Appendix A Your Portable Disk Drive Entire books could be written on the subject of disk drives, but this short appendix describes only a few matters of prudence and simple curiosity. The 3-1/2" Diskette Technology has advanced considerably in diskettes for personal computers. The diskettes used in you r PDD have more potential storage capacity than 5-1/4" Diskettes (which are still the standard for desktop personal computers), yet require less than half the area. A 3-1/2" diskette holds about 100 kbytes of data when used in your PDD; other disk drives can squeeze as much as 1.44 megabytes onto the same diskettes. Further, the 3-1/2" diskette is a step forward in ruggedness and reliability. No part of the magnetic surface is exposed, as a spring-loaded metal flap conceals the "window" in the firm plastic case. Be warned, however, that these diskettes are not hermetically sealed: avoid dust and dirt; do not leave diskettes scattered on a desktop under papers, etc.; and do not carry them in pockets of clothing that sheds (sports jackets, etc.) The other constant enemy of diskette data is magnetic fields. The wordless warnings on diskette boxes show horseshoe magnets, but remember that speakers, motors, and office paper-clip holders all contain magnets. You needn't be paranoid, but don't be careless either. The Disk-Drive Hardware The PDD comprises two basic moving parts: the motor and the head. The motor keeps the diskette spinning at a constant rate. The head glides over the diskette to read and write the data. The head can be in one of 40 different positions relative to the edge of the diskette; each of these 40 positions is called a "track". If you examine the magnetic surface of a used diskette, you may be able to distinguish lines of the tracks from friction with the head. Page 68 Appendix A Page 69 Diskettes must be "formatted" before the drive can successfully read or write data. We recommend formatting an entire box of diskettes as soon as the are purchased. Formatting uses up some of the diskette's storage ability for magnetic timing marks, so that the drive controller hardware can determine where on the track the head is at any point. After formatting, there are 2,560 bytes of capacity left on each track of a diskette. Further, it divides each track into two "sectors" of 1,280 bytes each. (Other drives, with more capacity, squeeze more bytes per track). The PDD Operating System It may surprise you that your PDD has software built into it - and contains both RAM and ROM hardware - in order to control the disk hardware. The PDD is known as an "intelligent" disk drive, since it decides on its own which sectors are available and which are full, whether a file with a certain name exists on the disk, etc. Thus, the POWR-DOS software merely sends codes conforming to those rules to allow the computer and the drive to communicate. There are advantages and disadvantages to this arrangement. The principle advantage is that code in the computer requires less room. The principle disadvantage is that the disk drive is more or less limited by its internal operating system - What it's designers intended for its use. In this case, this limitation means that the files on your PDD must be primarily sequential rather than random access. While it is possible to do random access, it is far less convenient than sequential access. There are several other interesting observations about the PDD operating system: * The PDD O/S uses a "directory" to keep track of files on a diskette. Every diskette has a directory, which makes up a sector on the outside track. Thus, an empty diskette holds 79 more sectors (of 1,280 bytes for a total of 101,120 bytes) * The directory cannot hold more than 40 names, limiting the diskette to 40 files maximum, regardless of size. * Each file uses at least one sector, no matter how small the file is. Appendix A Page 70 * The PDD does not create "empty" files, i.e., files of zero length. If you open a file for output and fail to write to it, it will not appear in the directory after closing it. * The PDD can only deal with one file at a time, whether for input or for output. * PDD file names can be longer than six characters and an extension. The actual limit appears to be something more like 24 characters, which can also include lower case. However, POWR-DOS limits names to six characters for compatibility with your computer's files, and to reduce the amount of space required for the POWR-DOS software.