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From: ephraim@wang.UUCP
Newsgroups: comp.sys.mac
Subject: Re: SCSI Hard Disks (Delphi Digest V2 #52)
Message-ID: <1637@wang.UUCP>
Date: Fri, 21-Nov-86 08:13:16 EST
Article-I.D.: wang.1637
Posted: Fri Nov 21 08:13:16 1986
Date-Received: Sun, 23-Nov-86 02:58:40 EST
Distribution: net
Organization: Wang Labs, Lowell MA
Lines: 35

Steve Brecher writes:
> to: news@cit-vax.Caltech.Edu (Usenet netnews)
> Subject: SCSI hard disks
>  
> >   Does anyone know what brand of drive is used for the major
> > SCSI disc drives. i.e dataframe, Apple etc..
> >  I would especially like to know what drive apple uses since
> > their drive is said to support filetags.
>  
> File tag support is a function of the controller rather than of the drive
> itself.  The Apple HD20SC uses a Seagate 225N with embedded controller; it
> probably has custom ROMs to support tags.  It does support tags, 'though Apple
> has indicated that tags will probably not be supported by future products.

This is very misleading.  The only thing that the controller provides for
tag support is funny-sized sectors.  For HFS tags support, 512 data bytes
plus 20 tag bytes = 532 bytes/sector.  The ST 225N *does not* need custom
ROMs to do this.  The current firmware supports unusual sector sizes as
a standard feature.  Some other disks provide at least as much support.
The Rodime RO 651 and 652, for example, support *any* sector size from 
128 to 1024 bytes.

Most of the work in providing sector tags falls to the disk driver in
the Mac.  When writing, the tags have to be retrieved from low memory
globals (for the first sector) or calculated (for successive sectors
of a multi-sector write) and inserted in the data stream.  When reading,
the tags have to be stripped out and the last set (from a multi-sector
read) deposited in the globals.

Apple Tech Support, BTW, is actively *discouraging* developers from
supporting tags.  They will not answer technical questions about them
and say that Apple may drop tag support in future products.

Ephraim Vishniac
decvax!wanginst!wang!ephraim