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Paging
In computer operating systems, memory paging is a memory management scheme that allows the physical Computer memory, memory used by a program to be non-contiguous. This also helps avoid the problem of memory fragmentation and requiring compaction to reduce fragmentation. Paging is often combined with the related technique of allocating and freeing Page (computer memory), ''page frames'' and storing pages on and retrieving them from Computer data storage#Secondary storage, secondary storage in order to allow the aggregate size of the address spaces to exceed the physical memory of the system. For historical reasons, this technique is sometimes referred to as ''swapping''. When combined with virtual memory, it is known as Virtual memory#Paged virtual memory, ''paged virtual memory''. In this scheme, the operating system retrieves data from secondary storage in Block (data storage), blocks of the same size (pages). Paging is an important part of virtual memory implementations in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Virtual Memory
In computing, virtual memory, or virtual storage, is a memory management technique that provides an "idealized abstraction of the storage resources that are actually available on a given machine" which "creates the illusion to users of a very large (main) memory". The computer's operating system, using a combination of hardware and software, maps memory addresses used by a program, called '' virtual addresses'', into ''physical addresses'' in computer memory. Main storage, as seen by a process or task, appears as a contiguous address space or collection of contiguous segments. The operating system manages virtual address spaces and the assignment of real memory to virtual memory. Address translation hardware in the CPU, often referred to as a memory management unit (MMU), automatically translates virtual addresses to physical addresses. Software within the operating system may extend these capabilities, utilizing, e.g., disk storage, to provide a virtual address space ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Memory Management Unit
A memory management unit (MMU), sometimes called paged memory management unit (PMMU), is a computer hardware unit that examines all references to computer memory, memory, and translates the memory addresses being referenced, known as virtual memory addresses, into physical addresses in main memory. In modern systems, programs generally have addresses that access the theoretical maximum memory of the computer architecture, 32 or 64 bits. The MMU maps the addresses from each program into separate areas in physical memory, which is generally much smaller than the theoretical maximum. This is possible because programs rarely use large amounts of memory at any one time. Most modern operating systems (OS) work in concert with an MMU to provide virtual memory (VM) support. The MMU tracks memory use in fixed-size blocks known as ''pages''. If a program refers to a location in a page that is not in physical memory, the MMU sends an interrupt to the operating system. The OS selects a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Memory Segmentation
Memory segmentation is an operating system memory management technique of dividing a computer's primary memory into segments or sections. In a computer system using segmentation, a reference to a memory location includes a value that identifies a segment and an offset (memory location) within that segment. Segments or sections are also used in object files of compiled programs when they are linked together into a program image and when the image is loaded into memory. Segments usually correspond to natural divisions of a program such as individual routines or data tables so segmentation is generally more visible to the programmer than paging alone. Segments may be created for program modules, or for classes of memory usage such as code segments and data segments. Certain segments may be shared between programs. Segmentation was originally invented as a method by which system software could isolate software processes ( tasks) and data they are using. It was intended to in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Control Register
A control register is a processor register that changes or controls the general behavior of a CPU or other digital device. Common tasks performed by control registers include interrupt control, switching the addressing mode, paging control, and coprocessor control. History The early CPU lacked dedicated control registers, and relied on a limited set of internal signals and flags. When IBM developed a paging version of the System/360, they added 16 control registers to the design for what became the 360/67. IBM did not provide control registers on other S/360 models, but made them a standard part of System/370, although with different register and bit assignments. As IBM added new features to the architecture, e.g., DAS, S/370-XA, S/370-ESA, ESA/390, they added additional fields to the control registers. With z/Architecture, IBM doubled the control register size to 64 bits. Control registers in IBM 360/67 On the 360/67, CR0 and CR2 are used by address translation, CR 4- ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Page (computer Memory)
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 h ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Operating System
An operating system (OS) is system software that manages computer hardware and software resources, and provides common daemon (computing), services for computer programs. Time-sharing operating systems scheduler (computing), schedule tasks for efficient use of the system and may also include accounting software for cost allocation of Scheduling (computing), processor time, mass storage, peripherals, and other resources. For hardware functions such as input and output and memory allocation, the operating system acts as an intermediary between programs and the computer hardware, although the application code is usually executed directly by the hardware and frequently makes system calls to an OS function or is interrupted by it. Operating systems are found on many devices that contain a computerfrom cellular phones and video game consoles to web servers and supercomputers. , Android (operating system), Android is the most popular operating system with a 46% market share, followed ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Kernel (operating System)
A kernel is a computer program at the core of a computer's operating system that always has complete control over everything in the system. The kernel is also responsible for preventing and mitigating conflicts between different processes. It is the portion of the operating system code that is always resident in memory and facilitates interactions between hardware and software components. A full kernel controls all hardware resources (e.g. I/O, memory, cryptography) via device drivers, arbitrates conflicts between processes concerning such resources, and optimizes the use of common resources, such as CPU, cache, file systems, and network sockets. On most systems, the kernel is one of the first programs loaded on startup (after the bootloader). It handles the rest of startup as well as memory, peripherals, and input/output (I/O) requests from software, translating them into data-processing instructions for the central processing unit. The critical code of the kernel is usua ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
IBM M44/44X
The IBM M44/44X was an experimental computer system from the mid-1960s, designed and operated at IBM's Thomas J. Watson Research Center at Yorktown Heights, New York. It was based on a modified IBM 7044 (the 'M44'), and simulated multiple 7044 virtual machines (the '44X'), using both hardware and software. Key team members were Dave Sayre and Rob Nelson. This was a groundbreaking machine, used to explore paging, the virtual machine concept, and computer performance measurement. It was purely a research system, and was cited in 1981 by Peter Denning as an outstanding example of experimental computer science. The term ''virtual machine'' probably originated with the M44/44X project, from which it was later appropriated by the CP-40 team to replace their earlier term ''pseudo machine''. Unlike CP-40 and later CP/CMS control programs, M44/44X did ''not'' implement a complete simulation of the underlying hardware (i.e. full virtualization). CP-40 project leader Robert Creasy o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Solid-state Drive
A solid-state drive (SSD) is a type of solid-state storage device that uses integrated circuits to store data persistently. It is sometimes called semiconductor storage device, solid-state device, or solid-state disk. SSDs rely on non-volatile memory, typically NAND flash, to store data in memory cells. The performance and endurance of SSDs vary depending on the number of bits stored per cell, ranging from high-performing single-level cells (SLC) to more affordable but slower quad-level cells (QLC). In addition to flash-based SSDs, other technologies such as 3D XPoint offer faster speeds and higher endurance through different data storage mechanisms. Unlike traditional hard disk drives (HDDs), SSDs have no moving parts, allowing them to deliver faster data access speeds, reduced latency, increased resistance to physical shock, lower power consumption, and silent operation. Often interfaced to a system in the same way as HDDs, SSDs are used in a variety of devices, ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Memory Fragmentation
In computer storage, fragmentation is a phenomenon in the computer system which involves the distribution of data in to smaller pieces which storage space, such as computer memory or a hard drive, is used inefficiently, reducing capacity or performance and often both. The exact consequences of fragmentation depend on the specific system of storage allocation in use and the particular form of fragmentation. In many cases, fragmentation leads to storage space being "wasted", and programs will tend to run inefficiently due to the shortage of memory. Basic principle In main memory fragmentation, when a computer program requests blocks of memory from the computer system, the blocks are allocated in chunks. When the computer program is finished with a chunk, it can free it back to the system, making it available to later be allocated again to another or the same program. The size and the amount of time a chunk is held by a program varies. During its lifespan, a computer program can requ ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
