On Mon, 21 Feb 2000, Miles Nordin wrote: 

> On Sun, 20 Feb 2000, Chris Tribo wrote: 
> 
> > ATAPI is an earlier (slower) version of today's IDE, correctly called EIDE.
> 
> The speed progression of IDE is
>  PIO
>  DMA
>  Ultra33-DMA
>  Ultra66-DMA
> 
> The ``Ultra'' modes require the more expensive thin-wire ribbon cables
> like the Macs ship with, and they also do CRC's on each transfer so if
> your cabling is inadequate you'll get errors printed on the console,
> rather than the data corruption that you get with PIO or DMA mode and
> crappy cables.

Note that Ultra33 devices are cable-compatible with older interfaces -
performance may drop, but the cables fit. Ultra66 aren't. If you have an
Ultra66 drive, as I understand it, you need an Ultra66 interface.

> ATAPI is ATA Programming Interface, sort of like how scsipi in NetBSD is
> the software command component of SCSI.  ATAPI is a way of sending some of
> the SCSI commands over the inferior IDE electrical standard, and indeed
> SCSI drivers like cd, sd, and uk attach to both the atapibus and the
> scsibus in NetBSD (while Linux uses redundant code for this). This ATAPI
> kludge is how CD-ROM's were made to work on the IDE bus, which is
> otherwise a descendent of the hard-disk-only PeeCee ST506 standard.

As I understand it, the "P" means Packet, not Programming. IDE was
designed for disk drives, while SCSI was designed for disks and tapes and
more. ATAPI added to ATA support for tapes and more. ;-) I don't think
it's so much that it's a way of sending SCSI commands, but as a way of
adding a richer command set, and SCSI was a convenient, tested model.

Take care,

Bill