Ultra Direct Memory Access (UDMA)

What is UDMA (Ultra Direct Memory Access)?

Ultra Direct Memory Access, or UDMA, refers to the data transfer protocol on a mass storage device, usually involving the ATA devices.

Technically speaking, according to the Ultra DMA protocol, the devices connected to the computer system are allowed to access the Random Access Memory (RAM) of the system directly without needing the Central Processing Unit (CPU).

Understanding UDMA (Ultra Direct Memory Access)

What is UDMA (Ultra Direct Memory Access)

Ultra Direct Memory Access is the specific practice of transferring data between the computer system and the devices connected to it.

UDMA is also referred to by different names, such as:

Ultra DMA is the data transfer protocol designed by Intel and Quantum and was introduced for the first time in 1998.

It supports the specific Advanced Technology Attachment or ATA 4 and Advanced Technology Attachment Packet Interface or ATAPI 4 standards.

However, with the release and approval of newer ATA standards later on, the Ultra DMA standards also support the following:

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It is better than the earlier Direct Memory Access interface and operates twice as fast.

Though most of the computer systems today support Ultra DMA, a few may not. Typically, there are a few specific signs that indicate whether or not a computer system supports Ultra DMA or has it configured correctly. These are:

If you find any of these issues in your computer system, then it is highly likely that the Ultra DMA feature in your system is not configured correctly.

You must also keep in mind that the hard drives with a fast UDMA mode are most commonly used on the motherboards that are designed to support slower Ultra DMA modes.

It is for this particular reason that the faster UDMA modes in most of the drives are disabled before they are shipped.

Therefore, you have to enable it before you want to use it. You can do this from the utility disk that usually comes with the drive. If you do not have it, you can follow these steps:

Also, check that your system has all of the requirements in order to reap the benefits offered by the Ultra DMA protocol and transfer data between the connected devices from your computer at a higher speed. Therefore check that:

In addition to that, you must also ensure that the host adapter, irrespective of whether it is motherboard-based or slot-based, has the Windows device drivers installed. This will help in achieving maximum performance.

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If it is not, then you can check and change it by following these steps:

It is also very important to keep in mind that the length of the cable for Ultra DMA should be shorter in comparison to the ones used on the regular DMA interfaces.

Ideally, it should be even less than 12 inches, or 30 cm, but certainly not more than 18 inches for the systems to work fine.

For higher data transfer speeds than 66 MB/s, you should typically use a shorter and specially designed 80-wire, 40-pin cable.

This will eliminate the chances of getting a lot of Cyclic Redundancy Check or CRC errors.

What Are UDMA Speeds?

Well, it depends on the version of UDMA you are talking about. However, it is a more advanced technology in comparison to Enhanced Integrated Drive Electronics (EIDE) and Small Computer System Interface (SCSI), offering almost two to four times or higher the speed and throughput than them, ranging between 16.7 MB/s and 167 MB/s.

There are different Ultra DMA modes, each supporting different data transfer speeds, minimum cycle timings, and defining standards. For your better understanding, here are the details of each of the different UDMA modes:

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At this point you should also note that the Ultra DMA modes that are higher and faster than UDMA mode 2 need to lower the following aspects to transfer data smoothly:

This is typically achieved by using an 80-conductor, 40-pin cable for transferring data between the computer and the ATA devices.


The Ultra Direct Memory Access standard of data transfer allows faster moving of data over the ATA interfaces between the PC and the ATA devices.

It is much faster and offers higher output than the EIDE and SCSI and is also relatively more affordable than them. Different versions of it come with different capabilities.