What is UltraDMA Mode ?

UltraDMA, ATA/ATAPI-4, Ultra-ATA, or Ultra-DMA/33

     A protocol developed by Quantum Corporation and Intel that supports burst mode data transfer rates of 33.3 MBps. This is twice as fast as the previous disk drive standard for PCs, and is necessary to take advantage of new, faster Ultra ATA disk drives.
     The latest improvement of the EIDE device interface. Also known as ATA/ATAPI-4, Ultra-ATA, or Ultra-DMA/33, this is the latest advancement to the ATA specifications (draft): ATA/ATAPI-4. Which, among other improvements, supports UltraDMA. This interface has a theoretical maximum data transfer rate (burst, not sustained) of 33MB/sec. The ATA/ATAPI 4 specification also introduces error checking, which ensures data integrity at the higher speeds.
     Ultra-DMA doubles the maximum transfer speed of the ATA-3 standard.

UltraDMA Mode	Cycle Time (nanoseconds)	Maximum Transfer Rate (Mb/s)	Defining Standard
Mode 0 UDMA-0	240 ns	16.7 Mbps	ATA/ATAPI-4
Mode 1 UDMA-1	160 ns	25.0 Mbps	ATA/ATAPI-4
Mode 2 UDMA-2	120 ns	33.3 Mbps	ATA/ATAPI-4
Mode 3 UDMA-3	90 ns	44.4 Mbps	ATA/ATAPI-5
Mode 4 UDMA-4	60 ns	66.7 Mbps	ATA/ATAPI-5
Mode 5 UDMA-5	40 ns	100.0 Mbps	ATA/ATAPI-6?

As you can see, UltraDMA/33 achieves nearly double the transfer rate of the existing ATA interface while not increasing the cycle time of the ATA bus clock over the rate currently used for PIO Mode 4.

This apparent miracle is achieved by:

Improving timing windows in the protocol on the ATA bus
Reducing propagation delays by pipelining data transfers
Transferring data in synchronous (strobed) mode

In addition, UltraDMA/33 improves data integrity by using a Cyclic Redundancy Check (CRC) to flag any data transfer errors that may be made over the ATA bus (Note: In this application CRC is only used to improve data integrity for ATA bus transfers, it is not used to improve the data integrity of either disk drives or host systems).
UltraDMA/33 requires no physical change to the cable, receivers, or drivers of the ATA bus. Furthermore, UltraDMA/33 protocol and commands have been designed to be compatible with existing ATA devices and systems. Thus, new UltraDMA/33 devices will be backwards compatible with most older (non UltraDMA/33) systems. Older ATA-1/2/3 devices will also be forwards compatible with systems that have UltraDMA/33 capability.

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