Unfortunately, DEC Unix doesn't know anything about Linux, so
sharing a single disk between the two OSes is not entirely trivial.
However, it is not a difficult task if you heed the tips in this
section. The section assumes you are using aboot version 0.5 or
newer.
First and foremost: never use any of the Linux partitioning
programs (minlabel or fdisk) on a disk that is also used by
DEC Unix. The Linux minlabel program uses the same partition
table format as DEC Unix disklabel, but there are some
incompatibilities in the data that minlabel fills in, so DEC Unix
will simply refuse to accept a partition table generated by
minlabel. To setup a Linux ext2 partition under DEC Unix,
you'll have to change the disktab entry for your disk. For the
purpose of this discussion, let's assume that you have an rz26 disk (a
common 1GB drive) on which you want to install Linux. The disktab
entry under DEC Unix v3.2 looks like this (see file
/etc/disktab):
rz26|RZ26|DEC RZ26 Winchester:\
:ty=winchester:dt=SCSI:ns#57:nt#14:nc#2570:\
:oa#0:pa#131072:ba#8192:fa#1024:\
:ob#131072:pb#262144:bb#8192:fb#1024:\
:oc#0:pc#2050860:bc#8192:fc#1024:\
:od#393216:pd#552548:bd#8192:fd#1024:\
:oe#945764:pe#552548:be#8192:fe#1024:\
:of#1498312:pf#552548:bf#8192:ff#1024:\
:og#393216:pg#819200:bg#8192:fg#1024:\
:oh#1212416:ph#838444:bh#8192:fh#1024:
The interesting fields here are o?, and p?, where
? is a letter in the range a-h (first through 8-th
partition). The o value gives the starting offset of the
partition (in sectors) and the p value gives the size of the
partition (also in sectors). See disktab(4) for more info. Note
that DEC Unix likes to define overlapping partitions. For the entry
above, the partition layout looks like this (you can verify this by
adding up the various o and p values):
a b d e f
|---|-------|-----------|-----------|-----------|
c
|-----------------------------------------------|
g h
|-----------------|-----------------|
DEC Unix insists that partition a starts at offset 0 and that
partition c spans the entire disk. Other than that, you can
setup the partition table any way you like.
Let's suppose you have DEC Unix using partition g and want to
install Linux on partition h with partition b being a
(largish) swap partition. To get this layout without destroying the
existing DEC Unix partition, you need to set the partition types
explicitly. You can do this by adding a t field for each
partition. In our case, we add the following line to the above
disktab entry.
:ta=unused:tb=swap:tg=4.2BSD:th=resrvd8:
Now why do we mark partition h as "reservd8" instead of "ext2"?
Well, DEC Unix doesn't know about Linux. It so happens that partition
type "ext2" corresponds to a numeric value of 8, and DEC Unix uses the
string "reservd8" for that value. Thus, in DEC Unix speak, "reservd8"
means "ext2". OK, this was the hard part. Now we just need to
install the updated disktab entry on the disk. Let's assume the disk
has SCSI id 5. In this case, we'd do:
disklabel -rw /dev/rrz5c rz26
You can verify that everything is all right by reading back the
disklabel with disklabel -r /dev/rrz5c. At this point, you
may want to reboot DEC Unix and make sure the existing DEC Unix
partition is still alive and well. If that is the case, you can shut
down the machine and start with the Linux installation. Be sure to
skip the disk partitioning step during the install. Since we already
installed a good partition table, you should be able to proceed and
select the 8th partition as the Linux root partition and the 2nd
partition as the swap partition. If the disk is, say, the second SCSI
disk in the machine, then the device name for these partitions would
be /dev/sdb8 and /dev/sdb2, respectively (note that
Linux uses letters to name the drives and numbers to name the
partitions, which is exactly reversed from what DEC Unix does; the
Linux scheme makes more sense, of course ;-).
aboot First big caveat: with the SRM firmware, you can boot one and
only one operating system per disk. For this reason, it is generally
best to have at least two SCSI disks in a machine that you want to
dualboot between Linux and DEC Unix. Of course, you could also boot
Linux from a floppy if speed doesn't matter or over the network, if
you have a bootp-capable server. But in this section we assume
you want to boot Linux from a disk that contains one or more DEC Unix
partitions.
Second big caveat: installing aboot on a disk shared with
DEC Unix renders the first and third partition unusable (since those
must have a starting offset of 0). For this reason, we recommend
that you change the size of partition a to something that is just
big enough to hold aboot (1MB should be plenty).
Once these two caveats are taken care of, installing aboot is
almost as easy as usual: since partition a and c will
overlap with aboot, we need to tell swriteboot that this is
indeed OK. We can do this under Linux with a command line of the
following form (again, assuming we're trying to install aboot on
the second SCSI disk):
swriteboot -f1 -f3 /dev/sdb bootlx
The -f1 means that we want to force writing bootlx even
though it overlaps with partition 1. The corresponding applies for
partition 3.
This is it. You should now be able to shutdown the system and boot Linux from the harddisk. In our example, the SRM command line to do this would be:
boot dka5 -fi 8/vmlinux.gz -fl root=/dev/sdb8