While most of the drives we use now are Ultra DMA, many of us remember using "mode 3" or "mode 4" hard drives. These were ATA-2 drives, using PIO mode 3 or mode 4. Here's a table of the evolution of the ATA Interface since 1993:

Interface Type ATA-2 PIO 3 ATA-2 PIO 4 UDMA/33 UDMA/66

Bus Speed 13.3MB/s 16.6MB/s 33.3MB/s 66.6MB/s

Year 1993 1994 1997 1999

As drive technology got better and areal densities started rising at rates faster than 20% per year, the need for a new standard emerged. One that would allow 2 devices to share an ATA chain comfortably, and still have room to breathe. With current hard drives reaching sustained transfers up in the 10MB/sec range, obviously a 16.6MB/sec bus would not have sufficed. Before these speeds were attained, however, Quantum developed the Ultra-ATA/33 standard (UDMA/33). To obtain the 33MB/sec speeds, the clock signal was kept the same, but data was now transferred on the rising AND falling edges of the clock, rather than just the rising. While this was cozy for a little while, it was not long before drive technology was growing faster than the new interface could handle.