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266 Chap. 8 File Processing and External SortingMedium 1996 1997 2000 2004 2006 2008 2011RAM $45.00 7.00 1.500 0.3500 0.1500 0.0339 0.0138Disk 0.25 0.10 0.010 0.0010 0.0005 0.0001 0.0001USB drive – – – 0.1000 0.0900 0.0029 0.0018Floppy 0.50 0.36 0.250 0.2500 – – –Tape 0.03 0.01 0.001 0.0003 – – –Solid State – – – – – – 0.0021Figure 8.1 Price comparison table for some writable electronic data storagemedia in common use. Prices are in US Dollars/MB.Access Memory (RAM), while secondary storage refers to devices such as harddisk drives, solid state drives, removable “USB” drives, CDs, and DVDs. Primarymemory also includes registers, cache, and video memories, but we will ignorethem for this discussion because their existence does not affect the principal differencesbetween primary and secondary memory.Along with a faster CPU, every new model of computer seems to come withmore main memory. As memory size continues to increase, is it possible that relativelyslow disk storage will be unnecessary? Probably not, because the desire tostore and process larger files grows at least as fast as main memory size. Pricesfor both main memory and peripheral storage devices have dropped dramaticallyin recent years, as demonstrated by Figure 8.1. However, the cost per unit of diskdrive storage is about two orders of magnitude less than RAM and has been formany years.There is now a wide range of removable media available for transferring dataor storing data offline in relative safety. These include floppy disks (now largelyobsolete), writable CDs and DVDs, “flash” drives, and magnetic tape. Optical storagesuch as CDs and DVDs costs roughly half the price of hard disk drive spaceper megabyte, and have become practical for use as backup storage within the pastfew years. Tape used to be much cheaper than other media, and was the preferredmeans of backup, but are not so popular now as other media have decreased inprice. “Flash” drives cost the most per megabyte, but due to their storage capacityand flexibility, quickly replaced floppy disks as the primary storage device fortransferring data between computer when direct network transfer is not available.Secondary storage devices have at least two other advantages over RAM memory.Perhaps most importantly, disk, “flash,” and optical media are persistent,meaning that they are not erased from the media when the power is turned off. Incontrast, RAM used for main memory is usually volatile — all information is lostwith the power. A second advantage is that CDs and “USB” drives can easily betransferred between computers. This provides a convenient way to take informationfrom one computer to another.