Karbosguide.com - Module 5b3.

About the EIDE interface - continued


The contents:

  • Transfer speeds and protocols
  • Improving PIO protocol
  • The max. disk size

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  • Transfer speeds and protocols

    EIDE exists with different protocols, like PIO (Programmed Inpu/Output) 3, PIO 4, UDMA/33, and UDMA/66. They are backwards compatible, therefore always choose the latest.

    The protocol is significant to the transfer speed, since it sets the standard for the drives external speed. The protocol controls the interface between drive and motherboard.

    An overview

    Each EIDE unit communicate according to a specific protocol. Here you see the four best known:

    Protocol for
    EIDE interface
    Year Maximum theoretical transfer
    PIO 3 or
    Multi-word DMA Mode 1
    1993 13.3 MB/second
    PIO 4 or
    Multi-word DMA Mode 2
    1994 16.6 MB/second
    Ultra DMA (ATA/33) 1997 33.0 MB/second
    Ultra DMA (ATA/66) 1999 66.0 MB/second
    Ultra DMA (ATA/100) 2000 100.0 MB/second


    Improving PIO

    Traditionally PIO data transfers rely heavily on the CPU to do all the work, processing each and every little task of reading or writing data.

    With the ATA-2 specification, including DMA-support solved this problem. Hence data are transported between RAM and hard disk without the supervision of the CPU.

    ATA-2 was to be known as EIDE.


    The max. disk size

    Another problem solved by this new protocol was the 2 GB limit of disk size. LBA (Logical Block Addressing) was the new setup. LBA is an addressing and translating scheme that replaces the CHS system (Cylinder Head Sector). This new scheme enables the BIOS to address up to 8.4 GB hard disk using 24 bit long addresses.

    The 8.4 GB limit was later broken by the new FAT32 file system.


    Only one protocol per channel?

    The two main channels (primary and secondary EIDE) can work with each one protocol. As an example, the primary channel may host UDMA/66 drives, while the secondary channel hosts PIO4 based CD-ROMs.

    However, sometimes the slave/master channels only have room for one protocol. Be aware of this potential problem. Is there only room for one common protocol? In that case the "winner" will invariably be the slowest of the ones connected.


    Harddisks always on the best protocol

    It is important that you connect your hard disk to an EIDE channel, which runs ATA/100 (or better).

    Ultra DMA requires the installation of drivers. Windows 95 before OSR2, due to age, did not recognize Ultra DMA, while Windows 98 naturally does.

    The motherboard vendor ASUS (as well as others) provides an excellent, simple patch program on CD. You run it just once. Then the drivers are stored in the right locations. After one or two re-boots everything works.

    Big disks

    Another problem can arise if you connect two hard disks to the system board. Despite the suppliers assurance that "it is very simple," it does not always work. Therefore, it is important to start with one sufficiently large hard disk.

    Please also read the article Problems with assigning two EIDE harddisks


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    To learn more

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

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

    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.

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