'D' option during boot

I just installed the 6.3 30-day evaluation system, and I ran into a small
problem. During the “Detecting devices” part of booting from the CD, the
system would hang. However, when I specified the ‘D’ option to disable DMA,
it worked fine. Once I had it on the hard drive, the D option was not
necessary, and things are working great. I am curious as to why I had to do
this. Any comments would be appreciated.