Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!cs.utexas.edu!uunet!cbmvax!jesup
From: jesup@cbmvax.commodore.com (Randell Jesup)
Newsgroups: comp.periphs.scsi
Subject: Re: Trouble in SCSI-Land: On PC-computers, it isn't really standard!
Message-ID: <10232@cbmvax.commodore.com>
Date: 18 Mar 90 05:52:51 GMT
References: <1990Mar15.042028.16024@Octopus.COM>
Reply-To: jesup@cbmvax (Randell Jesup)
Organization: Commodore, West Chester, PA
Lines: 39

In article <1990Mar15.042028.16024@Octopus.COM> pete@octopus.COM (Pete Holzmann) writes:
>Based on experiences during the past 6 months, I've come to the sorry 
>conclusion that SCSI is not defined well enough as an interface standard,
>at least in the PC world. [If SCSI is the base I/O scheme for peripherals,
>as in the Mac, etc, then I'm sure things ought to be much better...]

	Well, on the Amiga it's rapidly becoming the standard I/O interface.
In addition, Commodore and it's 3rd-parties have agreed on two standards: one
for storing partitioning information on a drive (including executables for
custom filesystems used by partitions on that drive).  The other standard
is a common I/O device interface to allow software to send direct SCSI
commands (this is an extension of the base-level disk driver interface
standard, and is even supported by some non-scsi drives).  The low-level
drivers are either in ROM on the controller, or loaded off your boot disk
automatically.

	This allows some nice things: we can take a drive from one system,
attach it to another (on a different manufacturers controller), and it comes
up with all partitions (you can even boot off it).  We can use anyone's
partitioning program to partition a drive, regardless of controller/driver.

	Since we can use direct SCSI commands, partitioning programs can
use MODE_SENSE, INQUIRY, READ_CAPACITY, etc to get all the info needed to
set up the drive.

>2) There is no standard way for a DMA-based SCSI controller to deal with
>    the '386 or '486 CPU's, which make a habit of changing memory maps
>    on the fly.

	This requires some OS support I suspect - for example, a
VirtualToPhysical(addr,&length) that returns the physical address for
DMA, and modifies length (if needed) to avoid any discontinuities.

	Almost all controllers on the Amiga are DMA.

-- 
Randell Jesup, Keeper of AmigaDos, Commodore Engineering.
{uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com  BIX: rjesup  
Common phrase heard at Amiga Devcon '89: "It's in there!"