4BSD file system structure

[Chris Torek]

In article <20 at ocpt.ccur.com> pechter at ocpt.ccur.com
(Bill Pechter <pechter>) writes:
>... there was a boot prom on Vax 11/750's that read in the boot block
>from cyl 0, sector 0, track 0 -- but that was the only Vax that did that.

Yes, the 750 did this; but it is not the only one.  The 8200, for
instance, also reads block 0 and jumps to it; and microVAXen with
RD-series disks read block zero, but rather than executing the result,
they interpret it as a set of additional blocks to read, and other
mysterious things.

Other computer systems use similar bootstraps.  Still others (such as
the Power-6/32) do something even weirder: read a Unix file system,
open the file `/etc/fstab', and read the first line.  (Gak!) As a
result of the latter, the 4.3BSD-tahoe distribution for the tahoe
includes a `fake' first line in /etc/fstab, that names /dev/xfd0a.
The first few lines of /etc/fstab on one of the UCB machines:

	/dev/xfd0a:/: /			ufs	xx	1 1
	/dev/dk0a /			ufs	rw	1 1
	/dev/dk0b none			swap	sw	0 0
	/dev/dk0b /tmp			mfs	xx,-s=24000 0 0
	/dev/dk8b /tmp			ufs	rw	1 2
	/dev/dk3b /usr			ufs	rw	1 2

(/tmp exists twice; the `mfs' entry is fake, as yet.)

>It was a pretty limited (i.e. AWFUL) way to do things.  DEC had to go to 
>the console tape mode on the 11/750 when they went to Vaxclusters...

Actually, it is a fairly reasonable way to boot from a disk, since
it does not hard-code anything too terribly (unlike CCI's `assume you
have a 4.2BSD file system' scheme).  If *everything* is in ROM, it
becomes difficult to fix.
-- 
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain:	chris at cs.umd.edu	Path:	uunet!mimsy!chris