Booting: The Kernel
This is part of a series of articles that covers the
booting of an OSR5 machine. See Booting
OSR5 for other related articles.
When you type the name of a kernel or just press enter to get
the kernel defined by defbootstr, /boot first checks to see that you haven't
typed a built in command that it understands, or that what you
typed is an alias (remember, it
has already read /stand/etc/default/boot) that will expand to
something that it can understand; if it is either an alias or a
direct reference (like "hd(40)unix") it then looks to see if the
kernel or other program you want you want is on the disk. Other
program? Yes, if you aren't loading a kernel, you'll be running
something else like bootos.
If you have given the name of a program to load, /boot checks
the magic number of the file.
For example, if you type "/etc/default/boot" at the boot prompt,
you'll be told that it has a "bad magic number"; in other words it
is not a program that can be executed by /boot. Only standalone programs can be run. If
you have the opportunity, try this:
# remounts /stand as read/write
cp /bin/ls /stand
# remounts /stand as readonly
shutdown -g1 -y
At the Boot: prompt, type "ls". You will be told that "ls" has a
bad magic number. It isn't a standalone program, so /boot won't
Built in commands usually affect some later behavior of /boot or
the kernel that it loads, although a notable exception is "dir",
which acts much like "ls" except that it has no flags: you can't do
"dir -l", for example. Another is "debug", which will be very
familiar to those old enough to remember machines that always
booted to a debug console, or MSDOS "debug", which served exactly
the same purpose as this. Probably the only thing anyone would use
this for nowadays is low-level disk format if the disk bios didn't
provide any other way to do it. If you have been doing this long
enough to remember when that was standard operating procedure, you
won't be surprised to learn that "g C800:6" would transfer you to
drive formatting code hopefully located at that address.
There's no harm in taking a peek at debug: just type
"debug" at the Boot: prompt. A "?" will show you the commands it
understands. You can examine registers with "r", and print bytes
(just bytes; this is not a disassembler) with "p". Just don't
change anything; when you've had enough, type "g" to return to the
"link" is another oddball: when you type "link", it looks like
it takes over the loading of the kernel, but actually /boot still
does the work, and passes control to link just after the loading.
This can be seen by using an emergency boot floppy made with "mkdev
fd" (which by default would not have the "link" program on the
disk). If you type "link" at that floppy's Boot: prompt, boot will
ask you what package(s) you want to link, will then load the unix
kernel, and only then will fail because it is unable to load and
pass control to link.
Other potentially useful commands:
This lets you change the default btld device (normally it is
fd0). Of course, you can always specify specific devices to link,
but if you had a strange machine where you needed to link from the
second floppy, this could be useful to tuck into the /stand/.bootrc
This is one that gets passed to the kernel and would let you use
a dumb terminal (or even a Windows PC with terminal emulation!)
attached to the COM1 port as your system console. This is hardly
ever done nowadays, but when monitors and video cards were fairly
expensive, it was common. If you did use this, you'd probably also
want to specify the SERIAL keyword in /etc/default/boot to set
This command gives you pretty tight control over the behaviour
of the CPU cache. Again, it's unusual to need this nowadays, but
the "man HW boot" page explains all about this.
Once you finally do type the name of a kernel, /boot actually
sizes memory before it
starts to load it. You can get /boot to do just the memory sizing
by typing "mem=/p" at the Boot: prompt.
If you do that, you'll have another surprise when you
actually do boot: after loading the kernel, /boot stops and tells
you that the kernel is loaded and that you need to press RETURN to
Memory sizing has a large section of the "man HW boot" page
devoted to it. Each "." that it prints represents 1 megabyte of
memory. If you have done "mem=/p", you'll see something like this
after all the dots:
The "/s" means the memory is "special", the /n means it is not
DMAable. The "mem=" command can
also be used to tell /boot what memory to use.
If you want to know what memory was used (and how it is used)
after booting, run "hw -r ram".
After loading your kernel, /boot passes control to it. The
information that boot has about memory size, etc. has been placed
at a special memory address where the kernel will know to look for
it. That information is documented in /usr/include/sys/bootinfo.h;
if you examine this you'll see that /boot actually passes quite a
bit of information.
After memory sizing, the kernel's .text (executable code)
section is loaded. This procedure prints a "." for each 4K of
Jim Mohr, in SCO
Companion says that each dot represents 12k, but my experience
says that isn't so. For example, on this 5.0.4 machine, "size
/stand/unix" tells me that the text size of this kernel is 1797680
bytes and the data segment is 205484 bytes. With each dot being 4k,
/boot should print 439 dots for the text segment and 50 dots for
data, and that is just what it does.
I suspect that this behaviour might have been changed
after Jim published. He mentioned that the booting had the
appearance (though not the reality) of being slow due to the 12k
chunks; perhaps this perception caused the design
After .text comes .data (initialized variables), and then space
is set aside for stack variables ("loading .bss"). At this point
control is passed to the kernel unless btld's are to be linked
Boot Time Loadable Drivers
If you have essential equipment such as a hard disk controller
that is not supported by any of the standard drivers included with
the installation kernel, you need a boot time loadable driver.
Generally, this is simple: you just type "link" at the Boot: prompt
or construct a "defbootstr=" line that includes the link. For
example, if you needed to link two drivers, you might have to
defbootstr link="alad ida"
Where this can get confusing is when link objects to something
about the driver and asks you what to do about it. The problems
- Major number already in use
The major number is the first of the comma separated numbers
that show up when you do a long listing of a device node. See
Devices for more
You shouldn't have a conflict, and if you do, it probably means
that the people providing the driver aren't familiar with SCO Unix-
and if that's the case, how much do you trust their driver? The
"Master" file that a BTLD provides normally tells the system to
simply assign the next major number. If a driver insists upon using
a specific number, it had better have an awfully compelling reason,
and the instructions for its installation should note that and tell
you what to do.
- Interrupt already in use
This, too, is a situation that shouldn't happen. If this is
older hardware where there is no choice, you may have to force
something else to use a different interrupt to avoid the conflict.
This can be done in most BIOSes by forcing PCI devices not to use
certain interrupts- generally terminology such as "Reserved for
legacy device" is what you'd want.
- Driver name already exists
Unless the driver is actually replacing an existing driver, this
is really bad, and there may not be a solution. If it is designed
to replace something in the SCO kernel, the documentation should
say so and assure you that telling link to replace the existing
driver with the btld is indeed what you want to do.
If the name clash is just bad luck, and if you have access to a
running Unix machine where you can mount the BTLD disk, you may be
able to change the name of the driver to avoid the conflict. You
should read the man page for "btld" carefully; at a minimum you'd
be changing the name of several directories, and editing the Master
and System files to change the name there. This is probably not
something you want to do if you haven't had some previous
experience with BTLD's and are certain of the effects. On the other
hand, if this is stopping your install, what have you got to
After this, /boot gives over control to the kernel.
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See also OSR504 boot STOPS after "Loading kernel ... .text"
© September 1999 A.P. Lawrence. All rights
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