By using Direct Memory Access (DMA), the processor is not tied up. The original DMA specification moved data at 16.6 MB per second. Ultra DMA (the 2 is implied) doubled this to 33 MB/sec. Mode 3 didn't really live long, however Ultra DMA 4 or UDMA 4 is frequently called Ultra ATA/66, named from the transfer rate of 66.7 MB/sec. As of this writing, Ultra ATA/133 is shipping. The fact that it may or may not become 'official' with ATA/ATAPI-6 won't matter to most of us.
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