assigns the same residents to their apartments without room changes (static allocation for hard real-time tasks). Some high-end buildings (advanced RTOS like QNX or RTEMS) have shared resources while still safely limiting access to only those who need it (user-space memory isolation).
Summary of Key Concepts
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β Virtual memory offers flexibility and protection, but at the cost of latency and complexity.
β Real-time systems usually avoid full virtual memory due to timing unpredictability.
β When needed, MMUs and memory locking techniques help balance performance and safety.
β MPUs are often preferred for predictable region-based memory protection in embedded RTOS environments.
Detailed Explanation
In summary, while virtual memory provides flexibility and enhances security by offering various management capabilities, it also introduces latency and adds complexity to systems. As such, real-time systems often shy away from employing full virtual memory due to the unpredictability it introduces to response times. In situations where virtual memory is necessary, using MMUs for management and memory locking techniques can help ensure that performance remains adequate while preserving safety. Additionally, for systems that prioritize predictable and deterministic behavior, MPUs are frequently chosen for their regional memory protection benefits, well-suited for embedded real-time operating systems.
Examples & Analogies
Consider a factory that needs to balance its production line's flexibility with the need for timely delivery (virtual memory benefits and downsides). A well-functioning factory adapts to changes in order size (flexibility) but mishandled parts (latency from full virtual memory) could disrupt output. To avoid this, some machines might be specially designated to handle critical tasks without interruptions (MMUs and memory locking) while reserving others for more flexible roles. A simpler factory layout (MPUs for predictable safety) facilitates smooth operations, ensuring effective production without delays.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
-
Virtual and Physical Addresses: Virtual addresses are used by programs, while physical addresses denote actual memory locations in hardware. An essential component is the Page Table, which maps virtual to physical addresses, and the MMU (Memory Management Unit), responsible for address translation, utilizing TLB (Translation Lookaside Buffer) for efficiency.
-
Benefits and Limitations: Virtual memory in embedded and real-time systems can provide memory protection, process isolation, dynamic memory management, and code sharing. However, it may cause unpredictable latency, higher overhead, and is unsuitable for low-end microcontrollers lacking MMUs.
-
Virtual Memory Mechanisms: Three main techniques are discussedβPaging simplifies memory allocation by dividing memory into fixed-size pages, Segmentation organizes memory into variable-sized segments, and Memory Mapping facilitates direct file/device memory access, crucial in embedded systems.
-
MMU and MPU Roles: The MMU extends full virtual memory capabilities with protection, while the MPU offers region protection without paging, often employed in low-end RTOS systems.
-
Real-Time Considerations: The section emphasizes the challenges of page faults leading to potential deadline violations, the infeasibility of swapping in hard real-time systems, and TLB misses causing delays. A cautious approach to virtual memory is advisable in soft real-time systems only.
-
Embedded Use Cases: Examples span embedded Linux systems, secure bootloaders, multimedia devices, and POSIX RT applications using memory-locking techniques to enhance realtime performance.
-
Hybrid Approaches: Many systems use a combination of MPUs for region protection and static allocation methods to maintain deterministic performance for real-time tasks that avoid full virtual memory.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
-
Embedded Linux systems often leverage virtual memory for multitasking, crucial in devices like routers and smart TVs.
-
Secure bootloaders can map memory regions with specific permissions to prevent unauthorized access.
-
Multimedia devices use memory mapping to efficiently handle large data sets like video frames.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
π΅ Rhymes Time
-
Virtual memory, so very nice / Gives us protection, that's the price / Latency, it might create / But we aim to mitigate.
π Fascinating Stories
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Imagine a library where books (memory) are stored not just on shelves (physical memory) but also on extra floors (virtual memory). Librarians (MMUs) help you find any book you need, whether it's on the ground floor or up high!
π§ Other Memory Gems
-
To remember the benefits of Virtual memory, think PIDS: Protection, Isolation, Dynamic Management, Sharing.
π― Super Acronyms
MMU = Memory Mapping Unit; MPU = Memory Protection Unit. Remember the distinction
- Mapping is Full
- Protection is Basic.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Virtual Address
Definition:
The address used by programs to access memory.
-
Term: Physical Address
Definition:
The actual location in hardware memory.
-
Term: Page Table
Definition:
A data structure that maps virtual addresses to physical pages.
-
Term: MMU (Memory Management Unit)
Definition:
A hardware component responsible for address translation.
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Term: TLB (Translation Lookaside Buffer)
Definition:
A cache that speeds up the address translation process by storing recent translations.
-
Term: Paging
Definition:
A memory management scheme that eliminates the need for contiguous allocation of physical memory.
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Term: Segmentation
Definition:
A memory management technique that divides memory into variable-sized segments.
-
Term: Memory Mapping
Definition:
The process of mapping files or devices directly into memory space.
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Term: MPU (Memory Protection Unit)
Definition:
A hardware component that provides memory protection without paging.