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[the following article is from the August 1985 issue of COMPUTE! Magazine]
 
Monster Memory
 

(Continued, page 3 of 3)   If you're an experienced programmer, you might be puzzled by the search times described above. Even in superfast machine language on the 520ST's 68000 microprocessor, how can the computer search through nine million words in less than four seconds? When you request a search, the 520ST simply consults the index of search tables on the disc and rapidly locates each occurrence of the search phrase. In effect, the searching has already been done for it by the VAX.       The answer is that the computer can't. Instead, it refers to an extremely sophisticated index of search tables on the CD.       The search tables were compiled with a VAX minicomputer by Activenture, Inc., the company which is developing the CD-ROM software for Atari. First, the VAX built a dictionary by identifying every unique word in the encyclopedia -more than 140,000 words. Then it compiled search tables which cross-reference the dictionary to every occurrence of each word in the encyclopedia. In conventional book form, the finished index would occupy about 20 volumes. That means the index is nearly as long as the encyclopedia itself.       When you request a search, the 520ST simply consults the index of search tables on the disc and rapidly locates each occurrence of the search phrase. In effect, the searching has already been done for it by the VAX.       Retrieving the information is very fast, too, though not quite as fast as with hard disks. Because CDs were designed for storing music, which is played sequentially, their random-access capabilities are not as good as they could be. Still, they're much faster than most floppy disk drives. Data is stored on a CD in 270,000 records containing 2,048 bytes each; the average access time for a record is about one second. The greatest access time- which happens when the head must move from an extreme outside track to an inside track or vice versa-is three to four seconds. To keep this from happening very often, data is recorded on a CD on the inside tracks first, moving out ward. This takes advantage of the CD-ROM's very rapid track-to track access time. To read a nearby track, the player merely tilts a tiny mirror to refocus the laser rather than repositioning the entire head. (Each track of microscopic pits is only two microns-two millionths of a meter-wide).       Once the information is located, the CD-ROM feeds it to the computer at a rate of 75 records (150K) per second. To make sure the data arrives without errors, the CD-ROM's error-correction capabilities have been improved over that of a regular audio CD player. An audio player can be expected to pass one bad bit for every 10,000 bits-inaudible when listening to Beethoven or the Beatles, but not nearly good enough for computer storage. So CD-ROMs employ an error-checking and correction scheme that allocates an additional 288 bytes for each 2,048-byte record. That much overhead-more than 14 percent-would be wasteful on a floppy or hard disk, but CDs have room to burn. The improved error rate on a CD-ROM is at least as good as with a hard disk: about one bit error for every 1,000,000,000,000 to 1,000,000, 000,000,000 bits (one trillion to one quadrillion).

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see also: Report From The Summer Consumer Electronics Show
 
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