
Think!

It's the SCSI card doing the DMA transfers NOT the disks...

The SCSI card can do DMA transfers containing data from any of the SCSI devices
it is attached when it wants to.

An important feature of SCSI is the ability to detach a device. This frees the
SCSI bus for other devices. This is typically used in a multi-tasking OS to
start transfers on several devices. While each device is seeking the data the
bus is free for other commands and data transfers. When the devices are
ready to transfer the data they can aquire the bus and send the data.

On an IDE bus when you start a transfer the bus is busy until the disk has seeked
the data and transfered it. This is typically a 10-20ms second lock out for other
processes wanting the bus irrespective of transfer time.
