Cheet Sheets 1995 February

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Text File | 1995-02-07 | 13.5 KB | 372 lines

~Disk Drives. An explanation of what they are and how they work. Written by Wayne Roberts back in 1989 !!! So some of the information is definately out of date, and remember this is way before the CD-ROM was used on home computers... Before the 386 was born, just around the time when VGA was introduced and the time when a 40Mbyte hard drive was a luxury !! ~What are disks ? Disks allow the computer to permanently or temporarily remember, or store, information. Unlike information in ROM (Read Only Memory) chips, this information can be readily changed. There are several types of disks. Two examples of disk storage follow :- ~Floppy Disks (or diskettes). A floppy disk works in a similar way in which a cassette does but is structured like a record. The computer's disk drive writes information on it just like a tape player records music on tape. When the computer puts information on a disk, it is said to be writing to the disk. When the computer uses this information, it is said to be reading from the disk. The floppy disk itself is portable. Currently there are two popular sizes of floppy disk - the 5.25" and the newer smaller 3.5" size. In the past 8" and 3" have been used but these sizes have slowly diminishing. Floppy disks are used with a floppy disk drive. This drive reads and writes to the disk and must be the same size as the disk being used. Besides the obvious physical difference, a 3.5" disk is more durable than the older 5.25" disk. And the 3.5" disk can surprisingly hold often as much as twice as much information than 5.25" disks. 3.5" disks are rapidly becoming the standard size disk for computer systems. The disk's storage capacity, or amount of information that can be written to a disk, is called disk memory. Disks have memory just as computers do. For instance, a standard 5.25" disk can hold 360 Kilo bytes, while the new 3.5" can hold 720K. The newest IBM PC, the PS2, uses 3.5" disks which can hold 1.44 Mbytes of information instead of the older 720K standard. As well as the amount of memory a disk has there are many other different aspects of a floppy disk. Below is a brief explanation of these. The density of the disk can be either Single, Double, High, or Quad, the latter of these being the best as more information can be stored on them. Disks can also either be single or double sided. The double sided often being able to store a higher amount of information. The diskette itself is divided up into tracks and sectors. The more the tracks and sectors the greater the storage size. Eg. If a 3.5" floppy disk is Double Sided, has 80 tracks, 18 sectors and each sector is 512 bytes of information then the memory available on the disk would be 1.44Mbytes (see below). Sides x tracks x sectors x bytes per sector = memory on disk ` 2 x 80 x 18 x 512 = 1,474,560 (1.44M) ~Hard Disk Drives. Hard disks are often referred to as fixed drives or non removable disk, the hard disk drive is just like a floppy disk drive except that it is not portable. It is also stiff rather than floppy, hence its name and is always situated in the computer. The advantages to a hard disk drive include speed and the amount of information that can be stored on it. The smallest hard disks are much faster than floppies and can hold over 10 to 20 times as much information as a single floppy. The following tables shows some typical technical disk information that can be acquired through disk utility programs or by simply typing chkdsk (Check disk). The information given in the tables is kept as simple as possible thus making them easier to follow. +-----------------------------------------------------------------+ |Drive C:(Hard Disk) | | | |Basic storage capacity: | | | | 20 million characters (MegaBytes) | | 7% of disk space is free | | | |Logical dimensions: | | | | Sectors are made up of 512 bytes | | Tracks are made up of 17 sectors per disk side | | There are 4 disk sides | | The disk space is made up of 610 cylinders | | Space for files is allocated in clusters of 4 sectors | | There are 10,337 clusters | | Each cluster is 2,048 bytes (2 Kbytes) | | The disk's root directory can hold 512 files | +-------------------------------------------------------------------+ +-------------------------------------------------------------------+ |Drive A:(Floppy disk) | | | |Type of disk: | | | | Double-sided, Quad-density, 9-sector. (DS-QD) | | | |Basic storage capacity: | | | | 720 thousand characters (Kilobytes) | | 69% of disk space is free | | | |Logical dimensions: | | | | Sectors are made up of 512 bytes | | Tracks are made up of 9 sectors per disk side | | There are 2 disk sides | | The disk is made up of 80 tracks | | Space for files is allocated in clusters of 2 sectors | | There are 713 clusters | | Each cluster is 1,024 bytes (1Kbytes) | +-------------------------------------------------------------------+ ~Other storage devices. As well as floppy and hard drives there are many other types of storage devices that are often removable from the hardware. These are normally tape backup up systems but are also disk and even CD based. Names given to these types of storage are Bernoulli, Tape streamers, removable disk, Fault Tolerant systems, WORM or CD-ROM (or disks). CD-ROM is becoming wider used for storage of information as the research into optical storage has been slowly renewed. At the moment optical storage of information is done using two methods WORM and CD-ROM. WORM (Write Only Read Many) drives can have a capacity of up to two Gigabytes, whereas compact-disk technology can have capacities of up to 600 Mbytes, but at the moment it is of a read-only nature. Both WORM's and CD's are removable and retail at a price between £5 and £50 when blank. Recently the first re-usable optical disk has been pronounced and are expected on the market by the end of 1989. ~General features of Hard and Floppy Disks. Things to consider when buying a hard disk include the type of magnetic medium on it. Usually this was a ferric oxide coating similar to that used on cassette tapes. On some hard disks a thin coating or pure metal which when plated on to the disk provide a hard surface. This makes the disk less prone to damage and speeds up the Disk Access Time. Disk Access Time - This is the time interval between the command is given to receive data from the computer memory or disk to the moment the transfer is completed. Consequently when a read or write command is received, any point to be accessed within one second, whether it is about to pass by the head or has just passed (in the latter case, the drive was for the designated point to come around again in one revolution). The access time for a floppy disk is generally between 90 milliseconds (1 millisecond= 1/1000 of a second) and 500 milliseconds. It is also possible to read from a certain spot on the disk and move directly from there to another spot. This can be done very quickly - between 3 and 50 milliseconds. In most systems the retrieval of data involves three indentifiable tasks :- a) Seek time - This is the time taken to move the read/write head to the required address of the data. b) Rotational Delay - This is when a read/write instruction is issued, the head is not usually positioned over the sector where the required data is stored, there is some rotational delay while the disk rotates into the proper position. On average, the time taken will be half a revolution of the disk pack. This average time is known as the "latency" of the disk. c) Data Transfer Time - This is the time taken to read a block of data into main memory. Disk Controllers - Each disk drive must have means of communicating with the computer that it serves, this is done by the disk controller. The purpose of the controller is to take commands from an application (such as "get next record") and, in conjunction with the operation system, translate them into commands that will be understood by the disk drive. When selecting a controller for a disk drive there are two major considerations you should take into account. a) The controller and its disk drive must use the same recording convention. b) The controller and its disk drive must use the same interface protocol. Sequential and Random Access files - Tape systems can record or read data only in sequence. In other words, they can only read recorded data in order, and we call this a sequential file. A structure of a disk does not have this drawback. It has a structure which allows data to be quickly recorded onto or read from any position on the disk. This is called a random access or a direct access file. There are many occasions when one wants to locate and use information quickly on a computer or word processor. This random access file is very convenient for successively handling data that was not produced in a particular order. Advantages of Disk Drives (Floppy and Hard). The necessity of a Hard disk drive is not so if you are using a computer with a lot of memory or you are running programs that do not take up disk space. A second floppy drive is often a substitute for a hard drive as the cost difference can be a lot, but the speed and size of programs that are run is often a let down. Large programs such as ORACLE and MS EXCEL need a hard disk to run as they can not run off floppy, plus they would run extremely slow. Probably the most obvious advantage of a hard disk is that it can store a great deal more information and software applications. Eg. A floppy disk can hold probably hold one software application whereas the hard disk could hold anything from 8 software applications plus user files. So, in short, Speed, price and storage capacity decides whether you buy a hard disk rather than a second floppy drive. ~What sort of drives are there you can buy ? When buying a disk system you need to decide whether you want a 3.5" drive or a 5.25" drive. Probably your best choice would be a 3.5" drive as they are quickly becoming the standard size disk and if you use 5.25" disks elsewhere then it would be advisable to purchase a 5.25" portable drive for transferring of data. 5.25" floppy disks are just over half the price of 3.5" disks and can store about half the amount of information, thus in the long run 3.5" disks are a greater value for money. ~Disk Management. Storage systems are very prone to corrupt if they are not looked after in the proper ways. This section of the leaflet illustrates some points which need to be taken towards keeping your storage systems error free. a) Floppy disk need more care taken of them than actual drives as they are very delicate. Care of 5.25" is commonly illustrated on disk sleeves, (see overleaf), they are normally as follows :- i) Never touch disk surface. ii) Never leave near magnetic materials (eg. speakers,monitors). iii) Never bend the disk. iv) Never write on disk with hard nibbed instruments (pencils,biros). v) Store in temperatures between 10-50C (50-122F). vi) Store in envelopes and/or storage boxes. b) The Floppy and hard disks need to compressed using a disk utility program. This tidies up the disk by moving programs to the beginning of the disk where free spaces have been left by programs deleted, all these free spaces are then put to the end of the disk. A compressor also lowers the chances of incurring bad sectors on a disk. You can also, by using a compress program, analyse the disk surface for bad sectors etc. c) The floppy disk drive needs to be cleaned regularly using a disk head cleaner. About once a month would be an ideal period in which you should clean your drive heads. d) The hard disk should also have care taken that it is not damaged in any way. Moving the system without parking the heads could quickly corrupt the disks operation. On most systems you have a command "PARKHEAD", which parks the heads on the hard drive and allows you to move the system without damaging the drive in any way. e) Their are many errors that can occur when using disk drives, some of the more common include :- 1) NOT READY ERROR READING DRIVE A - Which means that either disk door is not closed or floppy disk is not inserted properly. 2) GENERAL FAILURE ERROR READING DRIVE A - Which means that the disk is of a wrong format or it can not be read. 3) INVALID DRIVE - Which means that drive specified does not exist. 4) BAD COMMAND OR FILENAME - Which means that command entered does not exist or can't be found. 5) NON-SYSTEM DISK OR DISK ERROR - REPLACE DISK AND PRESS A KEY - When re-booting computer if a system disk (with DOS commands etc.) is not present then this error will occur. Other DOS errors that may occur could be found in a DOS user manual and other disk maintenance hints can be found, in detail, in books. One such book is Hard-Disk Management Published by TAB Books Ltd. ~There you are, a little look back to 6 years ago, when PC's were ~a little less used in the home. It's nice to see what we were using ~years ago......