KarbosGuide.com. Module 5b4.

About the EIDE interface - continued

The contents:

  • What does Ultra DMA offer?
  • Looking at a good harddisk
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  • What does Ultra DMA offer?

    The real EIDE improvement was accomplished with the introduction of the Ultra DMA or Ultra ATA (UDMA). It is an interface patented by Quantum but supported by all motherboard and disk drive manufacturers.

    New protocol

    The technology involves an improvement of the interface - the governing electronics which deliver the hard disk data to the system board. Quantum succeeded in eliminating the bottle neck in existing electronics to deliver data to the EIDE hard disks.

    The UDMA hard disk is no faster it self, but the data paths have been optimized. Within the new protocol, the speed is doubled by allowing twice the data transfer per clock cycle.

    Is 33 MB per second realistic?

    Introducing ATA/33, it sounded great but exaggerated to talk about a 33 MB per second transfer rate, well knowing that no EIDE hard disk at this time could deliver more than 7 MB per second. Then PIO4, which can move 16 MB per second, should suffice? No, not so - the secret is in the EIDE host controller.


    The host controller, among other things, must retrieve data from the drive and deliver them to the PCI bus. Or it must retrieve data from the bus and deliver them to the disk.

    The host controller has certain administrative duties to handle between reading to/from the disk. And they take some time. One clock cycle in the EIDE controller lasts 400 micro seconds. Of these, 275 are spent on "administrative overhead" - handling protocol commands, etc. The remaining 125 micro seconds are used to read from the hard disk. Therefore, a maximum transfer rate of 33 MB per second is necessary to keep up with the hard disk's capacity.

    40% better

    Actual measurements show that Ultra DMA disks yield up to 40% better performance than comparable PIO 4 disks. That was a clear improvement - even though the disks could not deliver the advertised 33 MB per second.

    Another new feature in Ultra DMA was the CRC (Cyclical Redundancy Check) - automatic error correction for better data protection and verification.

    EIDE on the motherboard

    The system board and with that the chip set, must be set up for Ultra DMA in order for you to utilize such a disk. As always, check the chip set, when you buy a new PC. Since it provides solid performance improvement at no extra cost, it is important that it supports Ultra DMA.

    Note: With the increasing magnetic density on harddisk platters, we use the bandwidth of the UDMA ATA/66 or Serial ATA interface.


    For the EIDE hard disk to function in the Ultra DMA protocol, the following conditions must be met:

  • The hard disk must be the Ultra DMA type.

  • The system board must have a chip set which supports Ultra DMA with the latest protocol.

  • BIOS must "log on" the hard disk with Ultra DMA protocol. You can verify that in the start up screen.

  • Drivers for the chip set must be installed in Windows .

    A good Ultra DMA disk

    Here you see my (old) Maxtor harddisk (Diamond Max 91728D8 from 1998/99). It holds 17 GB. It rotates with a modest 5400 RPM, which makes it noise-free and it does not produces a lot of heat.

    Yet it is quite fast due to the magnetic density, which is 4.32 GB per platter. This thing holds 4 platters:

    The harddisk is configured as a slave unit on the primary EIDE channel as described later.

    More modern harddisks from Maxtor take exactly the same physical dimensions; internally, they just hold 30, 45 or 60 GB.

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    To learn more

    Read more the boot process and system bus in Module 2b

    Read about file systems in module 6a

    Read about I/O buses in module 2c

    Read about the motherboard chip set in module 2d

    Read about RAM in module 2e

    Read Module 5c about SCSI, USB etc.

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

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