Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!uunet!crdgw1!crdos1!davidsen
From: davidsen@crdos1.crd.ge.COM (Wm E Davidsen Jr)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Dell 310 flame (of sorts)
Summary: Solving the wrong problem
Message-ID: <1141@crdos1.crd.ge.COM>
Date: 16 Oct 89 15:53:13 GMT
References: <8634@spool.cs.wisc.edu> <8779@spool.cs.wisc.edu> <[2406.5]comp.ibmpc;1@point.UUCP>
Reply-To: davidsen@crdos1.UUCP (bill davidsen)
Organization: GE Corp R&D Center
Lines: 43

In article <[2406.5]comp.ibmpc;1@point.UUCP>, wek@point.UUCP (Bill Kuykendall) writes:

|  The key to the discussion above is probably the Mylex board's bus speed, not
|  the Dell's.  Many clone makers run the bus at the clock speed of the CPU. 
|  Dell limits the bus speed to 8mz, to ensure compatibility with add-in cards.

  I agree that the problem being addressed is probably the wrong one.
Someone in a non-business environment can look at really high bus
speeds, and live with the possibility of failuer of some boards. A
machine designed for reliability and targeted at the business market is
more likely to have an 8MHz bus.

  I am not sure what's holding back the transfer rate in the original
poster's machine, but let's look at what isn't... namely the bus. An RLL
controller has 26 sectors, or 512 bytes, rotating at 60 rps, for a
maximum transfer rate (1:1) of 798720 bytes/sec. At 6 cyc/byte that
takes a bus speed of 4.8MHz, at 8 cyc/byte 6.4MHz. You say ESDI? Okay,
32 sectors times 512 bytes times 60 rev/sec is 983040 bytes/sec max at
1:1. Assuming 8 cyc/byte that takes a bus speed of 7.9 MHz.

  But wait! All those assume a 8 bit controller! Cut the required bus
speed in half for anything made for the AT bus. Sure doesn't look as if
the bus is a problem, unless someone has the system configured for more
than 8 wait states!

  Here's the point: the AT bus is plenty fast enough to keep up with the
transfer rates of any hard disk. If there's a problem it's in the
controller or the setup. Figuring 8 cyc/transfer, the AT bus will do
2MB/s transfer, and that is more than the sustained one revolution rate
of any popular disk type.

  Systems like Mylex, which allow the controller to sit on a 32 bit bus
are a good implementation. While the AT bus will keep up with popular
disks, there is nothing wrong with saving cycles, particularly on
non-DOS operating systems which don't sit and wiat for the disk i/o to
end before running a process. This allows the AT bus to run at a nice
reliable rate, adequate for low speed peripherals, such as serial and
printers.
-- 
bill davidsen	(davidsen@crdos1.crd.GE.COM -or- uunet!crdgw1!crdos1!davidsen)
"The world is filled with fools. They blindly follow their so-called
'reason' in the face of the church and common sense. Any fool can see
that the world is flat!" - anon