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ATA/ATAPI-4

The next significant enhancement to the ATA standard after ATA-2 saw the ATA Packet Interface (ATAPI) feature set merged with the conventional ATA command set and protocols to create ATA/ATAPI-4. This standard was published by ANSI in 1998 as NCITS 317-1998, AT Attachment with Packet Interface Extensions. (Note the change to "NCITS" in the document number, from the "X3" used in earlier ATA standards; see here for an explanation of these organization names.)

Aside from combining ATA and ATAPI, this standard defined several other significant enhancements and changes:

  • Ultra DMA Modes: High-speed Ultra DMA modes 0, 1 and 2, defining transfer rates of 16.7, 25 and 33.3 MB/s were created.
  • High-Performance IDE Cable: An improved, 80-conductor IDE cable was first defined in this standard. It was thought that the higher-speed Ultra DMA modes would require the use of this cable in order to eliminate interference caused by their higher speed. In the end, the use of this cable was left "optional" for these modes. (It became mandatory under the still faster UDMA modes defined in ATA/ATAPI-5.)
  • Cyclical Redundancy Checking (CRC): This feature was added to ensure the integrity of data sent using the faster Ultra DMA modes. Read more about it here.
  • Advanced Commands Defined: Special command queuing and overlapping protocols were defined.
  • Command Removal: The command set was "cleaned up", with several older, obsolete commands removed.

Of course, the Ultra DMA modes were the most exciting part of this new standard. Ultra DMA modes 0 and 1 were never really implemented by hard disk manufacturers, but UDMA mode 2 made quite a splash, as it doubled the throughput of the fastest transfer mode then available. Ultra DMA mode 2 was quickly dubbed "Ultra DMA/33", and drives conforming to ATA/ATAPI-4 are often called "Ultra ATA/33" drives, which technically does not exist.

Next: ATA/ATAPI-5

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