Subject: Re: Oooooh... IDE to Q-bus/Unibus adapters.
To: Allison J Parent <>
From: Thor Lancelot Simon <>
List: port-vax
Date: 07/07/1999 14:28:42
On Wed, Jul 07, 1999 at 10:34:37AM -0400, Allison J Parent wrote:
> <
> <huh? i'd tend to disagree!! IDE forces the host to perform all the work! 
> And PIO SCSI doesn't???
> Both can run with DMA, and unless your running wide scsi or ultrascsi
> most IDE drives can be IO'd pretty fast!  Keep in mind that IDE the 
> DATA path is naturally 16bits wide and the commands/status do not share 
> the same address.

That's as may be, but IDE _is_ missing a lot of the nice features of
SCSI that provide I/O concurrency.  No disconnection, no command queueing
(tagged or simple), etc.  And of course there's no good error reporting,
and no reliable way to flush the drive's cache when you're ready to turn
the power off!

No disconnection and no queueing mean that you spend a lot more time
waiting for data to come back from the disks than you do with SCSI --
the "no disconnection" bit means that if you try to sidestep that with
multiple disks, you have to put each disk on its own *bus*...

Also, better SCSI host adapters have very smart engines on them that can
let you queue up many transfers _on the host adapter_ and go back to what
you were doing.  IDE requires much more host CPU involvement than that,
even if you're doing DMA for the actual data.

Thor Lancelot Simon	                            
	"And where do all these highways go, now that we are free?"