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70-399: The Silicon Disk System was the first commercially available RAM disk for microcomputers . It was written by Jerry Karlin in 1979/80. Karlin was joined by Peter Cheesewright, and their company Microcosm Research Ltd. marketed the product for a number of years. The product was available as a standalone and also bundled with a number of different microcomputers and RAM-board products. Later,

140-428: A CPU-style MMU. Digital signal processors have similarly generalized over the years. Earlier designs used scratchpad memory fed by direct memory access , but modern DSPs such as Qualcomm Hexagon often include a very similar set of caches to a CPU (e.g. Modified Harvard architecture with shared L2, split L1 I-cache and D-cache). A memory management unit (MMU) that fetches page table entries from main memory has

210-541: A RAM drive is generally orders of magnitude faster than other forms of digital storage, such as SSD , tape , optical , hard disk , and floppy drives. This performance gain is due to multiple factors, including access time, maximum throughput , and file system characteristics. File access time is greatly reduced since a RAM drive is solid state (no moving parts). A physical hard drive, optical (e.g, CD-ROM , DVD , and Blu-ray ) or other media (e.g. magnetic bubble , acoustic storage , magnetic tape ) must move

280-600: A RAM drive named /RAM . IBM added a RAM drive named VDISK.SYS to PC DOS (version 3.0) in August 1984, which was the first DOS component to use extended memory . VDISK.SYS was not available in Microsoft 's MS-DOS as it, unlike most components of early versions of PC DOS, was written by IBM. Microsoft included the similar program RAMDRIVE.SYS in MS-DOS 3.2 (released in 1986), which could also use expanded memory . It

350-473: A backing store. Memoization is an optimization technique that stores the results of resource-consuming function calls within a lookup table, allowing subsequent calls to reuse the stored results and avoid repeated computation. It is related to the dynamic programming algorithm design methodology, which can also be thought of as a means of caching. A content delivery network (CDN) is a network of distributed servers that deliver pages and other Web content to

420-524: A cache benefits one or both of latency and throughput ( bandwidth ). A larger resource incurs a significant latency for access – e.g. it can take hundreds of clock cycles for a modern 4 GHz processor to reach DRAM. This is mitigated by reading large chunks into the cache, in the hope that subsequent reads will be from nearby locations and can be read from the cache. Prediction or explicit prefetching can be used to guess where future reads will come from and make requests ahead of time; if done optimally,

490-424: A cache for frequently accessed data, providing high speed local access to frequently accessed data in the cloud storage service. Cloud storage gateways also provide additional benefits such as accessing cloud object storage through traditional file serving protocols as well as continued access to cached data during connectivity outages. The BIND DNS daemon caches a mapping of domain names to IP addresses , as does

560-525: A disk drive was much faster than the disk drives. especially before hard drives were readily available on such machines. The Silicon Disk was launched in 1980, initially for the CP/M operating system and later for MS-DOS . The 128kB Atari 130XE (with DOS 2.5) and Commodore 128 natively support RAM drives, as does ProDOS for the Apple II . On systems with 128kB or more of RAM, ProDOS automatically creates

630-413: A few specialized "ultra-lightweight" Linux distributions which are designed to boot from removable media and stored in a ramdisk for the entire session. There have been RAM drives which use DRAM memory that is exclusively dedicated to function as an extremely low latency storage device. This memory is isolated from the processor and not directly accessible in the same manner as normal system memory. Some of

700-514: A pair of Serial ATA ports, allowing it to function as a single drive or masquerade as a pair of drives that can easily be split into an even faster RAID 0 array." In 2009, Acard Technology produced the ACARD ANS-9010BA 5.25 Dynamic SSD SATA-II RAM Disk, max 64GB. It uses a single SATA-II port. Both variants are equipped with one or more CompactFlash card interface located in the front panel, allowing non-volatile data being stored on

770-470: A partition on a physical hard drive rather than accessing the data bus normally used for secondary storage. Though RAM drives can often be supported directly in the operating system via special mechanisms in the OS kernel , it is generally simpler to access a RAM drive through a virtual device driver. This makes the non-disk nature of RAM drives invisible to both the OS and applications. Usually no battery backup

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840-474: A reliable backup. In 2009, DDRdrive, LLC produced the DDRDrive X1, which claims to be the fastest solid state drive in the world. The drive is a primary 4GB DDR dedicated RAM drive for regular use, which can back up to and recall from a 4GB SLC NAND drive. The intended market is for keeping and recording log files . If there is a power loss the data can be saved to an internal 4GB ssd in 60 seconds, via

910-476: A resolver library. Write-through operation is common when operating over unreliable networks (like an Ethernet LAN), because of the enormous complexity of the coherency protocol required between multiple write-back caches when communication is unreliable. For instance, web page caches and client-side network file system caches (like those in NFS or SMB ) are typically read-only or write-through specifically to keep

980-620: A selling feature. Later, the ASDG RRD was made available as shareware carrying a suggested donation of 10 dollars. The shareware version appeared on Fred Fish Disks 58 and 241. AmigaOS itself would gain a Recoverable Ram Disk (called "RAD") in version 1.3. Many Unix and Unix-like systems provide some form of RAM drive functionality, such as /dev/ram on Linux , or md(4) on FreeBSD . RAM drives are particularly useful in high-performance, low-resource applications for which Unix-like operating systems are sometimes configured. There are also

1050-465: A specialized cache, used for recording the results of virtual address to physical address translations. This specialized cache is called a translation lookaside buffer (TLB). Information-centric networking (ICN) is an approach to evolve the Internet infrastructure away from a host-centric paradigm, based on perpetual connectivity and the end-to-end principle , to a network architecture in which

1120-504: A specific function are the D-cache , I-cache and the translation lookaside buffer for the memory management unit (MMU). Earlier graphics processing units (GPUs) often had limited read-only texture caches and used swizzling to improve 2D locality of reference . Cache misses would drastically affect performance, e.g. if mipmapping was not used. Caching was important to leverage 32-bit (and wider) transfers for texture data that

1190-451: A system writes data to cache, it must at some point write that data to the backing store as well. The timing of this write is controlled by what is known as the write policy . There are two basic writing approaches: A write-back cache is more complex to implement since it needs to track which of its locations have been written over and mark them as dirty for later writing to the backing store. The data in these locations are written back to

1260-459: A tag matching that of the desired data, the data in the entry is used instead. This situation is known as a cache hit . For example, a web browser program might check its local cache on disk to see if it has a local copy of the contents of a web page at a particular URL . In this example, the URL is the tag, and the content of the web page is the data. The percentage of accesses that result in cache hits

1330-410: A user, based on the geographic locations of the user, the origin of the web page and the content delivery server. CDNs began in the late 1990s as a way to speed up the delivery of static content, such as HTML pages, images and videos. By replicating content on multiple servers around the world and delivering it to users based on their location, CDNs can significantly improve the speed and availability of

1400-492: A website or application. When a user requests a piece of content, the CDN will check to see if it has a copy of the content in its cache. If it does, the CDN will deliver the content to the user from the cache. A cloud storage gateway, also known as an edge filer, is a hybrid cloud storage device that connects a local network to one or more cloud storage services , typically object storage services such as Amazon S3 . It provides

1470-401: Is a variant of LRU designed for the situation where the stored contents in cache have a valid lifetime. The algorithm is suitable in network cache applications, such as ICN, content delivery networks (CDNs) and distributed networks in general. TLRU introduces a new term: time to use (TTU). TTU is a time stamp on content which stipulates the usability time for the content based on the locality of

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1540-435: Is a web cache that is shared among all users of that network. Another form of cache is P2P caching , where the files most sought for by peer-to-peer applications are stored in an ISP cache to accelerate P2P transfers. Similarly, decentralised equivalents exist, which allow communities to perform the same task for P2P traffic, for example, Corelli. A cache can store data that is computed on demand rather than retrieved from

1610-482: Is an example of disk cache, is managed by the operating system kernel . While the disk buffer , which is an integrated part of the hard disk drive or solid state drive, is sometimes misleadingly referred to as "disk cache", its main functions are write sequencing and read prefetching. Repeated cache hits are relatively rare, due to the small size of the buffer in comparison to the drive's capacity. However, high-end disk controllers often have their own on-board cache of

1680-435: Is known as the hit rate or hit ratio of the cache. The alternative situation, when the cache is checked and found not to contain any entry with the desired tag, is known as a cache miss . This requires a more expensive access of data from the backing store. Once the requested data is retrieved, it is typically copied into the cache, ready for the next access. During a cache miss, some other previously existing cache entry

1750-428: Is made up of a pool of entries. Each entry has associated data , which is a copy of the same data in some backing store . Each entry also has a tag , which specifies the identity of the data in the backing store of which the entry is a copy. When the cache client (a CPU, web browser, operating system ) needs to access data presumed to exist in the backing store, it first checks the cache. If an entry can be found with

1820-548: Is needed due to the temporary nature of the information stored in the RAM drive, but an uninterruptible power supply can keep the system running during a short power outage. Some RAM drives use a compressed file system such as cramfs to allow compressed data to be accessed on the fly, without decompressing it first. This is convenient because RAM drives are often small due to the higher price per byte than conventional hard drive storage. The first software RAM drive for microcomputers

1890-409: Is read or written for the first time is effectively being buffered; and in the case of a write, mostly realizing a performance increase for the application from where the write originated. Additionally, the portion of a caching protocol where individual writes are deferred to a batch of writes is a form of buffering. The portion of a caching protocol where individual reads are deferred to a batch of reads

1960-476: Is requested that is stored near data that has already been requested. In memory design, there is an inherent trade-off between capacity and speed because larger capacity implies larger size and thus greater physical distances for signals to travel causing propagation delays . There is also a tradeoff between high-performance technologies such as SRAM and cheaper, easily mass-produced commodities such as DRAM , flash , or hard disks . The buffering provided by

2030-714: Is sometimes referred to as a virtual RAM drive or software RAM drive to distinguish it from a hardware RAM drive that uses separate hardware containing RAM, which is a type of battery-backed solid-state drive . Historically primary storage based mass storage devices were conceived to bridge the performance gap between internal memory and secondary storage devices. In the advent of solid-state devices this advantage lost most of its appeal. However, solid-state devices do suffer from wear from frequent writing. Primary memory writes do not so or in far lesser effect. So RAM devices do offer an advantage to store frequently changing data, like temporary or cached information. The performance of

2100-446: Is the speed of the actual mechanics of the drive motors, heads, or eyes. Third, the file system in use, such as NTFS , HFS , UFS , ext2, etc., uses extra accesses, reads and writes to the drive, which although small, can add up quickly, especially in the event of many small files vs. few larger files (temporary internet folders, web caches, etc.). Because the storage is in RAM, it is volatile memory , which means it will be lost in

2170-413: Is typically removed in order to make room for the newly retrieved data. The heuristic used to select the entry to replace is known as the replacement policy . One popular replacement policy, least recently used (LRU), replaces the oldest entry, the entry that was accessed less recently than any other entry. More sophisticated caching algorithms also take into account the frequency of use of entries. When

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2240-693: The GC-RAMDISK , max 8GB, which was the second generation creation for the i-RAM. It has a maximum of 8 GB capacity, twice that of the i-RAM. It used the SATA-II port, again twice that of the i-RAM. One of its best selling points is that it can be used as a boot device. In 2007, ACard Technology produced the ANS-9010 Serial ATA RAM disk, max 64 GB. Quote from the tech report: The ANS-9010 "which has eight DDR2 DIMM slots and support for up to 8 GB of memory per slot. The ANS-9010 also features

2310-468: The i-RAM , max 4 GB, which functioned essentially identically to the Rocket Drive, except upgraded to use the newer DDR memory technology, though also limited to a maximum of 4 GB capacity. For both of these devices, the dynamic RAM requires continuous power to retain data; when power is lost, the data fades away. For the Rocket Drive, there was a connector for an external power supply separate from

2380-503: The page cache associated with a prefetcher or the web cache associated with link prefetching . Small memories on or close to the CPU can operate faster than the much larger main memory . Most CPUs since the 1980s have used one or more caches, sometimes in cascaded levels ; modern high-end embedded , desktop and server microprocessors may have as many as six types of cache (between levels and functions). Some examples of caches with

2450-569: The RAM drive to be copied on the CompactFlash card in case of power failure and low backup battery. Two pushbuttons located on the front panel allows the user to manually backup / restore data on the RAM drive. The CompactFlash card itself is not accessible to the user by normal means as the CF card is solely intended for RAM backup and restoration. The CF card's capacity has to meet / exceed the RAM module's total capacity in order to effectively work as

2520-679: The Silicon Disk System was sold by Microcosm Ltd . Initially, it was available for the CP/M operating system. Versions for the MP/M , CP/M-86 , and MP/M-86 operating systems followed. Following the launch of the IBM PC, a version for the MS-DOS and PC DOS operating systems was produced. This computing article is a stub . You can help Misplaced Pages by expanding it . RAM disk It

2590-418: The amount of information that needs to be transmitted across the network, as information previously stored in the cache can often be re-used. This reduces bandwidth and processing requirements of the web server, and helps to improve responsiveness for users of the web. Web browsers employ a built-in web cache, but some Internet service providers (ISPs) or organizations also use a caching proxy server, which

2660-409: The backing store only when they are evicted from the cache, a process referred to as a lazy write . For this reason, a read miss in a write-back cache will often require two memory backing store accesses to service: one for the write back, and one to retrieve the needed data. Other policies may also trigger data write-back. The client may make many changes to data in the cache, and then explicitly notify

2730-486: The cache ahead of time. Anticipatory paging is especially helpful when the backing store has a long latency to read the first chunk and much shorter times to sequentially read the next few chunks, such as disk storage and DRAM. A few operating systems go further with a loader that always pre-loads the entire executable into RAM. A few caches go even further, not only pre-loading an entire file, but also starting to load other related files that may soon be requested, such as

2800-448: The cache is a network-level solution. Therefore, it has rapidly changing cache states and higher request arrival rates; moreover, smaller cache sizes impose different requirements on the content eviction policies. In particular, eviction policies for ICN should be fast and lightweight. Various cache replication and eviction schemes for different ICN architectures and applications have been proposed. The time aware least recently used (TLRU)

2870-452: The cache is divided into two partitions called privileged and unprivileged partitions. The privileged partition can be seen as a protected partition. If content is highly popular, it is pushed into the privileged partition. Replacement of the privileged partition is done by first evicting content from the unprivileged partition, then pushing content from the privileged partition to the unprivileged partition, and finally inserting new content into

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2940-404: The cache to write back the data. Since no data is returned to the requester on write operations, a decision needs to be made whether or not data would be loaded into the cache on write misses. Both write-through and write-back policies can use either of these write-miss policies, but usually they are paired. Entities other than the cache may change the data in the backing store, in which case

3010-400: The cache, the faster the system performs. To be cost-effective, caches must be relatively small. Nevertheless, caches are effective in many areas of computing because typical computer applications access data with a high degree of locality of reference . Such access patterns exhibit temporal locality, where data is requested that has been recently requested, and spatial locality, where data

3080-629: The computer, and the option for an external battery to retain data during a power failure. The i-RAM included a small battery directly on the expansion board, for 10-16 hours of protection. Both devices used the SATA 1.0 interface to transfer data from the dedicated RAM drive to the system. The SATA interface was a slow bottleneck that limited the maximum performance of both RAM drives, but these drives still provided exceptionally low data access latency and high sustained transfer speeds, compared to mechanical hard drives. In 2006, Gigabyte Technology produced

3150-517: The content and information from the content publisher. Owing to this locality-based time stamp, TTU provides more control to the local administrator to regulate in-network storage. In the TLRU algorithm, when a piece of content arrives, a cache node calculates the local TTU value based on the TTU value assigned by the content publisher. The local TTU value is calculated by using a locally-defined function. Once

3220-525: The copy in the cache may become out-of-date or stale . Alternatively, when the client updates the data in the cache, copies of those data in other caches will become stale. Communication protocols between the cache managers that keep the data consistent are associated with cache coherence . On a cache read miss, caches with a demand paging policy read the minimum amount from the backing store. A typical demand-paging virtual memory implementation reads one page of virtual memory (often 4 KB) from disk into

3290-415: The data item to its residing storage at a later stage or else occurring as a background process. Contrary to strict buffering, a caching process must adhere to a (potentially distributed) cache coherency protocol in order to maintain consistency between the cache's intermediate storage and the location where the data resides. Buffering, on the other hand, With typical caching implementations, a data item that

3360-408: The data item to realize a performance increase by virtue of being able to be fetched from the cache's (faster) intermediate storage rather than the data's residing location. With write caches, a performance increase of writing a data item may be realized upon the first write of the data item by virtue of the data item immediately being stored in the cache's intermediate storage, deferring the transfer of

3430-403: The data stored in a cache might be the result of an earlier computation or a copy of data stored elsewhere. A cache hit occurs when the requested data can be found in a cache, while a cache miss occurs when it cannot. Cache hits are served by reading data from the cache, which is faster than recomputing a result or reading from a slower data store; thus, the more requests that can be served from

3500-439: The data stored on the RAM drive is created from data permanently stored elsewhere, for faster access , and is re-created on the RAM drive when the system reboots. Apart from the risk of data loss, the major limitation of RAM drives is capacity, which is constrained by the amount of installed RAM. Multi-terabyte SSD storage has become common, but RAM is still measured in gigabytes. RAM drives use normal system memory as if it were

3570-522: The disk cache in RAM. A typical CPU reads a single L2 cache line of 128 bytes from DRAM into the L2 cache, and a single L1 cache line of 64 bytes from the L2 cache into the L1 cache. Caches with a prefetch input queue or more general anticipatory paging policy go further—they not only read the data requested, but guess that the next chunk or two of data will soon be required, and so prefetch that data into

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3640-423: The event of power loss, whether intentional (computer reboot or shutdown) or accidental (power failure or system crash). This is, in general, a weakness (the data must periodically be backed up to a persistent-storage medium to avoid loss), but is sometimes desirable: for example, when working with a decrypted copy of an encrypted file, or using the RAM drive to store the system's temporary files . In many cases,

3710-538: The feature through the life of Mac OS 9 . Mac OS X users can use the hdid , newfs (or newfs hfs ) and mount utilities to create, format and mount a RAM drive. A RAM drive innovation introduced in 1986 but made generally available in 1987 by Perry Kivolowitz for AmigaOS was the ability of the RAM drive to survive most crashes and reboots. Called the ASDG Recoverable Ram Disk, the device survived reboots by allocating memory dynamically in

3780-469: The first dedicated RAM drives were released in 1983-1985. An early example of a hardware RAM drive was introduced by Assimilation Process in 1986 for the Macintosh. Called the "Excalibur", it was an external 2MB RAM drive, and retailed for between $ 599 and $ 699 US. With the RAM capacity expandable in 1MB increments, its internal battery was said to be effective for between 6 and 8 hours, and, unusual for

3850-450: The focal point is identified information. Due to the inherent caching capability of the nodes in an ICN, it can be viewed as a loosely connected network of caches, which has unique requirements for caching policies. However, ubiquitous content caching introduces the challenge to content protection against unauthorized access, which requires extra care and solutions. Unlike proxy servers, in ICN

3920-645: The hard disk drive's data blocks. Finally, a fast local hard disk drive can also cache information held on even slower data storage devices, such as remote servers (web cache) or local tape drives or optical jukeboxes ; such a scheme is the main concept of hierarchical storage management . Also, fast flash-based solid-state drives (SSDs) can be used as caches for slower rotational-media hard disk drives, working together as hybrid drives or solid-state hybrid drives (SSHDs). Web browsers and web proxy servers employ web caches to store previous responses from web servers, such as web pages and images . Web caches reduce

3990-500: The information to a particular position before reading or writing can occur. RAM drives can access data with only the address, eliminating this latency . Second, the maximum throughput of a RAM drive is limited by the speed of the RAM, the data bus , and the CPU of the computer. Other forms of storage media are further limited by the speed of the storage bus, such as IDE (PATA), SATA , USB or FireWire . Compounding this limitation

4060-621: The latency is bypassed altogether. The use of a cache also allows for higher throughput from the underlying resource, by assembling multiple fine-grain transfers into larger, more efficient requests. In the case of DRAM circuits, the additional throughput may be gained by using a wider data bus. Hardware implements cache as a block of memory for temporary storage of data likely to be used again. Central processing units (CPUs), solid-state drives (SSDs) and hard disk drives (HDDs) frequently include hardware-based cache, while web browsers and web servers commonly rely on software caching. A cache

4130-456: The local TTU value is calculated the replacement of content is performed on a subset of the total content stored in cache node. The TLRU ensures that less popular and short-lived content should be replaced with incoming content. The least frequent recently used (LFRU) cache replacement scheme combines the benefits of LFU and LRU schemes. LFRU is suitable for network cache applications, such as ICN, CDNs and distributed networks in general. In LFRU,

4200-417: The network protocol simple and reliable. Search engines also frequently make web pages they have indexed available from their cache. For example, Google provides a "Cached" link next to each search result. This can prove useful when web pages from a web server are temporarily or permanently inaccessible. Database caching can substantially improve the throughput of database applications, for example in

4270-535: The privileged partition. In the above procedure, the LRU is used for the privileged partition and an approximated LFU (ALFU) scheme is used for the unprivileged partition. The basic idea is to cache the locally popular content with the ALFU scheme and push the popular content to the privileged partition. In 2011, the use of smartphones with weather forecasting options was overly taxing AccuWeather servers; two requests within

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4340-499: The process of caching and the process of buffering. Fundamentally, caching realizes a performance increase for transfers of data that is being repeatedly transferred. While a caching system may realize a performance increase upon the initial (typically write) transfer of a data item, this performance increase is due to buffering occurring within the caching system. With read caches, a data item must have been fetched from its residing location at least once in order for subsequent reads of

4410-412: The processing of indexes , data dictionaries , and frequently used subsets of data. A distributed cache uses networked hosts to provide scalability, reliability and performance to the application. The hosts can be co-located or spread over different geographical regions. The semantics of a "buffer" and a "cache" are not totally different; even so, there are fundamental differences in intent between

4480-431: The reverse order of default memory allocation (a feature supported by the underlying OS) so as to reduce memory fragmentation. A "super-block" was written with a unique signature which could be located in memory upon reboot. The super-block, and all other RRD disk "blocks" maintained check sums to enable the invalidation of the disk if corruption was detected. At first, the ASDG RRD was locked to ASDG memory boards and used as

4550-460: The same park would generate separate requests. An optimization by edge-servers to truncate the GPS coordinates to fewer decimal places meant that the cached results from the earlier query would be used. The number of to-the-server lookups per day dropped by half. While CPU caches are generally managed entirely by hardware, a variety of software manages other caches. The page cache in main memory, which

4620-658: The time, it was connected via the Macintosh floppy disk port. In 2002, Cenatek produced the Rocket Drive , max 4 GB, which had four DIMM slots for PC133 memory, with up to a maximum of four gigabytes of storage. At the time, common desktop computers used 64 to 128 megabytes of PC100 or PC133 memory. The one gigabyte PC133 modules (the largest available at the time) cost approximately $ 1,300 (equivalent to $ 2,202 in 2023). A fully outfitted Rocket Drive with four GB of storage would have cost $ 5,600 (equivalent to $ 9,486 in 2023). In 2005, Gigabyte Technology produced

4690-485: The use of a battery backup. Thereafter the data can be recovered back in to RAM once power is restored. A host power loss triggers the DDRdrive X1 to back up volatile data to on-board non-volatile storage. Cache (computing) In computing , a cache ( / k æ ʃ / KASH ) is a hardware or software component that stores data so that future requests for that data can be served faster;

4760-685: Was discontinued in Windows 7. DR-DOS and the DR family of multi-user operating systems also came with a RAM disk named VDISK.SYS. In Multiuser DOS , the RAM disk defaults to the drive letter M: (for memory drive). AmigaOS has had a built in RAM drive since the release of version 1.1 in 1985 and still has it in AmigaOS 4.1 (2010). Apple Computer added the functionality to the Apple Macintosh with System 7 's Memory control panel in 1991, and kept

4830-560: Was invented and written by Jerry Karlin in the UK in 1979/80. The software, known as the Silicon Disk System , was further developed into a commercial product and marketed by JK Systems Research which became Microcosm Research Ltd when the company was joined by Peter Cheesewright of Microcosm Ltd . The idea was to enable the early microcomputers to use more RAM than the CPU could directly address. Making bank-switched RAM behave like

4900-449: Was often as little as 4 bits per pixel. As GPUs advanced, supporting general-purpose computing on graphics processing units and compute kernels , they have developed progressively larger and increasingly general caches, including instruction caches for shaders , exhibiting functionality commonly found in CPU caches. These caches have grown to handle synchronization primitives between threads and atomic operations , and interface with

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