{"id":288,"date":"2020-08-18T19:23:47","date_gmt":"2020-08-18T20:23:47","guid":{"rendered":"http:\/\/www.linux-tutorial.info\/?page_id=77"},"modified":"2020-08-22T19:25:59","modified_gmt":"2020-08-22T20:25:59","slug":"this-is-the-page-title-toplevel-123","status":"publish","type":"page","link":"http:\/\/www.linux-tutorial.info\/?page_id=288","title":{"rendered":"CD-ROMS"},"content":{"rendered":"\nCD-ROMS<\/title>\n<p>\nLinux distribution media is becoming more and more prevalent on CD-ROMs. One\n<glossary>CD-ROM<\/glossary> takes a lot less space than 50 floppies or even one\nQIC tape, so the media is easier to handle. In fact, I would be hard-pressed to\nfind a version that is still being distributed on floppies. Added to this,\nCD-ROMs are significantly faster than either floppies or tape media.\n<\/p>\n<p>\nAnother important aspect of CD-ROMs when it comes to installing media is\ntheir size. Therefore, it is possible to get numerous products on the\n<glossary>CD-ROM<\/glossary>. You can get a single CD-ROM that contains a\nrunnable copy of Linux and the source code with room to spare.\n<\/p>\n<p>\nCD-ROMs, in fact CD-ROMs technology in general, have always fascinated me. I\nam amazed that you could get so much information into such a small place and\nstill have such quick access to your data.\n<\/p>\n<p>\nThe basic principle behind data storage on a <glossary>CD-ROM<\/glossary>\nis really nothing more than Morse code. A series of light and dark (dots and\ndashes) compose the encoded information on the disk. Commercial CD-ROMs,\nwhether music or data, almost universally have data on one side of the disk.\nAlthough there is nothing technologically preventing a CD-ROM from having a flip\nside, current convention limits data to just a single side. This is enough when\nyou consider that you can get more than 600Mb of data on a single CD-ROM. As the\ntechnology improves, the amount is steadily increasing. In addition, certain\nmanufacturers are working on dual-sided CD-ROMs.\n<\/p>\n<p>\nOn the surface of the disk are a series of dents, or holes, called “lands.”\nThe areas between the lands are called “pits.” A laser is projected onto the\nsurface of the disk and the light is either reflected by the pits or scattered\nby the lands. If reflected, the light reaches a light-sensing receptor, which\nthen sends an electrical <glossary>signal<\/glossary> that is received by the\ncontrol mechanism of the <glossary>CD-ROM<\/glossary> drive. Just as the pattern\nof alternating dots and dashes forms the message when using Morse code, it is\nthe pattern of reflected light and no light that indicates the data stored on\nthe disk.\n<\/p>\n<p>\nWhen I first thought of CD-ROMs, I conceptualized them as being like WORM\n(Write-Once, Read-Many) drives, which they are, somewhat. I visualized them as\nbeing a read-only version of a hard disk. However, after looking more closely at\nthe way data is stored, I saw that CD-ROMs have less in common with hard disks.\n<\/p>\n<p>\nAs you remember from our discussion of hard disks, each surface is composed\nof concentric rings called tracks, and each track is divided into sectors. The\ndisk spins at a constant speed as the heads move in and out across the drives\nsurface. Therefore, the tracks on the outer edges move faster than those on the\ninside.\n<\/p>\n<p>\nFor example, take a track that is a half-inch away from the center of the\ndisk. The diameter of the circle representing the track is one inch, so the\nradius of that circle is approximately 3.1415 inches. Spinning 60 times a\nsecond, the track goes at a speed of about 190 inches per second. Now, take a\ntrack at one inch from the center, or twice as far. The diameter of the circle\nrepresenting that track is 6.2830 inches. It, too, is going around at 60\nrevolutions per second. However, because it has to travel twice as far in each\nrevolution, it has to go twice as fast.\n<\/p>\n<p>\nA <glossary>CD-ROM<\/glossary>\nisn’t like that. CD-ROMS rotate in a manner called “constant linear velocity.”\nThe motor keeps the CD-ROM moving at the same speed, regardless of where the\nCD-ROM reader is reading from. Therefore, as the light detector moves inward,\nthe disks slows down so that each revolution takes the same amount of time per\ntrack.\n<\/p>\n<p>\nLets look at hard disks again. They are divided into concentric tracks that\nare divided into sectors. Because the number of sectors per track remains\nconstant, the sectors must get smaller toward the center of the disk (because\nthe circumference of the circle representing the track is getting smaller as you\nmove in).\n<\/p>\n<p>\nAgain, a <glossary>CD-ROM<\/glossary>\nisn’t like that. Actually, there is no reason why it should work that way. Most\nCD-ROMs are laid out in a single spiral, just like a phonograph record. There\nare no concentric circles, so there is no circumference to get smaller. As a\nresult, the sectors in a CD-ROM can remain the same size no matter what. The\nadded advantage of sectors remaining the same size means there can be more on\nthe disk and therefore more data for the user.\n<\/p>\n<p>\nCurrently, CD-ROMS have a capacity of <i>at least <\/i>650Mb, although I am\naware of some that are already more than 700Mb. Several companies are currently\nworking on technology to get even more out of them. The simplest technology\ninvolves making the <glossary>CD-ROM<\/glossary> writable on <i>both<\/i> sides,\nwhich simply doubles the capability. Others involve storing the data in\ndifferent layers on the CD-ROM and using light that is polarized differently to\nread the different layers.\n<\/p>\n<p>\nThe single-speed <glossary>CD-ROM<\/glossary>\ndrives are the oldest and have a transfer rate of about 150Kb a second.\nRecently (as of this writing), eight-speed CD-ROM drives are available. I often\nsee both four-speed and six-speed drives sold in machines, but the single- and\ndouble-speed drives are slowly dying out.\n<\/p>\n<p>\nMost CD-ROMS are formatted with the ISO-9660 format, which allows them to be\nread by a wide range of machines, including DOS\/Windows, Macintosh, as well as\nLinux and other <glossary>UNIX<\/glossary> dialects. The shortcoming, though, is\njust that: short, as in short file names. The ISO-9660 format only recognizes\nthe <glossary>DOS<\/glossary> 8.3 file names, so the long file names that we have\ngrown to love in UNIX are not possible. As a result, the Rock Ridge Extensions\nto the ISO-9660 format allow many of the typical UNIX file stuff, like longer\nfile names, file ownership, and even symbolic links, etc.\n<\/p>\n<p>\nAnother standard is the PhotoCD, which was developed by Kodak to store\nphotographic information. Not every <glossary>CD-ROM<\/glossary> drive can access\nthese CD-ROMs, which require special software. (As a side note, one system\n<glossary>administrator<\/glossary> working on the PhotoCD project at Kodak had\nLinux installed to do <glossary>network<\/glossary> monitoring!)<\/p>\n<p>\nCD-ROMs first became popular as <glossary>SCSI<\/glossary>\ndevices. However, because most people could not afford a SCSI <glossary>host<\/glossary>\nadapter, companies began producing <glossary>CD-ROM<\/glossary>\ndrives that ran on <glossary>IDE<\/glossary>\ncontrollers, and therefore looked just like a hard disk, or would run on their\nown controllers. Some even enabled you to run the CD-ROM off a sound card or\nthrough your parallel port. For details on how to get these to work correctly,\nsee the CD-ROM <glossary>HOWTO<\/glossary>.\n<\/p>\n","protected":false},"excerpt":{"rendered":"<p>CD-ROMS Linux distribution media is becoming more and more prevalent on CD-ROMs. One CD-ROM takes a lot less space than 50 floppies or even one QIC tape, so the media is easier to handle. In fact, I would be hard-pressed … <a href=\"http:\/\/www.linux-tutorial.info\/?page_id=288\">Continue reading <span class=\"meta-nav\">→<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-288","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/288","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=288"}],"version-history":[{"count":1,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/288\/revisions"}],"predecessor-version":[{"id":516,"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=\/wp\/v2\/pages\/288\/revisions\/516"}],"wp:attachment":[{"href":"http:\/\/www.linux-tutorial.info\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=288"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}