Classic Computer Magazine Archive COMPUTE! ISSUE 52 / SEPTEMBER 1984 / PAGE 30

Questions Beginners Ask

Tom R. Halfhill, Staff Editor

Are you thinking about buying a computer for the first time, but you don't know much about computers? Or maybe you just purchased a computer and are still a bit baffled. Each month in this column, COMPUTE! will answer some questions often asked by beginners.

Q How long does a floppy disk last? If I have some disks that are more than a year old, should I copy them onto new disks? How can you tell if a disk is wearing out?

A The lifetime of a floppy disk largely depends on how heavily it is used and how carefully it is handled.

It's doubtful that you would actually wear out a disk with normal home use. One well-known manufacturer of floppy disks (3M-Scotch) guarantees its products for 3.5 million passes per track. 3M calculates that if you updated the disk (rewrote every track) once every hour, it would take more than 200 years to exceed 3.5 million passes. Not many home computer owners would use a disk that heavily. Some disk-intensive business programs which run 40 hours a week might approach that many passes in much less time than 200 years, in which case an alternative should be considered—perhaps a hard disk or a RAM disk. But the casual or moderate user has little to worry about. At COMPUTE! we have some disks that get fairly heavy use for storing word processing files, and after two years they're still going strong.

A floppy disk's lifetime can be drastically shortened, however, if it isn't handled and stored correctly. In careless hands a disk might not even last ten minutes. Disks are particularly vulnerable to magnetic fields from monitors and TV sets, spilled drinks, cigarette ashes, extremes of heat and cold, heavy weights, ballpoint pens used to mark on labels, and even chemical fumes.

Unfortunately, the first clue which indicates a disk is going bad probably will be a lost program or a lost file. One day you'll confidently slip the disk into the drive and discover that a certain file won't load. That's why you should always keep at least one backup copy of important files on another disk or tape.

There is one clue you can watch for, though—if you regularly clean your disk drive's read/write head and notice one day that it's much dirtier than usual, it could mean that the magnetic coating on one of your disks is starting to flake off. It could also mean that you bought some poor quality disks.

Q On my Commodore 64, when using the command PRINT 7↑2, the computer will come up with the answer 49.0000001. Is this a bug in the computer?

A Not really. It's a rounding error caused by the way the Commodore 64 calculates exponentiation. When you ask the computer to figure 72, it doesn't actually multiply 7*7. Instead, it uses logarithmic tables. Rather than engaging in a long discussion of higher mathematics, let's just say that these tables sometimes lead the computer to an answer which is slightly off. If 49.0000001 isn't accurate enough for your purposes, you can either PRINT 7*7 or PRINT INT(7↑2), which extracts the integer from the result.

The Commodore 64 isn't the only microcomputer which suffers from this problem by any means. Certain math operations will result in very small rounding errors on anything from an Atari to an IBM PC. On an Atari with the BASIC cartridge, for example, PRINT 2^2 (equivalent to PRINT 2↑2 on a Commodore) comes out to 3.99999996, while PRINT 2*2 yields the correct 4. The problem was fixed with the BASIC built into the Atari 600XL and 800XL.

The quirk which leads to this kind of problem is that computers perform all of their math in binary. When floating-point (moving decimal point) numbers are converted to binary and then back to decimal, small errors can accumulate.

Q I recently found several old 60-minute cassette tapes. Are the 60- and 30-minute cassette tapes acceptable for program recording? Is there any command or program which can be used to erase these tapes? They contain music and talk.

A Theoretically, longer tapes are thinner and thus more susceptible to stretching and print-through (signals leaking through the tape and imprinting on adjacent layers of tape). But we've never had any problems saving programs on 30- or 60-minute cassettes. If they are good-quality tapes, they should work as well as the 10- or 20-minute cassettes generally sold for data recording. However, the longer tapes could be slightly less convenient if you store many programs on the same cassette. You'd have to wind the tape further to locate the program you want to load. But even this is a negligible problem if you jot down the tape counter numbers.

It isn't really necessary to erase the old material on the tape before recording your programs. The recorder's erase head will wipe it out as you record. However, your letter indicates you have an Atari 400 computer. The Atari Program Recorder has a stereo read/write head because the Atari can accommodate an audio track that plays through the TV speaker. When you load your program, you'll hear the old audio material along with the bleeps of the data. By typing POKE 65,0 before loading the tape, the bleeps will be silenced and you'll hear the audio only.

However, if you do want to erase a tape before using it, the best method is to use a bulk tape eraser (available at electronic supply stores). Another way is to insert the tape into an audio cassette recorder, press the record button, and disable the microphone by switching it off or sticking a null plug into the microphone jack. If the recorder has level controls, turn them all the way down.

With an Atari Program Recorder, you can erase tapes by pressing the record and play buttons and typing POKE 54018,52. This switches on the cassette motor from the computer. You can turn off the motor by typing POKE 54018,60.