Status of support for 4KB disk sectors

perryh at perryh at
Wed Jul 20 08:22:06 UTC 2011

Jeremy Chadwick <freebsd at> wrote:

> On Wed, Jul 20, 2011 at 02:39:28AM -0700, perryh at wrote:
> > IIRC, Plextor (and maybe some others) had a switch to select 512 or
> > 2048 as the default transfer size, precisely so that they could be
> > used as boot devices with systems that supported only 512.
> I don't think Plextor was around back then; they used to be called
> TEXEL back in the early 90s.  The only Sun SCSI CD drives I saw
> were external and caddy-based, so I mentally correlate them with
> NEC.  Back then I wasn't looking at brands as much as I do today,
> though.

I still have a non-Sun 512-2048 drive; turns out it is a (caddy-
based) Hitachi CDR-1750S rather than a Plextor.  So much for
remembering all the details from late in the Sun-3 era.  (Plextor
still rings a bell WRT the 512-2048 switch though; maybe some of
the early Plextor drives also provided one.)

Chuck Swiger <cswiger at> wrote:

> Come to think of it, I do remember that switch, yes.
> Do you happen to know whether this limitation was part of the Sun
> hardware, or of SunOS?  CMU had a lot of Sun3 machines and NeXT
> clusters, so I ended up mixing NeXT CD-ROM and the Canon? magneto-
> optical drives with Sun H/W, and vice versa.

Dunno if there were any hardware limitations, but most Sun-3
_bootroms_ predated CDROM support and thus could boot from a
CD only by being fooled into believing it was a normal MFM or
ESDI hard drive connected via an Adaptec ACB-4000 (SCSI-MFM)
or Emulex MD21 (SCSI-ESDI) bridge controller.  Remember those?

This only worked if the CD drive's transfer size matched the
expected hard drive sector size.  I think the SunOS sr driver took
the path of least resistance and issued an explicit "set transfer
size 512" before trying to access the drive, thus enabling off-brand
CD drives to work with the OS without running into any limitations
that might have existed in either the hardware or the lower-level
SCSI drivers, but that only worked after the OS had been booted :)

> SunOS wasn't the only O/S which was run on a m68k Sun box.  ;-)

I'm aware of a NetBSD port that may still exist even today.  Others?

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