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To: Hal Snyder <hal@post.vale.com>
Subject: Re: Multiple swaps slow down system? 
Cc: freebsd-hackers@freebsd.org
In-Reply-To: Your message of "Mon, 26 Aug 1996 09:15:14 IST."
             <01BB932F.47934380@jaguar> 
Date: Tue, 27 Aug 1996 08:27:18 +0500
From: Koshy <koshy@india.hp.com>
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>>>>>> "hs" == "Hal Snyder" >>>>>>

hs? The last time I looked at hd device drivers, SCSI had it all over IDE just
hs? because of DMA support - the old ST506 interface still used by IDE forced the
hs? CPU to handle all I/O going to/from the hard drive.  Has this changed?

IDE (ATA-2) can use DMA, in fact, future ATA standards make DMA support 
mandatory.  

SCSI still has some advantages over IDE though:

(A) the IDE bus is unterminated and has no parity checking.  The SCSI bus
    is terminated and also mandates parity checking.   With IDE you can
    never be sure of what you are reading over the bus.
(B) the IDE interface doesn't have disconnect/reconnect semantics so 
    transfers hold the bus till the device is finished.
(C) you can address only two devices per bus.
(D) only one device can be active per bus at a time; so putting two devices
    forces serialization of access between the two.
(E) Most of the IDE controllers in the market supporting two IDE interfaces 
    have only one set of pins for the data lines D0-D15; this forces only 
    one device across both busses to be active at a given time.  (Adding
    extra pins would cause chipset cost to go up dramatically, and single
    tasking operating systems like Win95 can't use the extra functionality
    anyway).

SCSI has none of these problems.

However, IDE devices sell for around half the price of comparable SCSI devices
in most parts of the world, so they are probably going to be around for a
while.

Koshy