Karbosguide.com - Module 4b5.

Harddisks - continued


The contents:

  • Hard disk limitations
  • Hard disk interface
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  • Hard disk limitations

    Up through the years, hard disks have suffered from a long list of software related limitations. Read about that under the file system.

    Hard disk speeds

    There are countless test programs and measuring methods to evaluate the various hard disks.

    Don't place too much stock in the sales person's presentation of seek times. Many hard disks are advertised with a number like 8 ms. That refers to a seek time, which is measured in milliseconds. There are many different seek times. That makes comparison difficult. You can measure in terms of:

    Average track to track speed

    How long does it take the actuator to move read/write heads from one track to another? There are typically 3000 tracks on a platter side. There, a track change could be to just one over. That might take 2 ms. Or, it could be up to 2999 tracks over. That might take 20 ms. On current hard disks, the average seek time will be between 8 and 14 ms.

    Change time between read and write

    That takes time as well.

    Wait time for the right sector

    When the arm moves to a track, it must wait for the right sector to appear under the head. That takes time also. On the average, the platter must rotate ½ revolution, to reach the right sector. This time is inversely proportional to the disk rotation speed. On modern hard disks it usually is between four and eight ms.


    When I test a hard disk, I emphasize practical applications. You can take a stopwatch and measure, for example how long it takes to start Windows 98 and Word 97 (possibly including a large file). That type of measurement can really tell you something about the hard disk's performance. However they must be under comparable circumstances. System board, CPU, and the driver program also influence the results.


    Hard disk interface

    Read here in more detail about the different interface types.

    MFM and RLL

    MFM and RLL are actually coding principles for the hard disks. They are not interface standards. The coding occurs from controller to the hard disk. As a coding principle, RLL was more effective than was MFM, so in the "old days" you could experiment using RLL controllers to MFM disks.

    WD 1003

    MFM as well as RLL are WD1003 compatible, meaning that the standards would work with the at that time most widely used controller chip from Western Digital.

    ST 506

    ST 506 is an interface, which was used both with RLL and MFM. There is serial connection from controller to disk. The ST 506 controller functions as a converter from the serial read/write head data to the 8 or 16 bit parallel bus. ST 506 was the most widespread controller standard before IDE.

    IDE

    Integrated Device Electronics. Under the IDE standard, the controller chip WD 1003 is mounted directly on the hard disk, not on the IDE adapter. This means that the conversion to parallel data is already done on the disk. Because of the short serial cable, this increases the transfer speed significantly relative to MFM and RLL. IDE is a simple adapter. The adapter itself contains only amplifying circuits to/from the I/O bus. Therefore it is inexpensive. The IDE controller does not care whether the hard disk works internally with MFM or RLL coding.

    ESDI

    ESDI was an improvement over the ST506 standard. An ESDI disk operates on a common 16 bit AT bus (ISA bus), but it is better put together than an ordinary IDE. This results in an almost doubling of the transfer speed between hard disk and controller/bus. ESDI was also different in many other ways. Among the features are a sector on the hard disk which identifies its number of tracks, cylinders, etc. This information is usually stored in CMOS.


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    Learn more

    Also see Module 4c about optical media (CDROM and DVD).

    Also read about EIDE and UDMA

    And about the most advanced and elegant controller principle of all: SCSI.

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.

    HD Tach - a fine program to test harddisks.

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