The two broad categories of computer memory are random-access memory and read-only memory. When you run a program, it stores its processes in RAM. RAM is volatile, which means if you close the program without saving a file, you lose whatever changes you made since the last time you saved the file. If you saved the file, the computer commits the data to ROM, which is not volatile, so even if you turn off the computer, you will not lose the data. Internal memory consists of both RAM and ROM, while external devices use only ROM. Computers ship with a finite amount of internal memory. You may see the storage capacity advertised along with the purchase price, but some of internal memory, or system memory, is reserved for the operating system and applications currently running on the computer.
The only way to increase internal memory is by replacing the internal memory chips with larger-capacity modules or by installing new memory chips in available memory-expansion slots, if the system's motherboard supports them. RAM is also used to store the files that define the file system structure to allow the computer to read and write to its devices properly.
Example of random-access memory: Synchronous, primarily used as main memory in, and. Random-access memory ( RAM ) is a form of that stores and currently being used. A memory device allows items to be or written in almost the same amount of time irrespective of the physical location of data inside the memory.
In contrast, with other direct-access data storage media such as, and the older and, the time required to read and write data items varies significantly depending on their physical locations on the recording medium, due to mechanical limitations such as media rotation speeds and arm movement. RAM contains and circuitry, to connect the data lines to the addressed storage for reading or writing the entry. Usually more than one bit of storage is accessed by the same address, and RAM devices often have multiple data lines and are said to be '8-bit' or '16-bit', etc. In today's technology, random-access memory takes the form of.
RAM is normally associated with types of memory (such as ), where stored information is lost if power is removed, although non-volatile RAM has also been developed. Other types of exist that allow random access for read operations, but either do not allow write operations or have other kinds of limitations on them. These include most types of and a type of called. Integrated-circuit RAM chips came into the market in the early 1970s, with the first commercially available DRAM chip, the, introduced in October 1970. 1 Megabit chip – one of the last models developed by in 1989 Early computers used, or for main memory functions. Ultrasonic delay lines could only reproduce data in the order it was written.
Could be expanded at relatively low cost but efficient retrieval of memory items required knowledge of the physical layout of the drum to optimize speed. Latches built out of, and later, out of discrete, were used for smaller and faster memories such as registers. Such registers were relatively large and too costly to use for large amounts of data; generally only a few dozen or few hundred bits of such memory could be provided. The first practical form of random-access memory was the starting in 1947.
It stored data as electrically charged spots on the face of a. Since the electron beam of the CRT could read and write the spots on the tube in any order, memory was random access. The capacity of the Williams tube was a few hundred to around a thousand bits, but it was much smaller, faster, and more power-efficient than using individual vacuum tube latches. Developed at the in England, the Williams tube provided the medium on which the first electronically stored-memory program was implemented in the (SSEM) computer, which first successfully ran a program on 21 June 1948.
In fact, rather than the Williams tube memory being designed for the SSEM, the SSEM was a to demonstrate the reliability of the memory. Was invented in 1947 and developed up until the mid-1970s. It became a widespread form of random-access memory, relying on an array of magnetized rings. By changing the sense of each ring's magnetization, data could be stored with one bit stored per ring. Since every ring had a combination of address wires to select and read or write it, access to any memory location in any sequence was possible.
Magnetic core memory was the standard form of memory system until displaced by solid-state memory in integrated circuits, starting in the early 1970s. (DRAM) allowed replacement of a 4 or 6-transistor latch circuit by a single transistor for each memory bit, greatly increasing memory density at the cost of volatility. Data was stored in the tiny capacitance of each transistor, and had to be periodically refreshed every few milliseconds before the charge could leak away. The Toscal BC-1411, which was introduced in 1965, used a form of DRAM built from discrete components. DRAM was then developed by in 1968. Prior to the development of integrated (ROM) circuits, permanent (or read-only) random-access memory was often constructed using driven by, or specially wound planes. Types of random-access memory The two widely used forms of modern RAM are (SRAM) and (DRAM).
In SRAM, a is stored using the state of a six transistor. This form of RAM is more expensive to produce, but is generally faster and requires less dynamic power than DRAM. In modern computers, SRAM is often used as. DRAM stores a bit of data using a transistor and capacitor pair, which together comprise a DRAM cell. The capacitor holds a high or low charge (1 or 0, respectively), and the transistor acts as a switch that lets the control circuitry on the chip read the capacitor's state of charge or change it. As this form of memory is less expensive to produce than static RAM, it is the predominant form of computer memory used in modern computers. Both static and dynamic RAM are considered volatile, as their state is lost or reset when power is removed from the system.
By contrast, (ROM) stores data by permanently enabling or disabling selected transistors, such that the memory cannot be altered. Writeable variants of ROM (such as and ) share properties of both ROM and RAM, enabling data to without power and to be updated without requiring special equipment. These persistent forms of semiconductor ROM include flash drives, memory cards for cameras and portable devices, and. (which can be either SRAM or DRAM) includes special circuitry to detect and/or correct random faults (memory errors) in the stored data, using. In general, the term RAM refers solely to solid-state memory devices (either DRAM or SRAM), and more specifically the main memory in most computers. In optical storage, the term is somewhat of a misnomer since, unlike or it does not need to be erased before reuse. Nevertheless, a DVD-RAM behaves much like a hard disc drive if somewhat slower.
Main article: The memory cell is the fundamental building block of. The memory cell is an that stores one of binary information and it must be set to store a logic 1 (high voltage level) and reset to store a logic 0 (low voltage level). Its value is maintained/stored until it is changed by the set/reset process. The value in the memory cell can be accessed by reading it.
In SRAM, the memory cell is a type of circuit, usually implemented using. This means that SRAM requires very low power when not being accessed, but it is expensive and has low storage density. A second type, DRAM, is based around a capacitor. Charging and discharging this capacitor can store a '1' or a '0' in the cell. However, the charge in this capacitor slowly leaks away, and must be refreshed periodically. Because of this refresh process, DRAM uses more power, but it can achieve greater storage densities and lower unit costs compared to SRAM. SRAM Cell (6 Transistors) Addressing To be useful, memory cells must be readable and writeable.
Read Only Memory
Within the RAM device, multiplexing and demultiplexing circuitry is used to select memory cells. Typically, a RAM device has a set of address lines A0. An, and for each combination of bits that may be applied to these lines, a set of memory cells are activated. Due to this addressing, RAM devices virtually always have a memory capacity that is a power of two. Usually several memory cells share the same address. For example, a 4 bit 'wide' RAM chip has 4 memory cells for each address. Often the width of the memory and that of the microprocessor are different, for a 32 bit microprocessor, eight 4 bit RAM chips would be needed.
Often more addresses are needed than can be provided by a device. In that case, external multiplexors to the device are used to activate the correct device that is being accessed.
Memory hierarchy. Main article: One can read and over-write data in RAM.
Many computer systems have a memory hierarchy consisting of, on-die caches, external, systems and or on a hard drive. This entire pool of memory may be referred to as 'RAM' by many developers, even though the various subsystems can have very different, violating the original concept behind the random access term in RAM. Even within a hierarchy level such as DRAM, the specific row, column, bank, channel, or organization of the components make the access time variable, although not to the extent that access time to rotating or a tape is variable. The overall goal of using a memory hierarchy is to obtain the highest possible average access performance while minimizing the total cost of the entire memory system (generally, the memory hierarchy follows the access time with the fast CPU registers at the top and the slow hard drive at the bottom). In many modern personal computers, the RAM comes in an easily upgraded form of modules called or DRAM modules about the size of a few sticks of chewing gum. These can quickly be replaced should they become damaged or when changing needs demand more storage capacity. As suggested above, smaller amounts of RAM (mostly SRAM) are also integrated in the and other on the, as well as in hard-drives, and several other parts of the computer system.
Other uses of RAM. Main article: Most modern operating systems employ a method of extending RAM capacity, known as 'virtual memory'. A portion of the computer's is set aside for a paging file or a scratch partition, and the combination of physical RAM and the paging file form the system's total memory. (For example, if a computer has 2 GB of RAM and a 1 GB page file, the operating system has 3 GB total memory available to it.) When the system runs low on physical memory, it can ' portions of RAM to the paging file to make room for new data, as well as to read previously swapped information back into RAM. Excessive use of this mechanism results in and generally hampers overall system performance, mainly because hard drives are far slower than RAM. Main article: Software can 'partition' a portion of a computer's RAM, allowing it to act as a much faster hard drive that is called a. A RAM disk loses the stored data when the computer is shut down, unless memory is arranged to have a standby battery source.
Shadow RAM Sometimes, the contents of a relatively slow ROM chip are copied to read/write memory to allow for shorter access times. The ROM chip is then disabled while the initialized memory locations are switched in on the same block of addresses (often write-protected). This process, sometimes called shadowing, is fairly common in both computers and. As a common example, the in typical personal computers often has an option called “use shadow BIOS” or similar. When enabled, functions that rely on data from the BIOS’s ROM instead use DRAM locations (most can also toggle shadowing of video card ROM or other ROM sections). Depending on the system, this may not result in increased performance, and may cause incompatibilities. For example, some hardware may be inaccessible to the if shadow RAM is used.
On some systems the benefit may be hypothetical because the BIOS is not used after booting in favor of direct hardware access. Free memory is reduced by the size of the shadowed ROMs.
Recent developments Several new types of, which preserve data while powered down, are under development. The technologies used include and approaches utilizing.
Amongst the 1st generation, a 128 ( 128 × 2 10 bytes) chip was manufactured with 0.18 µm technology in the summer of 2003. In June 2004, unveiled a 16 (16 × 2 20 bytes) prototype again based on 0.18 µm technology. There are two 2nd generation techniques currently in development: (TAS) which is being developed by, and (STT) on which, and several other companies are working. Built a functioning carbon nanotube memory prototype 10 (10 × 2 30 bytes) array in 2004. Whether some of these technologies can eventually take significant market share from either DRAM, SRAM, or flash-memory technology, however, remains to be seen.
Since 2006, ' (based on flash memory) with capacities exceeding 256 gigabytes and performance far exceeding traditional disks have become available. This development has started to blur the definition between traditional random-access memory and 'disks', dramatically reducing the difference in performance. Some kinds of random-access memory, such as 'EcoRAM', are specifically designed for, where is more important than speed. Memory wall The 'memory wall' is the growing disparity of speed between CPU and memory outside the CPU chip. An important reason for this disparity is the limited communication bandwidth beyond chip boundaries, which is also referred to as bandwidth wall. From 1986 to 2000, speed improved at an annual rate of 55% while memory speed only improved at 10%. Given these trends, it was expected that memory latency would become an overwhelming in computer performance.
CPU speed improvements slowed significantly partly due to major physical barriers and partly because current CPU designs have already hit the memory wall in some sense. Bardhan udry development microeconomics pdf book. Summarized these causes in a 2005 document. “First of all, as chip geometries shrink and clock frequencies rise, the transistor increases, leading to excess power consumption and heat. Secondly, the advantages of higher clock speeds are in part negated by memory latency, since memory access times have not been able to keep pace with increasing clock frequencies.
Third, for certain applications, traditional serial architectures are becoming less efficient as processors get faster (due to the so-called ), further undercutting any gains that frequency increases might otherwise buy. In addition, partly due to limitations in the means of producing inductance within solid state devices, (RC) delays in signal transmission are growing as feature sizes shrink, imposing an additional bottleneck that frequency increases don't address.” The RC delays in signal transmission were also noted in, which projected a maximum of 12.5% average annual CPU performance improvement between 2000 and 2014. A different concept is the processor-memory performance gap, which can be addressed by that reduce the distance between the logic and memory aspects that are further apart in a 2D chip. Memory subsystem design requires a focus on the gap, which is widening over time. The main method of bridging the gap is the use of; small amounts of high-speed memory that houses recent operations and instructions nearby the processor, speeding up the execution of those operations or instructions in cases where they are called upon frequently.
Multiple levels of caching have been developed to deal with the widening gap, and the performance of high-speed modern computers relies on evolving caching techniques. These can prevent the loss of processor performance, as it takes less time to perform the computation it has been initiated to complete. There can be up to a 53% difference between the growth in speed of processor speeds and the lagging speed of main memory access. In contrast, RAM can be as fast as 5766 MB/s vs 477 MB/s for an. Gallagher, Sean.
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Topics:. Summary: In interface design favor direct access to the user’s preferred item instead of forcing users to go through your content in a serial order. If you happened to be around in the 90s, when the web was invented, you may remember that.
In fact, “HTML” itself stands for “Hypertext Markup Language.” Hypertext made the web work as an interconnected media form: text that contains links (hyperlinks) to additional content that can be immediately accessed. The hypertext and hyperlink exemplify the direct-access paradigm and are a significant improvement over the more traditional, book-based model of sequential access. (Direct access can also be called random access, because it allows equally easy and fast access to any randomly selected destination. Somewhat like traveling by a Star Trek transporter instead of driving along the freeway and passing the exits one at a time, which is what you get with sequential access.) In a normal, physical book, the reader is supposed to read pages one by one, in the order in which they are provided by the author. For most books (fiction, at least), it makes little sense for the reader to turn directly page 256 and start reading there. Unless, of course, that is where the reader left off in their last reading session. Getting to page 256 in a 500-pages book poses a bit of a challenge, as we well know it, and each of us have their preferred method of dealing with it (be it a bookmark, a dog ear, or our own memory).
Tables of contents try to alleviate a book’s sequential-access problem by telling people what content is going to be found in the book and at which page. The user still has the problem of turning to the desired page number, but at least he doesn’t need to bother with parsing the content and deciding whether he’s found what he is looking for. By definition, however, the web embraces direct access.
Thus, it is disappointing to see sequential-access designs becoming increasingly popular nowadays. Costs and Benefits of Sequential Access But why is sequential access so bad? Simply because it forces the user to work harder than she needs to: she has to process all the content that sequentially precedes the piece of information that she is interested in.
Thus, sequential access increases. Sequential access increases interaction cost: the user has to inspect all the items that precede the item of interest in a list. With direct access, the user can focus on the element of interest without explicitly processing the items that come before it in the list.
Sequential access has two potential benefits:. Progressing linearly through an information space can be accomplished through particularly simple navigation controls: basically a “give me more” button.
However, designs like more than they help. You ought to design navigation controls that allow users more freedom without being overly complicated. If you know that users have been through the earlier steps in a sequence, you can build on that knowledge in explaining the next step. In practice, of course, and miss much of the information. So you can’t truly rely on users reading (much less understanding) all the earlier exposition, even if they have passed through it.
The benefits of sequential access are more hoped-for than they are real on most practical websites. In contrast, the costs are very real and are incurred every time. Examples of Sequential Access in User Interfaces Let’s take a look at a few examples of sequential access in modern interfaces. Carousels The carousel has always been a popular way to stick content on the front page without taking up too much space and has seen a resurgence with the advent of the iPad.
( and wanted to control the layout in the tiniest detail. As a result they often forewent vertical scrolling in favor of a card or carousel-like design.), but one big disadvantage is that they are based on sequential access: users must go through all the items in the carousel one by one in order to get to the last one. This interaction is inefficient and provides little: users generally have no information about what comes next. Although carousels may solve content-priority quarrels within the organization, they slow users down (at least in their more traditional incarnations). How can you make carousels more direct-access like?
If you cannot avoid them altogether, provide links to the stories in the carousels to let people select them in any order or, at least, present more than one item at once. Food52.com: Carousel items can be accessed directly by clicking the titles to the right of the image. Food52.com: The homepage contains a carousel that features 3 stories at the time on a desktop screen. This design has a lower interaction cost than one with 1 story per screen. That means that the interaction will be sped up (to access item number 5, users will have to change the carousel once with 3 items per screen instead of 5 times with 1 item per screen). Also, remember that carousels are ok only for short lists: users should be able to get to the last item in the list in 3–4 steps.
Search results or long lists never belong in carousels; as one of our users put it, “I don’t know what item 20 is, but I know that I will never find out.” Videos Even more than books, videos are the sequential-access medium par excellence: users must patiently watch a lot of video footage before getting to a piece of content that is relevant or interesting to them. That is why videos by themselves are not an ideal medium for instructional or informational content; although they can work great in conjunction with text, if they are the only method available to users, they are terribly inefficient. How can we fix them? Not all material needs to be in video format. If you provide a video, make sure that you also provide, if not a transcription, at least a detailed text summary that allows people to quickly scan the information for relevant details. Long Pages With the advent of, uncommonly long pages proliferate not only on mobile, but also on the desktop. A long page that contains a variety of content forces the user to scroll down with the hope that they will find something relevant.
Yes, but only if tempted by the promise of relevant content. If the page information is made of different, loosely related pieces of information, users have no way of knowing whether they must scroll for more or they should stop. They often err on the side of minimizing effort and stop before reaching a relevant piece of information. How to fix the issue? Avoid excessively long pages altogether. If you cannot, at least provide a: a linked page table of contents at the top of the page.
The mini-IA (whether from accordions or jump links) will tell people what to expect on the page, it will allow them to form a of the page, and will also facilitate direct access. Worldwildlife.org’s responsive design results in overly long pages that contain several types of content. The page has a mini-IA at the top in the large-screen version (left), but that mini-IA is unfortunately removed from the small-screen versions (right). Accessibility Screen readers and keyboard-only navigation exemplify another one of the pitfalls of sequential access.
These tools scan all the links on a page in a sequential manner. If the link of interest is somewhere in the middle of the link list, it may take forever to get to it and be able to select it. How can you improve the access to random content?
Code your designs to include to decrease the interaction cost (and the overall working-memory load.) Digital Magazines When iPad magazines first came around, they used to follow the physical-magazine mental model and eliminate all direct access and hyperlinks. Stories were referenced on the cover or in the table of contents, but they were not linked to. Luckily most publishers eventually realized that a lack of hyperlinks was a tremendous downside because it forced people to browse through stories as if they were using a paper version. King tubby dub conference. How can we fix the issue? Use hyperlinks. Selection from a Long List on Mobile On mobile devices we often encounter designs that favor selection versus typing; these designs are based on the assumption that typing is difficult with a small touchscreen keyboard.
As a result, users are sometimes forced to select an alternative from a long list of items — for instance, a list of years or countries. It is indeed generally easier to select than to type, but not if you have to scroll a lot to find the item of interest. (On desktops, often the names are all visible and it is ok to let people see them at once and select one from them.) How can we fix the issue?
Allow people to type 1–2 letters and offer suggestions based on those. Even though typing long words or phrases is painful, it’s not so bad just to enter the first character(s). People can still select, but now from a list that has been considerably narrowed down to only a few alternatives. (Like, this solution works if the names of the items in the list are known to the users, as it is often the case with countries, brands, and car makes.
) Letting users type the first few letters of the item that they are looking for (as in the Nordstrom app, right) is closer to a direct access implementation and far more efficient than forcing them to choose from a long list of alternatives (as in the autozone.com example, left). When Is Sequential Access Appropriate? Sequential access is the method of choice if you expect users to access all the content in a prescribed order. It forces users to accept your curated contribution and assumes that most users will be willing to do so. For works of fiction, many articles, or entertainment videos that assumption is accurate. If, on the other hand, people are likely to be unequally interested in all the content that you offer, use a direct access method to let them reach their goal faster.
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