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Page Table
A page table is a data structure used by a virtual memory system in a computer to store mappings between virtual addresses and physical addresses. Virtual addresses are used by the program executed by the accessing process, while physical addresses are used by the hardware, or more specifically, by the random-access memory (RAM) subsystem. The page table is a key component of virtual address translation that is necessary to access data in memory. The page table is set up by the computer's operating system, and may be read and written during the virtual address translation process by the memory management unit or by low-level system software or firmware. Role of the page table In operating systems that use virtual memory, every process is given the impression that it is working with large, contiguous sections of memory. Physically, the memory of each process may be dispersed across different areas of physical memory, or may have been moved ( paged out) to secondary storage ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Virtual Address Space And Physical Address Space Relationship
Virtual may refer to: * Virtual image, an apparent image of an object (as opposed to a real object), in the study of optics * Virtual (horse), a thoroughbred racehorse * Virtual channel, a channel designation which differs from that of the actual radio channel (or range of frequencies) on which the signal travels * Virtual function, a programming function or method whose behaviour can be overridden within an inheriting class by a function with the same signature * Virtual machine, the virtualization of a computer system * Virtual meeting, or web conferencing * Virtual memory, a memory management technique that abstracts the memory address space in a computer * Virtual particle, a type of short-lived particle of indeterminate mass * Virtual reality (virtuality), computer programs with an interface that gives the user the impression that they are physically inside a simulated space * Virtual world, a computer-based simulated environment populated by many users who can create a persona ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Page Fault
In computing, a page fault is an exception that the memory management unit (MMU) raises when a process accesses a memory page without proper preparations. Accessing the page requires a mapping to be added to the process's virtual address space. Furthermore, the actual page contents may need to be loaded from a back-up, e.g. a disk. The MMU detects the page fault, but the operating system's kernel handles the exception by making the required page accessible in the physical memory or denying an illegal memory access. Valid page faults are common and necessary to increase the amount of memory available to programs in any operating system that uses virtual memory, such as Windows, macOS, and the Linux kernel. Types Minor page fault If the page is loaded in memory at the time the fault is generated, but is not marked in the memory management unit as being loaded in memory, then it is called a ''minor'' or ''soft'' page fault. The page fault handler in the operating system m ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cache Locality
In computer science, locality of reference, also known as the principle of locality, is the tendency of a processor to access the same set of memory locations repetitively over a short period of time. There are two basic types of reference locality temporal and spatial locality. Temporal locality refers to the reuse of specific data and/or resources within a relatively small time duration. Spatial locality (also termed ''data locality'')"NIST Big Data Interoperability Framework: Volume 1"urn:doi:10.6028/NIST.SP.1500-1r2 refers to the use of data elements within relatively close storage locations. Sequential locality, a special case of spatial locality, occurs when data elements are arranged and accessed linearly, such as traversing the elements in a one-dimensional Array data structure, array. Locality is a type of predictability, predictable behavior that occurs in computer systems. Systems which exhibit strong ''locality of reference'' are good candidates for performance optimiza ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hash Table
In computer science, a hash table is a data structure that implements an associative array, also called a dictionary or simply map; an associative array is an abstract data type that maps Unique key, keys to Value (computer science), values. A hash table uses a hash function to compute an ''index'', also called a ''hash code'', into an array of ''buckets'' or ''slots'', from which the desired value can be found. During lookup, the key is hashed and the resulting hash indicates where the corresponding value is stored. A map implemented by a hash table is called a hash map. Most hash table designs employ an Perfect hash function, imperfect hash function. Hash collision, Hash collisions, where the hash function generates the same index for more than one key, therefore typically must be accommodated in some way. In a well-dimensioned hash table, the average time complexity for each lookup is independent of the number of elements stored in the table. Many hash table designs also ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hash Collision
In computer science, a hash collision or hash clash is when two distinct pieces of data in a hash table share the same hash value. The hash value in this case is derived from a hash function which takes a data input and returns a fixed length of bits. Although hash algorithms, especially cryptographic hash algorithms, have been created with the intent of being Collision resistance, collision resistant, they can still sometimes map different data to the same hash (by virtue of the pigeonhole principle). Malicious users can take advantage of this to mimic, access, or alter data. Due to the possible negative applications of hash collisions in data management and computer security (in particular, cryptographic hash functions), collision avoidance has become an important topic in computer security. Background Hash collisions can be unavoidable depending on the number of objects in a set and whether or not the bit string they are mapped to is long enough in length. When there is a s ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Translation Lookaside Buffer
A translation lookaside buffer (TLB) is a memory CPU cache, cache that stores the recent translations of virtual memory address to a physical memory Memory_address, location. It is used to reduce the time taken to access a user memory location. It can be called an address-translation cache. It is a part of the chip's Memory management unit, memory-management unit (MMU). A TLB may reside between the Central processing unit, CPU and the CPU cache, between CPU cache and the main memory or between the different levels of the multi-level cache. The majority of desktop, laptop, and server processors include one or more TLBs in the memory-management hardware, and it is nearly always present in any processor that uses Memory paging, paged or Memory segmentation, segmented virtual memory. The TLB is sometimes implemented as content-addressable memory (CAM). The CAM search key is the virtual address, and the search result is a physical address. If the requested address is present in the TLB ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Single Address Space Operating System
In computer science, a single address space operating system (or SASOS) is an operating system that provides only one globally shared address space for all processes. In a single address space operating system, numerically identical (virtual memory) logical addresses in different processes all refer to exactly the same byte of data. In a traditional OS with private per-process address space, memory protection is based on address space boundaries ("address space isolation"). Single address-space operating systems make translation and protection orthogonal, which in no way weakens protection. The core advantage is that pointers (i.e. memory references) have global validity, meaning their meaning is independent of the process using it. This allows sharing pointer-connected data structures across processes, and making them persistent, i.e. storing them on backup store. Some processor architectures have direct support for protection independent of translation. On such architectures, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Dirty Bit
A dirty bit or modified bit is a bit that is associated with a block of computer memory and indicates whether the corresponding block of memory has been modified. The dirty bit is set when the processor writes to (modifies) this memory. The bit indicates that its associated block of memory has been modified and has not been saved to storage yet. When a block of memory is to be replaced, its corresponding dirty bit is checked to see if the block needs to be written back to secondary memory before being replaced or if it can simply be removed. Dirty bits are used by the CPU cache and in the page replacement algorithms of an operating system. Dirty bits can also be used in Incremental computing by marking segments of data that need to be processed or have yet to be processed. This technique can be used with delayed computing to avoid unnecessary processing of objects or states that have not changed. When the model is updated (usually by multiple sources), only the segments that ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Page Frame
A page, memory page, or virtual page is a fixed-length contiguous block of virtual memory, described by a single entry in a page table. It is the smallest unit of data for memory management in an operating system that uses virtual memory. Similarly, a page frame is the smallest fixed-length contiguous block of physical memory into which memory pages are mapped by the operating system. A transfer of pages between main memory and an auxiliary store, such as a hard disk drive, is referred to as paging or swapping. Explanation Computer memory is divided into pages so that information can be found more quickly. The concept is named by analogy to the pages of a printed book. If a reader wanted to find, for example, the 5,000th word in the book, they could count from the first word. This would be time-consuming. It would be much faster if the reader had a listing of how many words are on each page. From this listing they could determine which page the 5,000th word appears on, and how ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Write XOR Execute
W^X (write xor execute, pronounced ''W xor X'') is a security policy in operating systems and software frameworks. It implements executable space protection by ensuring every memory page (a fixed-size block in a program’s virtual address space, the memory layout it uses) is either writable or executable, but not both. Without such protection, a program can write (as data "W") CPU instructions in an area of memory intended for data and then run (as executable "X"; or read-execute "RX") those instructions. This can be dangerous if the writer of the memory is malicious. The terminology was first introduced in 2003 for Unix-like systems, but is today also used by some multi-platform systems (such as .NET). Other operating systems have adopted similar policies under different names (e.g., DEP in Windows). In Unix, W^X is typically controlled via the mprotect system call. It is relatively simple on processor architectures supporting fine-grained page permissions, such as SPARC, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
NX Bit
The NX bit (no-execute bit) is a processor feature that separates areas of a virtual address space (the memory layout a program uses) into sections for storing data or program instructions. An operating system supporting the NX bit can mark certain areas of the virtual address space as non-executable, preventing the processor from running any code stored there. This technique, known as executable space protection or Write XOR Execute, protects computers from malicious software that attempts to insert harmful code into another program’s data storage area and execute it, such as in a buffer overflow attack. The term "NX bit" was introduced by Advanced Micro Devices (AMD) as a marketing term. Intel markets this feature as the XD bit (execute disable), while the MIPS architecture refers to it as the XI bit (execute inhibit). In the ARM architecture, introduced in ARMv6, it is known as XN (execute never). The term NX bit is often used broadly to describe similar executable space p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Copy-on-write
Copy-on-write (COW), also called implicit sharing or shadowing, is a resource-management technique used in programming to manage shared data efficiently. Instead of copying data right away when multiple programs use it, the same data is shared between programs until one tries to modify it. If no changes are made, no private copy is created, saving resources. A copy is only made when needed, ensuring each program has its own version when modifications occur. This technique is commonly applied to memory, files, and data structures. In virtual memory management Copy-on-write finds its main use in operating systems, sharing the physical memory of computers running multiple processes, in the implementation of the fork() system call. Typically, the new process does not modify any memory and immediately executes a new process, replacing the address space entirely. It would waste processor time and memory to copy all of the old process's memory during the fork only to immediately dis ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
