Lecture

Lesson Plan 7 Addendum



Introduction to floppy drives.



Floppy drives started in the early 70's and earned their name from the construction of the platters. The original floppies were 7 1/2" in diameter and truly flopped when handled. Naturally one of the disadvantages of these units was the fact that when people handled them, the data would be destroyed easily. Show old flopppies. The next generation of floppies measured 5 1/4" and held an astounding 360kb of data. They were designated as DS-DD (double sided - double density) which means that the technology of imprinting the small bits of iron ferrite allowed closer spacing of the bits on the disk thus increasing its capacity. They were still subject to crimping and misuse. The next generation which came along was the one you may be familiar with, the hard sided floppy. The 3 1/4" floppy disk covered with a plastic shell enormously aided in the handling ease of the disk. The first disks were 720 kb, then 1.44 kb and finally 2.88 kb. The 2.88 went away from lack of sales.





Introduction to floppy disk drive geometry - floppy drives platters are arranged with 80 tracks. These tracks are concentric circles of iron ferite that do not spiral such as a record platter but are one closed loop around the platter. This track is divided into 18 sectors which are the basic unit of measure for any disk drive even today. Each sector contains 512 bytes of data. By doing some manipulation of the data, we find that 80 tracks x 18 sectors per track yields 1440 total sectors on each side of the disk. This is refered to as clusters and there are 2880 on both sides of the platter. Multiply 2880 clusters by 512 bytes of data per sector and we get the total capacity of a drive, 1.44 mbytes. Illustrate on board this geomtry. The disks are made out of mylar and imprinted with small bits of iron ferrite material. The iron ferrite is magnitized on one direction for a bit of data and magnetized on the opposite direction for a zero bit of data. The iron ferrite is read by a read/write head as the platter spins. By adding a voltage to the bit (or not adding a voltage) data is recorded on the disk. Each piece of data is assigned to each sector. If the data is smaller than 512 bytes, it still uses the whole sector. If it is larger than 512 bytes, the data is spread out among several sectors which may or may not be adjacent. When the data is not in adjacent sectors, they are considered to be fragmented sectors. The process of defragging a disk is the rearrangement of the data into adjacent sectors to speed data acquisition when required. More about that later.





Fat table - When you format a disk, it creates three items in the first sector of the disk. The first created is a Master Boot Record (MBR) of the files on the disk. It is linked to the File Allocation Table (FAT) that contains the location of the files on the geometry of the disk. This is followed by the boot sector that contains the information necessary to boot to the operating system (if it is aboot disk). The FAT and MBR are created with back up copies of themselves. When formatting (long) a flopy drive, the value of F6 (binary 111 0110) is entered into all tracks except the first one. A separator bit is entered between sectors.






Made out of Mylar - transparency plastic - covered with iron ferite particles. Each particle represents an alignment of magnetic fields - northsouth alignment = one bit of date - eastwest alignment represents a 0 bit of data.


Troubleshooting - if A-drive not found by BIOS - system halts - check cabling, drive, etc. Solid green light on - means cable backwards, Read errors - radial misalignment - care head cleaning kits are available - not worth money - kit costs more than new unit.


Format - two ways - from DOS (format a: /s) or from windows menu - click on System check box to make floppy bootable - adds three files. Quick format just changes FAT - doesn’t erase files. Formatting writes F6 or binary 11110110 to all sectors.


Cables - 2, 3, 4, or 5 connectors but can only handle two floppies - just different kinds of connections - slides or pins



(ESD) and special care must be taken when handling the disks. It is never a good idea to carry disks in a shirt pocket or brief case when the friction created by rubbing between items may create a static discharge leakage if not direct that will damage data. The most sensitive area of the disk just happens to be the first track near the outer edge of the disk. This is the most exposed area of the disk when travelling and also contains the boot sector. When you damage a disk by ESD, it will report that the disk in drive A is not formatted or sometimes wont even recognize the disk at all. Floppies are also subject to leakage in that small magnetic influences always present may over time, cause some data to be lost on the disk. This occurs when you store the disks near a monitor or TV that emits magnetic fields, or near electric motors,  or anything that may create an electromagnetic field. Radial misalignment occurs when the head of one disk drive is slightly off the line fo read/write that another disk drive is. When this occurs, a perfectly good disk may be read in one machine but not by another. The manufacturing tolerances leave room for doubt.

The first sector is exposed to more ESD and handling damage so by placing the boot files at the end of the dosk (near the spindle) it offers more protection from ESD and handling issues.

Maintenance and care - floopy drives must be maintained for optimal performance. There are several software tools used regularly. Scandisk, chkdsk, and format are very useful and prolong the life of disk drives.  We will establish monthly maintenance cycles and develop preventative and backup schedules in our lesson about proper troublshooting tips and then again when we make our performance notebooks.