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Path: utzoo!mnetor!seismo!ll-xn!cit-vax!mangler
From: mangler@cit-vax.Caltech.Edu (System Mangler)
Newsgroups: comp.unix.wizards
Subject: Re: BSD 4.2 minphys() < 64K
Message-ID: <1361@cit-vax.Caltech.Edu>
Date: Wed, 17-Dec-86 17:42:06 EST
Article-I.D.: cit-vax.1361
Posted: Wed Dec 17 17:42:06 1986
Date-Received: Thu, 18-Dec-86 03:59:30 EST
References: <376@wyszecki.munsell.UUCP> <1871@utah-gr.UUCP> <4763@mimsy.UUCP>
Organization: California Institute of Technology
Lines: 30
Summary: Two 16-bit MBA byte counts in one 32-bit register

In article <376@wyszecki.munsell.UUCP>, jwf@munsell.UUCP (Jim Franklin) writes:
> I would like to be able to blast a 1/4 megabyte to a disk in one i/o --
> the disk, disk controller, and device driver can all deal with this.

What kind of disk controller?

In article <4763@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes:
> The MBA byte count register is 32 bits wide,

The MBA byte count register contains two 16-bit byte counts:  the
number of bytes transferred to/from memory, and the number transferred
to/from the drive.  On an error, they may differ, due to buffering.
The 16-bit byte counts limit the MBA to at most 127 sectors at a time.

The massbus disk driver really ought to supply its own minphys to deal
with 516 byte sectors (which happen when you're writing the headers).

> At 63k-at-a-time, I get a raw data rate of
> 1.1Mb/s on a Vax 785 with Eagles on an Emulex SC788

Even a lowly 750 can do that, given Eagles.

> This amounts to less than 20 interrupts per second---quite trivial;

Lost revolutions, not interrupts, are the issue, and only when you're
doing fairly specialized things, like a database writing raw cylinders,
fast bad-block checking, or image backup to a 200-ips 6250-bpi streamer,
all of which probably require more CPU horsepower than a VAX anyway.

Don Speck   speck@vlsi.caltech.edu  {seismo,rutgers,ames}!cit-vax!speck