SunOS 4.x boot disk layout

[Jules Richardson]

On Sun, 2005-03-27 at 15:01 -0500, Sean 'Captain Napalm' Conner wrote:
> > Normally, yes.  I don't think root _had_ to be the a partition, or swap
> > b, but those certainly were the normal way.  c being the whole-disk
> > partition was, I think, not optional.
> 
>   I always wondered why that was (and I think NetBSD wants it the same
> way---I seem to recall setting up the partitions like that on the HP/Apollos
> I have (but that are currently in storage at the moment)).  It's such an odd
> arrangement (to me).

I think it's done to enable things like whole disk backups, where the
size of the usable data area on the disk might not always be the entire
addressable range of the drive (spare sectors, things optimised for
cylinder boundaries resulting in some wastage etc.).

Plus of course ST412 type drives don't have any notion of any "get drive
geometry" command like SCSI does.

Most machines of the era seem to support / require a "whole disk" entry
(from memory it's entry #6 rather than #3 on my Tek)

Why it's the third entry on Suns I don't know. Maybe it's something odd
like OS bootable tapes support a condensed table with less entries than
a disk or something...