From: wlsmith@valve.heart.rri.uwo.ca (Wayne Smith)
Subject: Re: IDE vs SCSI
Organization: The John P. Robarts Research Institute, London, Ontario
Nntp-Posting-Host: valve.heart.rri.uwo.ca
Lines: 35

In article <RICHK.93Apr15075248@gozer.grebyn.com> richk@grebyn.com (Richard Krehbiel) writes:
>>     Can anyone explain in fairly simple terms why, if I get OS/2, I might 
>>   need an SCSI controler rather than an IDE.  Will performance suffer that
>>   much?  For a 200MB or so drive?  If I don't have a tape drive or CD-ROM?
>>   Any help would be appreciated.

>So, when you've got multi-tasking, you want to increase performance by
>increasing the amount of overlapping you do.
>
>One way is with DMA or bus mastering.  Either of these make it
>possible for I/O devices to move their data into and out of memory
>without interrupting the CPU.  The alternative is for the CPU to move
>the data.  There are several SCSI interface cards that allow DMA and
>bus mastering.
 ^^^^^^^^^^^^
How do you do bus-mastering on the ISA bus?

>IDE, however, is defined by the standard AT interface
>created for the IBM PC AT, which requires the CPU to move all the data
>bytes, with no DMA.

If we're talking ISA (AT) bus here, then you can only have 1 DMA channel
active at any one time, presumably transferring data from a single device.
So even though you can have at least 7 devices on a SCSI bus, explain how
all 7 of those devices can to DMA transfers through a single SCSI card
to the ISA-AT bus at the same time.

Also, I'm still trying to track down a copy of IBM's AT reference book,
but from their PC technical manual (page 2-93):

"The (FDD) adapter is buffered on the I.O bus and uses the System Board
direct memory access (DMA) for record data transfers."
I expect to see something similar for the PC-AT HDD adapter.  
So the lowly low-density original PC FDD card used DMA and the PC-AT
HDD controller doesn't!?!?  That makes real sense.
