

There is a way in which a multi-tasking computer actually gives you
more CPU power then you had before, and that is with I/O overlap.
With I/O overlap, your CPU can continue to "think" while disk
operations are underway, whereas without overlap, your CPU sits idly
waiting for each disk operation to finish - and disk operations take
an *eternity*, compared to a fast CPU.

So, when you've got multi-tasking, you want to increase performance by
increasing the amount of overlapping you do.

One way is with DMA or bus mastering.  Either of these make it
possible for I/O devices to move their data into and out of memory
without interrupting the CPU.  The alternative is for the CPU to move
the data.  There are several SCSI interface cards that allow DMA and
bus mastering.  IDE, however, is defined by the standard AT interface
created for the IBM PC AT, which requires the CPU to move all the data
bytes, with no DMA.