Ultra DMA (UDMA) Modes

With the increase in performance of hard disks over the last few years, the use of programmed I/O modes became a hindrance to performance. As a result, focus was placed on the use of direct memory access (DMA) modes. In particular, bus mastering DMA on the PCI bus became mainstream due to its efficiency advantages. If you have not yet, you should read the description of the various DMA modes and how bus mastering DMA works; this will help you understand this page much better.

Of course, hard disks get faster and faster, and the maximum speed of multiword DMA mode 2, 16.7 MB/s, quickly became insufficient for the fastest drives. However, the engineers who went to work to speed up the interface discovered that this was no simple task. The IDE/ATA interface, and the flat ribbon cable it used, were designed for slow data transfer--about 5 MB/s. Increasing the speed of the interface (by reducing the cycle time) caused all sorts of signaling problems related to interference. So instead of making the interface run faster, a different approach had to be taken: improving the efficiency of the interface itself. The result was the creation of a new type of DMA transfer modes, which were called Ultra DMA modes.

The key technological advance introduced to IDE/ATA in Ultra DMA was double transition clocking. Before Ultra DMA, one transfer of data occurred on each clock cycle, triggered by the rising edge of the interface clock (or "strobe"). With Ultra DMA, data is transferred on both the rising and falling edges of the clock. (For a complete description of clocked data transfer and double transition clocking, see this fundamentals section.) Double transition clocking, along with some other minor changes made to the signaling technique to improve efficiency, allowed the data throughput of the interface to be doubled for any given clock speed.

In order to improve the integrity of this now faster interface, Ultra DMA also introduced the use of cyclical redundancy checking or CRC on the interface. The device sending data uses the CRC algorithm to calculate redundant information from each block of data sent over the interface. This "CRC code" is sent along with the data. On the other end of the interface, the recipient of the data does the same CRC calculation and compares its result to the code the sender delivered. If there is a mismatch, this means data was corrupted somehow and the block of data is resent. (CRC is similar in concept and operation to the way error checking is done on the system memory.) If errors occur frequently, the system may determine that there are hardware issues and thus drop down to a slower Ultra DMA mode, or even disable Ultra DMA operation.

The first implementation of Ultra DMA was specified in the ATA/ATAPI-4 standard and included three Ultra DMA modes, providing up to 33 MB/s of throughput. Several newer, faster Ultra DMA modes were added in subsequent years. This table shows all of the current Ultra DMA modes, along with their cycle times and maximum transfer rates:

The cycle time shows the speed of the interface clock; the clock's frequency is the reciprocal of this number (see here for more on clocking.) The maximum transfer rate is four times the reciprocal of the cycle time--double transition clocking means each cycle has two transfers, and each transfer moves two bytes (16 bits). Only modes 2, 4 and 5 have ever been used in drives; I'm not sure why they even bothered with mode 0, perhaps for compatibility. Ultra DMA mode 5 is the latest, and is implemented in all currently-shipping drives. It is anticipated that it will be included in the forthcoming ATA/ATAPI-6 standard.

Note: In common parlance, drives that use Ultra DMA are often called "Ultra ATA/xx" where "xx" is the speed of the interface. So, few people really talk about current drives being "Ultra DMA mode 5", they say they are "Ultra ATA/100".

Double transition clocking is what allows Ultra DMA mode 2 to have a maximum transfer rate of 33.3 MB/s despite having a clock cycle time identical to "regular DMA" multiword mode 2, which has half that maximum. Now, you may be asking yourself: if they had to go to double transition clocking to get to to 33.3 MB/s, how did they get to 66 MB/s, and then 100 MB/s? Well, they did in fact speed up the interface after all. :^) But the use of double transition clocking let them do it while staying at half the speed they would have needed. Without double transition clocking, Ultra DMA mode 5 would have required a cycle time of 20 nanoseconds instead of 40, making implementation much more difficult.

Even with the advantage of double transition clocking, going above 33 MB/s finally exceeded the capabilities of the old 40-conductor standard IDE cable. To use Ultra DMA modes over 2, a special, 80-conductor IDE cable is required. This cable uses the same 40 pins as the old cables, but adds 40 ground lines between the original 40 signals to separate those lines from each other and prevent interference and data corruption. I discuss the 80-conductor in much more detail here. (The 80-conductor cable was actually specified in ATA/ATAPI-4 along with the first Ultra DMA modes, but it was "optional" for modes 0, 1 and 2.)

Today, all modern systems that use IDE/ATA drives should be using one of the Ultra DMA modes. There are several specific requirements for running Ultra DMA:

1. Hard Disk Support: The hard disk itself must support Ultra DMA. In addition, the appropriate Ultra DMA mode must be enabled on the drive.
2. Controller Support: A controller capable of Ultra DMA transfers must be used. This can be either the interface controller built into the motherboard, or an add-in IDE/ATA interface card.
3. Operating System Support: The BIOS and/or operating system must support Ultra DMA transfers, and the hard disk must be set to operate in Ultra DMA in the operating system.

4. 80-Conductor Cable: For Ultra DMA modes over 2, an 80-conductor cable must be used. If an 80-conductor cable is not detected by the system, 66 MB/s or 100 MB/s operation will be disabled. See the discussion of the 80-conductor cable for more.

On new systems there are few issues with running Ultra DMA, because the hardware is all new and designed to run in Ultra DMA mode. With older systems, things are a bit more complex. In theory, new drives should be backwards compatible with older controllers, and putting an Ultra DMA drive on an older PC should cause it to automatically run in a slower mode, such as PIO mode 4. Unfortunately, certain motherboards don't function well when an Ultra DMA drive is connected, and this may result in lockups or errors. A BIOS upgrade from the motherboard manufacturer is a good idea, if you are able to do this. Otherwise, you may need to use a special Ultra DMA software utility (available from the drive manufacturer) to tell the hard disk not to try to run in Ultra DMA mode. The same utility can be used to enable Ultra DMA mode on a drive that is set not to use it. You should use the utility specific to whatever make of drive you have.

I discuss more issues and considerations for implementing Ultra DMA in the section on configuration.