* Ported to boot 386BSD by Julian Elischer, September 1992
* Copyright (c) 1992, 1991 Carnegie Mellon University
* Permission to use, copy, modify and distribute this software and its
* documentation is hereby granted, provided that both the copyright
* notice and this permission notice appear in all copies of the
* software, derivative works or modified versions, and any portions
* thereof, and that both notices appear in supporting documentation.
* CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
* CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
* ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
* Carnegie Mellon requests users of this software to return to
* Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU
* School of Computer Science
* Carnegie Mellon University
* Pittsburgh PA 15213-3890
* any improvements or extensions that they make and grant Carnegie Mellon
* the rights to redistribute these changes.
* Revision 2.2 92/04/04 11:33:55 rpd
********NOTE: This is not all relevant to the 386BSD version**********
AT386 Protected Mode Bootstrap Loader
=====================================
After the system is rebooted, the BIOS bootstrap routine reads Sector
1, Track 0 into memory at location 0000:7C00H. If location 0000:7DFEH
(last two bytes of that sector) contains the value AA55H, the BIOS
bootstrap routine will transfer control to 0000:7C00H. Otherwise, the
boot code in that sector is bad and the boot routine stops.
For DOS compatibility reason, one extra stage of boot is required if
the boot device is a hard disk. The first sector of the hard disk will
contain the MOS-DOS boot code and a boot record partition table.
When this sector is loaded into 0000:7C00H, it will relocate itself
to somewhere else and then load the first sector of the active
partition into 0000:7C00H. Both UNIX and DOS use the command "fdisk"
[ 386bsd does not have an 'fdisk' (yet) ]
to install this first sector into the hard disk and to manipulate
the hard disk partitions.
2. The First Stage Bootstrap Loader
--------------------------------
After startup, the first stage boot is loaded at 0000:7C00H. This
first stage boot will load itself and the second stage boot into
memory at location 0000:1000H. For floppy disks, the first cylinder
is reserved as the boot cylinder, and the boot code (first and second)
will be loaded from there. Currently, only double sided, high density
(15 sectors per track) floppies are supported. For hard disks, the
first 29 sectors of the active partition is reserved for boot code
which will be loaded by the first stage boot. All the disk types
recognized by BIOS are supported by this bootstrap loader.
[for 386bsd we load the second stage booter to 9000:0]
3. The Second Stage Bootstrap Loader
--------------------------------
After the boot code is loaded, the control is passed to the second
stage bootstrap loader "boot2()". In order to be able to load the
big kernel image (bigger than 512K or 640K, depends on the memory
configuration), the second stage boot loader will run on the protected
mode. This bootstarp loader does not have any stand alone device
drivers, all the I/O's are through the BIOS calls. Since the first
stage boot code will no longer be used at this moment, the memory
location of the first stage boot code (0000:1000H to 0000:1200H) will
be used as an internal buffer for BIOS calls. Immediately after this
internal buffer is the GDT table for the second stage boot loader.
Since this boot loader needs to switch back and forth between protected
and real mode in order to use BIOS calls, the limit of the boot code
and boot data segments must not be greater than 64K.
The boot loader loads the kernel image at memory location above 1 MB
to skip the memory hole between 521K/640K and 1MB. After the kernel
is loaded, the boot loader stores the information in the stack and
then passes control to kernel. Currently, the three information passed
fromm the boot loader to the kernel are type of the boot device, size
of the base memory and size of the extended memory.
[ 386bsd receives: howto, bootdev]
[ 386bsd is loaded where-ever the "MByte" bits of the load address specify,
so if you link it for FE100000 it will load to 1MB, but if you link
it for FE000000 it will load ad 0MB]
[for machines with only 512KB normal ram the kernel will need to be linked
for 1MB and the bootblocks modified to run below 512KB. (8000:0)]
Since the boot loader loads the kernel image at memory location above
1MB, the kernel has to start as protected mode. In addition, the
link editor description file (vuifile) has to indicate that
the text and data segments start above 1MB. Also, the boot loader
passes the infomation to the kernel through the stack.
[MOST of what is mentionned below is NOT relevant to 386bsd]
5. Disk Layout and Bad Block Handling
---------------------------------
The System V/386 Release 3.2 (AT) disk layout will be used as the disk
layout for the MACH System on the AT platform.
This disk layout is as follows:
* Reserve the first sector of cylinder 0 for the DOS boot record which
contains the master boot code (446 bytes) and the partition table.
(Refer to DOS Technical Reference Manual page 9-6 to 9-10).
* Reserve the first 29 sectors of the UNIX partition for the first
and the second stage bootstrap.
* Reserve the 30th sector of the UNIX partition for the pdinfo and
* Reserve the 31st to the 34th sectors of the UNIX partition for the
bad track and the bad block mapping tables.
* Reserve up to 253 consecutive tracks when required, beginning with
the 35th sector of the UNIX partition, for alternate tracks.
* Reserve up to 253 consecutive blocks, beginning with the first
sector after the alternate tracks area, for alternate blocks.
----------------------------------------------------
---------------- .......... --------------------
---------------- .......... --------------------
===============================================================
^ | BOOTSTRAP | CYL N, TRK M
| ----------------------------------------------------
| | |30 |31 |32 |33 |34 |
---------------------------------------------------- ---
N ---------------- .......... --------------------- |
I | .......... | Alternate Tracks
X ---------------- .......... --------------------- |
P ---------------------------------------------------- ---
R ---------------- .......... --------------------- |
T | .......... | Alternate Blocks
I ---------------- .......... -------------------- |
I ---------------------------------------------------- ---
O | Unix root partition starts from here |
N ---------------- -----------------
----------------------------------------------------
----------------------------------------------------
| ---------------------------------------------------
| ----------------------------------------------------
===============================================================
--------------- ........ --------------
----------------------------------------------------
The bad block handling mechanism is as follows:
* Use the alternate track in the alternate tracks area if the
track containing the target sector is bad.
* Use the alternate block in the alternate blocks area if the
Since the kernel image is loaded above 1 MB, the kernel must start
as protected mode. This means that this bootstrap loader will work
only when the corresponding changes on the kernel startup code are done.
The make command to generate this bootstrap loader is:
make -f boot.mk fdboot (floppy boot loader)
make -f boot.mk hdboot (wini boot loader)
[to make 386bsd bootblocks "make sd wd" (warning: they will be installed
in /dev/mdec.. take backups)]
projects / unix-history / blob