8. Elaborate Access Time and Seek Time Higher performance inhard disk drivescomes from devices which have faster performance characteristics. This device is actually affected by some other electrically based element that still causes a measurable delay when isolating and measuring that attribute.[3]These performance characteristics can be grouped into two categories:access timeanddata transfer time (or rate).Access time[edit]Theaccess timeorresponse timeof a rotating drive is a measure of the time it takes before the drive can actuallytransfer data. The factors that control this time on a rotating drive are mostly related to the mechanical nature of the rotating disks and movingheads. It is composed of a few independently measurable elements that are added together to get a single value when evaluating the performance of a storage device. The access time can vary significantly, so it is typically provided by manufacturers or measured in benchmarks as an average.[4][5]For SSDs this time is not dependent on moving parts, but rather electrical connections to solid state memory, so the access time is very quick and consistent.[6]Most testing and benchmark applications do not draw a distinction between rotating drives and SSDs so they both go through the same measurement process.The key components that are typically added together to obtain the access time are:[2][7] Seek time Rotational latency Other Command processing time Settle timeSeek time[edit]See also:Hard disk drive: Seek timeWith rotating drives, theseek timemeasures the time it takes the head assembly on the actuator arm to travel to the track of the disk where the data will be read or written.[7]The data on the media is stored in sectors which are arranged in parallel circular tracks (concentricorspiraldepending upon thedevice type) and there is an actuator with an arm that suspends a head that can transfer data with that media. When the drive needs to read or write a certain sector it determines in which track the sector is located. It then uses the actuator to move the head to that particular track. If the initial location of the head was the desired track then the seek time would be zero. If the initial track was the outermost edge of the media and the desired track was at the innermost edge then the seek time would be the maximum for that drive.[8][9]Seek times are not linear compared with the seek distance traveled because of factors of acceleration and deceleration of the actuator arm.[10]A rotating drive'saverage seek timeis the average of all possible seek times which technically is the time to do all possible seeks divided by the number of all possible seeks, but in practice it is determined by statistical methods or simply approximated as the time of a seek over one-third of the number of tracks.[7][8][11]Average seek time ranges from under 4msfor high-end server drives,[12]to 15ms for mobile drives, with the most common mobile drives at about 12ms[13]and the most common desktop drives typically being around 9ms.The first HDD[14]had an average seek time of about 600 ms, and by the middle 1970s, HDDs were available with seek times of about 25ms.[15]Some early PC drives used astepper motorto move the heads, and as a result had seek times as slow as 80120ms, but this was quickly improved byvoice coiltype actuation in the 1980s, reducing seek times to around 20ms. Seek time has continued to improve slowly over time.The other two less commonly referenced seek measurements aretrack-to-trackandfull stroke. The track-to-track measurement is the time required to move from one track to an adjacent track.[7]This is the shortest (fastest) possible seek time. In HDDs this is typically between 0.2 and 0.8ms.[6]The full stroke measurement is the time required to move from the outermost track to the innermost track. This is the longest (slowest) possible seek time.[8]With SSDs there are no moving parts, so a measurement of the seek time is only testing electronic circuits preparing a particular location on the memory in the storage device. Typical SSDs will have a seek time between 0.08 and 0.16ms.

2. Explain The Impact Printer and Dot Matrix Printer

In computing, aprinteris aperipheralwhich makes a persistent human-readable representation of graphics or text on paper or similar physical media.The world's first computer printer was a 19th-century mechanically driven apparatus invented byCharles Babbagefor hisdifference engine.[1]Impact printers[edit]Impact printersrely on aforcibleimpact to transfer ink to the media. The impact printer uses a print head that either hits the surface of the ink ribbon, pressing the ink ribbon against the paper (similar to the action of atypewriter), or hits the back of the paper, pressing the paper against the ink ribbon (theIBM 1403for example). All but thedot matrix printerrely on the use offully formed characters,letterformsthat represent each of the characters that the printer was capable of printing. In addition, most of these printers were limited to monochrome, or sometimes two-color, printing in a single typeface at one time, althoughboldingandunderliningof text could be done by "overstriking", that is, printing two or more impressions in the same character position. Impact printers varieties include, typewriter-derived printers, teletypewriter-derived printers, daisy wheel printers, dot matrix printers and line printers. Dot matrix printers remain in common use in businesses where multi-part forms are printed, such as car rental services.An overview of impact printing[5]contains a detailed description of many of the technologies used.

Dot Matrix printers:Dot matrix printers can either becharacter-based or line-based (that is, a single horizontal series of pixels across the page), referring to the configuration of the print head.In the 1970s & 80s, dot matrix printers were one of the more common types of printers used for general use, such as for home and small office use. Such printers normally had either 9 or 24 pins on the print head (early 7 pin printers also existed, which did not print descenders). 24-pin print heads were able to print at a higher quality. Once the price of inkjet printers dropped to the point where they were competitive with dot matrix printers, dot matrix printers began to fall out of favor for general use.Some dot matrix printers, such as the NEC P6300, can be upgraded to print in colour. This is achieved through the use of a four-colour ribbon mounted on a mechanism (provided in an upgrade kit that replaces the standard black ribbon mechanism after installation) that raises and lowers the ribbons as needed. Colour graphics are generally printed in four passes at standard resolution, thus slowing down printing considerably. As a result, colour graphics can take up to four times longer to print than standard monochrome graphics, or up to 8-16 times as long at high resolution mode.Dot matrix printers are still commonly used in low-cost, low-quality applications likecash registers, or in demanding, very high volume applications likeinvoiceprinting. The fact that they use an impact printing method allows them to be used to print multi-part documents usingcarbonless copy paper, like sales invoices andcredit cardreceipts, whereas other printing methods are unusable with paper of this type. Dot-matrix printers are now (as of 2005) rapidly being superseded even as receipt printers.