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| HowTo Linux Zone | Linux Zone Home | E-Mail Me | SRM Firmware Howto David Mosberger <mailto:davidm@azstarnet.com>
v0.5, 17 August 1996
This document describes how to boot Linux/Alpha using the SRM
firmware, which is the firmware normally used to boot DEC Unix. Gen&SHY;
erally, it is preferable to use MILO instead of aboot since MILO is
perfectly adapted to the needs of Linux. However, MILO is not always
available for a particular system and MILO does not presently have the
ability to boot over the network. In either case, using the SRM con&SHY;
sole may be the right solution.
Unless you're interested in technical details, you may want to skip
right to Section ``''.
1. How Does SRM Boot an OS?
All versions of SRM can boot from SCSI disks and the versions for
recent platforms, such as the Noname or AlphaStations can boot from
floppy disks as well. Network booting via bootp is supported. Note
that older SRM versions (notably the one for the Jensen) cannot boot
from floppy disks. Also, booting from IDE disk drives is unsupported.
Booting Linux with SRM is a two step process: first, SRM loads and
transfers control to the secondary bootstrap loader. Then the
secondary bootstrap loader sets up the environment for Linux, reads
the kernel image from a disk filesystem and finally transfers control
to Linux.
Currently, there are two secondary bootstrap loaders for Linux: the
raw loader that comes with the Linux kernel and aboot which is
distributed separately. These two loaders are described in more
detail below.
1.1. Loading The Secondary Bootstrap Loader
SRM knows nothing about filesystems or disk-partitions. It simply
expects that the secondary bootstrap loader occupies a consecutive
range of physical disk sector, starting from a given offset. The
information on the size of the secondary bootstrap loader and the
offset of its first disk sector is stored in the first 512 byte
sector. Specifically, the long integer at offset 480 stores the size
of the secondary bootstrap loader (in 512-byte blocks) and the long at
offset 488 gives the sector number at which the secondary bootstrap
loader starts. The first sector also stores a flag-word at offset 496
which is always 0 and a checksum at offset 504. The checksum is
simply the sum of the first 63 long integers in the first sector.
If the checksum in the first sector is correct, SRM goes ahead and
reads the size sectors starting from the sector given in the sector
number field and places them in virtual memory at address 0x20000000.
If the reading completes successfully, SRM performs a jump to address
0x20000000.
2. The Raw Loader
The sources for this loader can be found in directory
linux/arch/alpha/boot
of the Linux kernel source distribution. It loads the Linux kernel by
reading START_SIZE bytes starting at disk offset BOOT_SIZE+512 (also
in bytes). The constants START_SIZE and BOOT_SIZE are defined in
linux/include/asm-alpha/system.h. START_SIZE must be at least as big
as the kernel image (i.e., the size of the BOOT_SIZE must be at least
as big as the image of the raw bootstrap loader. Both constants
should be an integer multiple of the sector size, which is 512 bytes.
The default values are currently 2MB for START_SIZE and 16KB for
BOOT_SIZE. Note that if you want to boot from a 1.44MB floppy disk,
you have to reduce START_SIZE to 1400KB and make sure that the kernel
you want to boot is no bigger than that.
To build a raw loader, simply type make rawboot in /usr/src/linux.
This should produce the following files in arch/alpha/boot:
tools/lxboot:
The first sector on the disk. It contains the offset and size
of the next file in the format described above.
tools/bootlx:
The raw boot loader that will load the file below.
vmlinux.nh:
The raw kernel image consisting of the .text, .data, and .bss
segments of the object file in /usr/src/linux/vmlinux. The
extension .nh indicates that this file has no object-file
header.
The concatenation of these three files should be written to the disk
from which you want to boot. For example, to boot from a floppy,
insert an empty floppy disk in, say, /dev/fd0 and then type:
cat tools/lxboot tools/bootlx vmlinux >/dev/fd0
You can then shutdown the system and boot from the floppy by issueing
the command boot dva0.
3. The aboot Loader
When using the SRM firmware, aboot is the preferred way of booting
Linux. It supports:
· direct booting from various filesystems (ext2, ISO9660, and UFS,
the DEC Unix filesystem)
· booting of executable object files (both ELF and ECOFF)
· booting compressed kernels
· network booting (using bootp)
· partition tables in DEC Unix format (which is compatible with BSD
Unix partition tables)
· interactive booting and default configurations for SRM consoles
that cannot pass long option strings
3.1. Getting and Building aboot
The latest sources for aboot are available in this ftp directory
<ftp://ftp.azstarnet.com/pub/linux/axp/aboot>. The description in
this manual applies to aboot version 0.5 or newer.
Once you downloaded and extracted the latest tar file, take a look at
the README and INSTALL files for installation hints. In particular,
be sure to adjust the variables in Makefile and in include/config.h to
match your environment. Normally, you won't need to change anything
when building under Linux, but it is always a good idea to double
check. If you're satisfied with the configuration, simply type make
to build it (if you're not building under Linux, be advised that aboot
requires GNU make).
After running make, the aboot directory should contain the following
files:
aboot
This is the actual aboot executable (either an ECOFF or ELF
object file).
bootlx
Same as above, but it contains only the text, data and bss
segments---that is, this file is not an object file.
sdisklabel/writeboot
Utility to install aboot on a hard disk.
tools/e2writeboot
Utility to install aboot on an ext2 filesystem (usually used for
floppies only).
tools/isomarkboot
Utility to install aboot on a iso9660 filesystem (used by CD-ROM
distributors).
tools/abootconf
Utility to configure an installed aboot.
3.2. Floppy Installation
The bootloader can be installed on a floppy using the e2writeboot
command (note: this can't be done on a Jensen since its firmware does
not support booting from floppy). This command requires that the disk
is not overly fragmented as it needs to find enough contiguous file
blocks to store the entire aboot image (currently about 90KB). If
e2writeboot fails because of this, reformat the floppy and try again
(e.g., with fdformat(1)). For example, the following steps install
aboot on floppy disk assuming the floppy is in drive /dev/fd0:
fdformat /dev/fd0
mke2fs /dev/fd0
e2writeboot /dev/fd0 bootlx
3.3. Harddisk Installation
Since the e2writeboot command may fail on highly fragmented disks and
since reformatting a harddisk is not without pain, it is generally
safer to install aboot on a harddisk using the swriteboot command.
swriteboot requires that the first few sectors are reserved for
booting purposes. We suggest that the disk be partitioned such that
the first partition starts at an offset of 2048 sectors. This leaves
1MB of space for storing aboot. On a properly partitioned disk, it is
then possible to install aboot as follows (assuming the disk is
/dev/sda):
swriteboot /dev/sda bootlx
On a Jensen, you will want to leave some more space, since you need to
write a kernel to this place, too---2MB should be sufficient when
using compressed kernels. Use swriteboot as described in Section ``''
to write bootlx together with the Linux kernel.
3.4. CD-ROM Installation
To make a CD-ROM bootable by SRM, simply build aboot as described
above. Then, make sure that the bootlx file is present on the iso9660
filesystem (e.g., copy bootlx to the directory that is the filesystem
master, then run mkisofs on that directory). After that, all that
remains to be done is to mark the filesystem as SRM bootable. This is
achieved with a command of the form:
isomarkboot filesystem bootlx
The command above assumes that filesystem is a file containing the
iso9660 filesystem and that bootlx has been copied into the root
directory of that filesystem. That's it!
3.5. Building the Linux Kernel
A bootable Linux kernel can be built with the following steps. During
the make config, be sure to answer "yes" to the question whether you
want to boot the kernel via SRM.
cd /usr/src/linux
make config
make dep
make boot
The last command will build the file arch/alpha/boot/vmlinux.gz which
can then be copied to the disk from which you want to boot from. In
our floppy disk example above, this would entail:
mount /dev/fd0 /mnt
cp arch/alpha/boot/vmlinux.gz /mnt
umount /mnt
3.6. Booting Linux
With the SRM firmware and aboot installed, Linux is generally booted
with a command of the form:
boot devicename -fi filename -fl flags
The filename and flags arguments are optional. If they are not
specified, SRM uses the default values stored in environment variables
BOOT_OSFILE and BOOT_OSFLAGS. The syntax and meaning of these two
arguments is described in more detail below.
3.6.1. Boot Filename
The filename argument takes the form:
[n/]filename
n is a single digit in the range 1..8 that gives the partition number
from which to boot from. filename is the path of the file you want
boot. For example to boot from the second partition of SCSI device 6,
you would enter:
boot dka600 -file 2/vmlinux.gz
Or to boot from floppy drive 0, you'd enter:
boot dva0 -file vmlinux.gz
If a disk has no partition table , aboot pretends the disk contains
one ext2 partition starting at the first diskblock. This allows
booting from floppy disks.
As a special case, partition number 0 is used to request booting from
a disk that does not (yet) contain a file system. When specifying
"partition" number 0, aboot assumes that the Linux kernel is stored
right behind the aboot image. Such a layout can be achieved with the
swriteboot command. For example, to setup a filesystem-less boot from
/dev/sda, one could use the command:
swriteboot /dev/sda bootlx vmlinux.gz
Booting a system in this way is not normally necessary. The reason
this feature exists is to make it possible to get Linux installed on a
systems that can't boot from a floppy disk (e.g., the Jensen).
3.6.2. Boot Flags
A number of bootflags can be specified. The syntax is:
-flags "options..."
Where "options..." is any combination the following options (separated
by blanks). There are many more bootoptions, depending on what
drivers your kernel has installed. The options listed below are
therefore just examples to illustrate the general idea:
load_ramdisk=1
Copy root file system from a (floppy) disk to the RAM disk
before starting the system. The RAM disk will be used in lieu
of the root device. This is useful to bootstrap Linux on a
system with only one floppy drive.
floppy=str
Sets floppy configuration to str.
root=dev
Select device dev as the root-file system. The device can be
specified as a major/minor hex number (e.g., 0x802 for
/dev/sda2) or one of a few canonical names (e.g., /dev/fd0,
/dev/sda2).
single
Boot system in single user mode.
kgdb
Enable kernel-gdb (works only if CONFIG_KGDB is enabled; a
second Alpha system needs to be connected over the serial port
in order to make this work)
Some SRM implementations (e.g., the one for the Jensen) are
handicapped and allow only short option strings (e.g., at most 8
characters). In such a case, aboot can be booted with the single-
character boot flag "i". With this flag, aboot will prompt the user
to interacively enter a boot option string of up to 256 characters.
For example:
boot dka0 -fl i
aboot> 3/vmlinux.gz root=/dev/sda3 single
Since booting in that manner quickly becomes tedious, aboot allows to
define short-hands for frequently used commandlines. In particular, a
single digit option (0-9) requests that aboot uses the corresponding
option string stored in file /etc/aboot.conf. A sample aboot.conf is
shown below:
#
# aboot default configurations
#
0:3/vmlinux.gz root=/dev/sda3
1:3/vmlinux.gz root=/dev/sda3 single
2:3/vmlinux.new.gz root=/dev/sda3
3:3/vmlinux root=/dev/sda3
8:- root=/dev/sda3 # fs-less boot of raw kernel
9:0/vmlinux.gz root=/dev/sda3 # fs-less boot of (compressed) ECOFF kernel
-
With this configuration file, the command
boot dka0 -fl 1
corresponds exactly to the boot command shown above. It is quite easy
to forget what number corresponds to what option string. To alleviate
this problem, boot with option "h" and aboot will print the contents
of /etc/aboot.conf before issueing the prompt for the full option
string.
Finally, whenever aboot prompts for an option string, it is possible
to enter one of the single character flags ("i", "h", or "0"-"9") to
get the same effect as if that flag had been specified in the boot
command line. For example, you could boot with flag "i" and then type
"h" (followed by return) to remind yourself of the contents of
/etc/aboot.conf
3.6.2.1. Selecting the Partition of /etc/aboot.conf
When installed on a harddisk, aboot needs to know what partition to
search for the /etc/aboot.conf file. A newly compiled aboot will
search the second partition (e.g., /dev/sda2). Since it would be
inconvenient to have to recompile aboot just to change the partition
number, abootconf allows to directly modify an installed aboot.
Specifically, if you want to change aboot to use the third partition
on disk /dev/sda, you'd use the command:
abootconf /dev/sda 3
You can verify the current setting by simply omitting the partition
number. That is: abootconf /dev/sda will print the currently selected
partition number. Note that aboot does have to be installed already
for this command to succeed. Also, when installing a new aboot, the
partition number will fall back to the default (i.e., it will be
necessary to rerun abootconf).
Since aboot version 0.5, it is also possible to select the aboot.conf
partition via the boot command line. This can be done with a command
line of the form a:b where a is the partition that holds
/etc/aboot.conf and b is a single-letter option as described above
(0-9, i, or h). For example, if you type boot -fl "3:h" dka100 the
system boots from SCSI ID 1, loads /etc/aboot.conf from the third
partition, prints its contents on the screen and waits for you to
enter the boot options.
3.7. Booting Over the Network
Two prelimenary steps are necessary before Linux can be booted via a
network. First, you need to set the SRM environment variables to
enable booting via the bootp protocol and second you need to setup
another machine as the your boot server. Please refer to the SRM
documentation that came with your machine for information on how to
enable bootp. Setting up the boot server is obviously dependent on
what operating system that machine is running, but typically it
involves starting the program bootpd in the background after
configuring the /etc/bootptab file. The bootptab file has one entry
describing each client that is allowed to boot from the server. For
example, if you want to boot the machine myhost.cs.arizona.edu, then
an entry of the following form would be needed:
myhost.cs.arizona.edu:\
:hd=/remote/:bf=vmlinux.bootp:\
:ht=ethernet:ha=08012B1C51F8:hn:vm=rfc1048:\
:ip=192.12.69.254:bs=auto:
This entry assumes that the machine's Ethernet address is 08012B1C51F8
and that its IP address is 192.12.69.254. The Ethernet address can be
found with the show device command of the SRM console or, if Linux is
running, with the ifconfig command. The entry also defines that if
the client does not specify otherwise, the file that will be booted is
vmlinux.bootp in directory /remote. For more information on
configuring bootpd, please refer to its man page.
Next, build aboot with with the command make netboot. Make sure the
kernel that you want to boot has been built already. By default, the
aboot Makefile uses the kernel in
/usr/src/linux/arch/alpha/boot/vmlinux.gz (edit the Makefile if you
want to use a different path). The result of make netboot is a file
called vmlinux.bootp which contains aboot and the Linux kernel, ready
for network booting.
Finally, copy vmlinux.bootp to the bootsever's directory. In the
example above, you'd copy it into /remote/vmlinux.bootp. Next, power
up the client machine and boot it, specifying the Ethernet adapter as
the boot device. Typically, SRM calls the first Ethernet adapter
ewa0, so to boot from that device, you'd use the command:
boot ewa0
The -fi and -fl options can be used as usual. In particular, you can
ask aboot to prompt for Linux kernel arguments by specifying the
option -fl i.
4. Sharing a Disk With DEC Unix
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.
4.1. Partitioning the disk
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
;-).
4.2. Installing 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
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