Enabling Bus Mastering (DMA) Support
By default, ATAPI CD-ROM drives may operate in Programmed I/O (PIO) mode rather than DMA mode, which is also called Bus Mastering mode. The fact that PIO mode limits DTR to 16.7 MB/s versus the 33.3 or 66.7 MB/s DTR available with DMA is unimportant because no CD-ROM drive even approaches the DTR limit of PIO mode. What is important is that PIO mode causes much higher CPU utilization than DMA mode. A typical ATAPI CD-ROM drive operating in PIO mode may occupy 80% or more of the CPU when the drive is being accessed heavily, while the same drive operating under the same conditions in DMA mode may occupy only 1% to 5% of CPU time.
Accordingly, enabling DMA mode is usually a good idea, but doing so requires that the BIOS, operating system, chipset, and CD-ROM drive itself all support DMA mode. Most recent ATAPI CD-ROM drives support DMA mode. Most motherboards of late-model Pentium vintage or later also support DMA mode on their embedded ATAPI interfaces.
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