Ultra DMA 66
In case you missed our DMA 66 explanation in our Abit BP6
and Abit BE6
reviews, we'll give you a little Ultra DMA-66 primer.
DMA Direct Memory Access
Here's a good explanation of DMA from our FS Hard Drive Guide
DMA stands for Direct Memory Access. DMA, like its name defines, allows devices to transfer to and from the system memory directly, bypassing CPU intervention. The benefit here should be obvious, because the CPU can do its thing without being bothered by hard drive operations, but this is usually transparent to us users. In newer DMA devices, the device itself (e.g. hard drive in our case) does the actual work of transferring data to and from the system memory. This is called bus mastering, in contrast to older systems that had a separate DMA controller handling this task.
All current IDE hard disks should support DMA-33, but sometimes you have to click the DMA box in the hard disk properties menu. If you don't enable DMA, your hard disk CPU utilization will be extremely high.
What's the difference between DMA 33 and 66?
The "33" and "66" numbers indicate the maximum transfer rate. DMA-33 supports a 33.3MB/s maximum transfer rate and DMA-66 supports a 66.6MB/s maximum transfer rate. Does this mean that DMA-66 drives are twice as fast as DMA-33 drives? Not quite. Today's hard drives have sustained transfer rate speeds that are much lower than 33MB/s. Even with all the overhead, hard drives won't be able to take advantage of the full 66MB/s burst rate. How much faster is DMA 66? Good question. We'll answer that later in our benchmark sections.