From: richk@grebyn.com (Richard Krehbiel)
Subject: Re: IDE vs SCSI
In-Reply-To: wlsmith@valve.heart.rri.uwo.ca's message of Sat, 17 Apr 1993 20:42:47 GMT
Lines: 30
Organization: Grebyn Timesharing, Inc.

In article <1993Apr17.204247.6741@julian.uwo.ca> wlsmith@valve.heart.rri.uwo.ca (Wayne Smith) writes:

>   In article <wayne.02uv@amtower.spacecoast.org> wayne@amtower.spacecoast.orgX-NewsSoftware: GRn 1.16f (10.17.92) by Mike Schwartz & Michael B. Smith writes:
>
>   >> but I still want to know why it intrinsically better
>   >> (than IDE, on an ISA bus) when it comes to multi-tasking OS's when
>   >> managing data from a single SCSI hard drive.
>   >
>   >A SCSI controller that transfers data by DMA allows the cpu to request data
>   >from the hard drive and continue working while the controller gets the data
>   >and moves it to memory. 
>
>   IDE also uses DMA techniques.  I believe floppy controller also uses DMA,
>   and most A/D boards also use DMA.  DMA is no big deal, and has nothing to
>   do directly with SCSI.

IDE does not do DMA.  This is because it's like te PC AT hard disk
interface.  The controller moves a sector to some buffer memory on the
controller card, then the CPU moves the data from the buffer to main
memory where it's needed.  If IDE doesn't work this way, then it's not
compatible.

Heck, if IDE used DMA, then you'd hear all kinds of complaints about
ISA bus systems with more than 16M RAM not working with their IDE
drives.  16M is the DMA addressing limit of the ISA bus, and if IDE
did DMA there would be trouble.

(BTW, there are DMA-enabling signals in the IDE cable spec, but the
last report I heard was that they are never implemented, because it
would require a different kind of IDE adapter and different drivers.)
-- 
Richard Krehbiel                                 richk@grebyn.com
OS/2 2.0 will do for me until AmigaDOS for the 386 comes along...
