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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Interfaces and Configuration | Integrated Drive Electronics / AT Attachment (IDE/ATA) Interface | Official IDE/ATA Standards and Feature Sets ]
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:
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.