206 lines
8.9 KiB
Plaintext
206 lines
8.9 KiB
Plaintext
/*
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* Ported to boot 386BSD by Julian Elischer, September 1992
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*
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* Mach Operating System
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* Copyright (c) 1992, 1991 Carnegie Mellon University
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* All Rights Reserved.
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*
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* Permission to use, copy, modify and distribute this software and its
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* documentation is hereby granted, provided that both the copyright
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* notice and this permission notice appear in all copies of the
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* software, derivative works or modified versions, and any portions
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* thereof, and that both notices appear in supporting documentation.
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*
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* CARNEGIE MELLON ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS"
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* CONDITION. CARNEGIE MELLON DISCLAIMS ANY LIABILITY OF ANY KIND FOR
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* ANY DAMAGES WHATSOEVER RESULTING FROM THE USE OF THIS SOFTWARE.
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*
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* Carnegie Mellon requests users of this software to return to
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*
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* Software Distribution Coordinator or Software.Distribution@CS.CMU.EDU
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* School of Computer Science
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* Carnegie Mellon University
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* Pittsburgh PA 15213-3890
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*
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* any improvements or extensions that they make and grant Carnegie Mellon
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* the rights to redistribute these changes.
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*/
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********NOTE: This is not all relevant to the 386BSD version**********
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AT386 Protected Mode Bootstrap Loader
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=====================================
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1. Overview of Startup
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-------------------
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After the system is rebooted, the BIOS bootstrap routine reads Sector
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1, Track 0 into memory at location 0000:7C00H. If location 0000:7DFEH
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(last two bytes of that sector) contains the value AA55H, the BIOS
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bootstrap routine will transfer control to 0000:7C00H. Otherwise, the
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boot code in that sector is bad and the boot routine stops.
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For DOS compatibility reason, one extra stage of boot is required if
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the boot device is a hard disk. The first sector of the hard disk will
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contain the MOS-DOS boot code and a boot record partition table.
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When this sector is loaded into 0000:7C00H, it will relocate itself
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to somewhere else and then load the first sector of the active
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partition into 0000:7C00H. Both UNIX and DOS use the command "fdisk"
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to install this first sector into the hard disk and to manipulate
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the hard disk partitions.
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2. The First Stage Bootstrap Loader
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--------------------------------
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After startup, the first stage boot is loaded at 0000:7C00H. This
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first stage boot will load itself and the second stage boot into
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memory at location 0000:1000H. For floppy disks, the first cylinder
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is reserved as the boot cylinder, and the boot code (first and second)
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will be loaded from there. Currently, only double sided, high density
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(15 sectors per track) floppies are supported. For hard disks, the
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first 29 sectors of the active partition is reserved for boot code
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which will be loaded by the first stage boot. All the disk types
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recognized by BIOS are supported by this bootstrap loader.
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3. The Second Stage Bootstrap Loader
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--------------------------------
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After the boot code is loaded, the control is passed to the second
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stage bootstrap loader "boot2()". In order to be able to load the
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big kernel image (bigger than 512K or 640K, depends on the memory
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configuration), the second stage boot loader will run on the protected
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mode. This bootstarp loader does not have any stand alone device
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drivers, all the I/O's are through the BIOS calls. Since the first
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stage boot code will no longer be used at this moment, the memory
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location of the first stage boot code (0000:1000H to 0000:1200H) will
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be used as an internal buffer for BIOS calls. Immediately after this
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[For NetBSD, we have a different buffer which can hold an entire floppy
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track.]
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internal buffer is the GDT table for the second stage boot loader.
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Since this boot loader needs to switch back and forth between protected
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and real mode in order to use BIOS calls, the limit of the boot code
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and boot data segments must not be greater than 64K.
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The boot loader loads the kernel image at memory location above 1 MB
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to skip the memory hole between 521K/640K and 1MB. After the kernel
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is loaded, the boot loader stores the information in the stack and
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then passes control to kernel. Currently, the three information passed
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fromm the boot loader to the kernel are type of the boot device, size
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of the base memory and size of the extended memory.
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[NetBSD receives: howto, bootdev, esym.]
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[NetBSD is loaded where-ever the "MByte" bits of the load address specify;
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normally the load address is F8100000, so it loads at 1MB.]
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4. The UNIX Startup
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----------------
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Since the boot loader loads the kernel image at memory location above
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1MB, the kernel has to start as protected mode. In addition, the
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link editor description file (vuifile) has to indicate that
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the text and data segments start above 1MB. Also, the boot loader
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passes the infomation to the kernel through the stack.
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[MOST of what is mentionned below is NOT relevant to NetBSD.]
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5. Disk Layout and Bad Block Handling
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---------------------------------
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The System V/386 Release 3.2 (AT) disk layout will be used as the disk
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layout for the MACH System on the AT platform.
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This disk layout is as follows:
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* Reserve the first sector of cylinder 0 for the DOS boot record which
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contains the master boot code (446 bytes) and the partition table.
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(Refer to DOS Technical Reference Manual page 9-6 to 9-10).
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* Reserve the first 29 sectors of the UNIX partition for the first
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and the second stage bootstrap.
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* Reserve the 30th sector of the UNIX partition for the pdinfo and
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the vtoc tables.
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* Reserve the 31st to the 34th sectors of the UNIX partition for the
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bad track and the bad block mapping tables.
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* Reserve up to 253 consecutive tracks when required, beginning with
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the 35th sector of the UNIX partition, for alternate tracks.
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* Reserve up to 253 consecutive blocks, beginning with the first
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sector after the alternate tracks area, for alternate blocks.
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SEC
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1
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----------------------------------------------------
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| X | | CYL 0, TRK 0
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---------------- .......... --------------------
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| .......... |
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---------------- .......... --------------------
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| .......... |
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===============================================================
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^ | BOOTSTRAP | CYL N, TRK M
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| ----------------------------------------------------
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| | |30 |31 |32 |33 |34 |
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---------------------------------------------------- ---
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U | .......... | ^
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N ---------------- .......... --------------------- |
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I | .......... | Alternate Tracks
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X ---------------- .......... --------------------- |
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| .......... | V
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P ---------------------------------------------------- ---
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A | .......... | ^
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R ---------------- .......... --------------------- |
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T | .......... | Alternate Blocks
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I ---------------- .......... -------------------- |
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T | .......... | V
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I ---------------------------------------------------- ---
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O | Unix root partition starts from here |
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N ---------------- -----------------
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----------------------------------------------------
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----------------------------------------------------
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| ---------------------------------------------------
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| | |
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| ----------------------------------------------------
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V | |
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===============================================================
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| ........ |
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--------------- ........ --------------
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| ........ |
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----------------------------------------------------
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The bad block handling mechanism is as follows:
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* Use the alternate track in the alternate tracks area if the
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track containing the target sector is bad.
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* Use the alternate block in the alternate blocks area if the
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target sector is bad.
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6. How to make:
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-----------
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Since the kernel image is loaded above 1 MB, the kernel must start
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as protected mode. This means that this bootstrap loader will work
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only when the corresponding changes on the kernel startup code are done.
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The make command to generate this bootstrap loader is:
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make -f boot.mk fdboot (floppy boot loader)
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make -f boot.mk hdboot (wini boot loader)
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[To make NetBSD bootblocks "make install" (warning: they will be installed
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in /usr/mdec.. make backups).]
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