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COMPUTER BITS By JENNIFER BONNITCHA Floppy disc drives: formats & options Despite their limited storage capacity, floppy discs are still vital to your PC. You need them to transfer files to and from your computer and to back up data on your hard disc. A computer system can be divided into several main parts. First, the CPU (central processing unit) controls all parts of the computer system while carrying out the instructions of the currently active program. The main memory (Random Access Memory or RAM) ex- ecutes or runs the program by reading each instruction in sequence and performing the requested action. Any data used in the calculations and the results are stored in memory. The remainder of the computer system is concerned with the way in - - - - - - - - - (5¼ I n c h ) - - - - - - - - - - - Sealed protective jacket Liner Fig.1: a floppy disc is made of flexible mylar which is coated on both sides with a thin layer of metallic oxide particles and enclosed in a protective jacket. In operation, the read/write heads move across the disc surface at the head aperture. 68 SILICON CHIP which you communicate with the CPU and RAM. Data and programs are entered via some input device such as a keyboard or disc drive, while the results are displayed on an output device such as a monitor or printer. There are hundreds of operations to keep track of during the operation of something like a disc drive, so the I/O (Input/Output) interfaces take care of the details of connecting an input or output device to the CPU. Allowing the I/O interface to worry about these details translates to smaller programs (at least in theory), since the program need not concern itself with the exact whys and wherefores. Inside most system units there js sufficient space to mount two 5 ¼-inch floppy disc drives. Aithough a floppy disc drive can take milliseconds to jump from data area to data area on the disc, the CPU can carry out thousands of calculations in that time. Thus a program which needs to go back and forth to the disc during calculations will run at a very sluggish pace indeed. A hard disc drive rotates constantly at 3600rpm and it takes about three milliseconds for the read/write head to move from track to track. By contrast, a floppy disc rotates at 300rpm only during read/write operations and it takes about eight milliseconds to move. For most users, a hard disc is the usual place to store all those precious bytes but not everyone has the necessary dollars for this luxury. So from here on in, let's concentrate on the increasingly maligned floppy disc. connector on the signal ea ble from the floppy disc controller card. If you want the new drive to be drive 0, swap the connector with the one currently on the other disc drive. Switch 1 O 2 3 4 5 6 7 8 ; □ LJ □□□□□□ Position 1-7-8 2 3-4 5-6 System switches Function Number of 5'/4inch diskette drives installed Reserved for co-processor Amount of RAM on system board Type of monitor you are using Switch 2 Position 1-2-3-4 5-6-7-8 Function Amount of memory options installed Always in the off position Fig.2,3: these diagrams show how DIP switch 1 is used to select the number of floppy disc drives in an IBM PC. Check the manual for your particular machine, as there can be differences between models. Number of 5-1/4" diskette drives installed Switch I ODrives I Drive 2-Drives The floppy disc A floppy disc (Fig .1) is inade of flexible Mylar coated on both sides with a thin layer of metallic oxide particles and enclosed in a protective jacket. The centre hole ,is generally reinforced around its edge since it is here that the disc is clamped by the drive mechanism. The disc's recording surface can store information as binaryencoded data (ls and 0s), the value at each point on the disc depending on the magnetic polarity. In operation, the read/write heads move across the disc surface, either reading or writing data as required. Data is stored on the disc by delivering a series of magnetic pulses from the heads as they move across the disc surface. Conversely, data is read from the disc by deter- Switch 2 1 □□0000 □□ 11000000001 ,~□DODD □□ I l □□ODODDDI l □ DDOOD □□ l l □ DDODODOI mining the polarity of the magnetic field at various points on the disc. The disc drive or read/write head differs considerably for floppy disc drives, high capacity drives and fixed discs. Installation It's usually quite easy to add an extra floppy disc drive to your PC if you currently only have one drive. To do this, first remove the system top cover and faceplate, then slide the drive through the front panel. Depending on the make of your computer, the drive will either be secured solely by screws or you may need to attach plastic rails to the sides of the drive. Slide the drive into the system unit until it is flush with the front panel, then install the mounting screws. Now attach the power supply connector and plug in the spare The lower left section of the XT motherboard contains sockets for the memory chips while an 8-way DIP (dual in-line package) switch is located below and to the right of the expansion slots (the PC has two 8-way DIP switches). These switches define which options - such as the number of disc drives, the amount of memory or the presence of a maths co-processor - are installed or connected to the PC. Depending on the make and model of your computer, you may need to change these DIP switch settings. Figs.2 & 3 show the switch settings for the number of disc drives in the IBM PC while Figs.4 & 5 are for the IBM XT. Remember, however, that these figures are a ppropriate to the "genuine article" (ie, IBM machines) only. If you have a PC-compatible, you are strongly advised to check the manual to determine the correct switch positions. It is a good idea to write down the current DIP switch settings before you make any changes. That way, if you encounter difficulties with the new installation, you can at least go back to the old configuration. Formatting When you have moved the DIP switches to their appropriate positions, replace the system unit cover. Now the fun of testing your installation prowess begins! All discs must be formatted before use. The process of formatting prepares the disc for use by DOS by defining a structure of concentric tracks and a filing system to keep track of just what is stored on the disc. The formatting process also analyses any defective areas on the disc and prepares the disc for file storage by establishing a directory and the File Allocation Table (FAT). When Format divides the disc into tracks, each track is then further divided into sectors. The disc capacity is measured in kilobytes (Kb), with 1Kb represenNOVEMBER 1989 69 Switch I 2 3 5 4 I Drive 6 7 8 of~ (j □ □ □ □ □ □ Position Function For normal operation is off Co-processor Amount of RAM on system board lype of monitor you are using Number or 5¼-inch diskette drives installed 2 3-4 5-6 7-8 ting 1024 bytes. Generally one byte is equivalent to one character so it is a simple matter to calculate the amount of space required to store one page of single-spaced A4 typing. In practice, 80 columns of type over 25 lines requires approximately 2Kb of storage space. Early IBM disc drives were single-sided only with a storage capacity of 160Kb. The data was recorded on 40 concentric tracks, with each track containing eight sectors and each sector capable of storing 512 bytes of information. Thus, 40 tracks x 8 sectors/track x 512 bytes/sector gave a formatted capacity of 163,840 bytes. DOS 1.1 extended disc I/O support to the double-sided disc with a formatted capacity of 327,680 bytes. Introduction of the XT also saw the introduction of DOS 2.0. This enabled the formatting of either single sided or double sided discs using either 8 or 9 sectors per track. Thus, a single sided disc could now store 184,320 bytes and a double sided disc could store 368,640 bytes of data (assuming nine sectors per track). 2 DrNeS I IOOOOOOU□ I ID DODOO □□ Fig.4,5: the XT machine only has a single configuration switch. Positions 7 & 8 select the number of floppy disc drives installed as shown above. Further advances, both to the operating system and to drive technology, now mean that the 5¼-inch disc on the AT is capable of storing 1,228,800 bytes (1.2 Megabytes) of information - a far cry from the original 160Kb! DOS 3.0 formats the high capacity disc to 80 tracks with 15 sectors per track. Subsequent introduction of the 3 ½-inch micro-disc has pushed disc storage limits further by storing the same amount of information in an even smaller area. The difference between the storage capacity of a hard disc and a floppy disc is a function of the number of tracks and sectors. The 360Kb floppy disc has 9 sectors per track with 40 tracks per side; the XT's 10Mb hard disc consists of two platters and each of the four surfaces formats to 306 tracks. Finally, the AT's 20Mb disc has four surfaces, each of which format to 615 tracks. Both the XT and AT hard discs have 17 sectors per track compared with the 15 sectors per track for the high capacity floppy disc (see Table 1). The AT-style computer generally Table 1: Floppy Disc Formats Sides 1 1 2 2 2 Tracks Sectors/Track 40 8 40 9 40 8 40 9 15 80 Storage Capacity (Bytes) 163,840 184,320 327,680 368,640 1,228,800 Table 1: floppy disc capacity has increased steadily with advances in system and drive technology. The 5¼-inch floppy disc for the AT machine is now capable of storing 1.2Mb of information. 70 SILICON CHIP has a high capacity disc drive which is capable of reading and writing to the low capacity 360Kb disc. However the 360Kb drive cannot format, read from or write to the high capacity disc. A good rule of thumb when formatting a 360Kb disc in a high capacity drive is to use a fresh disc with the "/4" option. Format switches The Format command has several switches available for use. Note that individual manufacturers may further customise the options available with the Format command. Fig.6 shows the result of formatting a typical disc. The Format command is as follows: FORMAT Drive:/S/1/8/V/B/4 Note that everything after "Drive:" is optional, depending on how you wish to format the disc. Here's what the various options mean: Drive: this specifies the drive containing the disc you want to format. IS supported by all DOS versions, this switch tells DOS to format the disc and include the operating system files. These system files are IQ.SYS (IBMIO.SYS); MSDOS.SYS (IBMDOS.SYS); & COMMAND.COM. The *.SYS files are hidden and are "seen" only when the CHKDSK command is used. when this switch is includ/1 ed, DOS 1.1 and above formats the floppy disc as single sided. If it is not included in the command, DOS formats discs as double sided, double density. DOS 2.0 and above uses /8 this switch to format a floppy disc with 8 sectors per track instead of the usual 9. It may not be used when formatting a hard disc. DOS 2 .0 and a hove IV enables the writing of a magnetic label to the disc. DOS prompts the user to Sector 4 Sector 5 Sector 3 Sector 7 Sector 2 Track 39 Head aperture provides access to each sector on a track as the disk spins. Fig.6: this 40-track disc has been formatted into eight sectors for a capacity of 327,680 bytes. Each sector appears in sequence at the head aperture as the disc spins. /B enter a volume label of up to 11 characters. Note that the label serves no practical purpose; it simply identifies the disc. with this switch, DOS 2.0 and above formats a disc with 8 sectors per track and creates two dummy files in the root directory to leave space for later addition of the operating system files. It is also used to create a disc onto'which any version of DOS (1.0, 1.1 or 2.0) can be placed. It may NOT be used in formatting the hard disc or with the /S or /V switches. /4 this option, introduced in DOS 3.0, is for. formatting 360Kb discs in a high capacity drive. Caution: 360Kb discs formatted in a high capacity drive may not be read or written to reliably in the 360Kb drive. Generally, however, there are few problems. If you own an AT, you can always consider installing an additional 360Kb drive in your system unit. Dummy data When a disc is formatted, any data stored on the disc is destroyed. The general procedure is that DOS Table 2: Format Options Disk Capacity 320/360Kb 1.2Mb Hard Disk Parameters /S /1 /8 N /B /4 /SN /SN Table 2: the format options available for various disc capacities. Each option is explained in the text. puts dummy data on the disc and then reads it back one track at a time. Any defective tracks are flagged as bad tracks in the File Allocation Table (FAT) to prevent them from being used for subsequent data storage. When writing and reading is finished, the Format command writes a new DOS boot sector then initialises the first FAT by filling it with zeros. The FAT begins with a Media Descriptor Byte (MDB) which tells DOS and programs like CHKDSK what type of disc it is, together with the Bad Track information. Generally, DOS maintains two copies of the FAT so that if one is physically damaged, it can use the other. Note that DOS, in the event of conflict, can't decide which of the two FATs is correct. The blank Root Directory follows the last copy of the FAT since this is the main directory from which all others branch and yes, a floppy disc may have a directory structure. The Root Directory on the floppy disc typically holds from 64 to 224 entries, depending on the disc type. From the Root Directory to the end of the disc, data storage takes place. The CHKDSK command The CHKDSK command is used to determine the total number of bytes available on the disc. It must be issued from a drive or directory where system files are present. For example, if the system files are the C drive, the command is: C:)CHKDSK A: The system then displays the following information: 362496 bytes total disc space 362496 bytes available on disc xxxxxx bytes total memory xxxxxx bytes free Note that if a disc is not formatted, the directory and file allocation table are not present. Thus, if you enter the CHKDSK command, DOS will report the •'Diskette not initialised" error message. Note also that the figures for "bytes total memory" and "bytes free" do not refer to the floppy disc but to the amount of available RAM in your computer. ~ NOVEMBER 1989 71