Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!bionet!ames!purdue!decwrl!sun!imagen!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: Hard drive synchronization with the ST
Message-ID: <1470@atari.UUCP>
Date: 1 May 89 20:55:15 GMT
References: <890427.09095442.063770@SFA.CP6> <1467@atari.UUCP> <17759@cup.portal.com> <629@stag.math.lsa.umich.edu>
Reply-To: apratt@atari.UUCP (Allan Pratt)
Organization: Atari (US) Corporation, Sunnyvale, California
Lines: 29

Some people have mentioned that their non-Atari hard disks don't always 
auto-boot.  Something came to my attention recently, and this might be
part of the explanation:

The boot sequence on the ST starts with a "move.w #$2700,sr" instruction
(to ensure IPL 7) and then "reset" to reset external devices. 
Unfortunately, the "reset" instruction does not pull down reset as long
as the SCSI spec requires.  Furthermore, the time between letting go of
the reset button and the time the "reset" instruction gets executed is
too short -- that is, you get "reset" (the button is down), then "not
reset" (the instructions are executing), then "reset" (the reset
instruction), and the "not reset" time is shorter than the SCSI
minimum-time-between-resets specification.

A host adapter to a true, picky SCSI device would have to lengthen any
reset it gets to the minimum SCSI length, and possibly remove the brief
"glitch" it sees while the two instructions are executing. 

Atari hard disks can take it (or the host adapter is doing the fiddling)
because, of course, they work.  I regret (retroactively) that this
timing problem exists.  The fact that the reset instruction is too short
isn't really our fault: it grounds "reset" for 124 clocks in the 68000,
512 clocks on the 68020 and 68030 (but remember, clocks are likely to be
shorter).  However, the too-short time between resets *is* under
software control. 

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt