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From: garyo@masscomp.UUCP
Newsgroups: comp.sys.amiga
Subject: Disk Controllers
Message-ID: <1242@masscomp.UUCP>
Date: Fri, 21-Nov-86 17:11:08 EST
Article-I.D.: masscomp.1242
Posted: Fri Nov 21 17:11:08 1986
Date-Received: Sun, 23-Nov-86 02:07:13 EST
References: <2043@well.UUCP> <758@ulowell.UUCP> <1986Nov19.104846.321@utcs.uucp> <2077@well.UUCP>
Reply-To: garyo@masscomp.UUCP (Gary Oberbrunner)
Organization: MASSCOMP - Westford, Ma
Lines: 38
Summary: Disk controller speeds NOT independent of I/O method

In article <2077@well.UUCP> perry@well.UUCP (Perry S. Kivolowitz) writes:
>
>[Brand X is a DMA controller, Brand Y is a slower Serial/Parallel controller]:
>...the throughput through  the AmigaDOS file system is low enough
>that neither controller will  run  at full speed (ie brand X will never get
>to really take advantage of the 300KB/sec  since disk i/o time is dominated
>by seeeeeeeeeeeeeeeeking).
>
>Those wanting to design THE  high performance disk controller for the amiga
>will have to look elsewhere for their performance boosts. We (ASDG) are.
>
>Perry
>

Most of this is true.  Normally, disk rate shouldn't be much improved
by using DMA, since most of the time is spent seeking.  However, with a
non-DMA controller the overhead during the actual transfer is MUCH higher,
since the device must do CPU interrupts per byte transferred.  Thus not
only is the transfer slowed by using the CPU, it's doubly slowed
because other computation cannot proceed while all these interrupts are
happening.

This will be much worse for image data, which presumably is stored semi-
contiguously on the disk.  In fact, for a non-dma SCSI controller reading
an optical disk, megabytes of image data could be read in without a single
disk seek.  In this case a DMA device would be a GREAT DEAL faster, for
the reasons mentioned above.

Give me a DMA SCSI controller any day.  It has all the advantages:
standard interface, fast, cheap, and flexible.  What more could one want?

				- Gary Oberbrunner

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