The Macintosh family of computers does not support true DMA transfers via
the SCSI chip.
The SCSI chip can operate in two modes: normal mode and pseudo-DMA mode.
- In normal mode, the SCSI driver software manages the handshaking to
communicate with a peripheral SCSI device.
- In pseudo-DMA mode, the SCSI chip's internal logic handles all the SCSI bus
handshake signals automatically.
(NOTE: For more information on the SCSI chip and DMA, see pages 5-24,
12-25, and 20-21 of the "Macintosh Family Hardware Reference" manual from
Addison Wesley, (ISBN# 0-201-19255-1).