i386/doc/README

   1 /*
   2  * Mach Operating System
   3  * Copyright (c) 1990 Carnegie-Mellon University
   4  * Copyright (c) 1989 Carnegie-Mellon University
   5  * All rights reserved.  The CMU software License Agreement specifies
   6  * the terms and conditions for use and redistribution.
   7  */
   8
   9                 AT386 Protected Mode Bootstrap Loader
  10                 =====================================
  11
  12 1. Overview of Startup
  13    -------------------
  14
  15         After the system is rebooted, the BIOS bootstrap routine reads Sector
  16         1, Track 0 into memory at location 0000:7C00H.  If location 0000:7DFEH
  17         (last two bytes of that sector) contains the value AA55H, the BIOS
  18         bootstrap routine will transfer control to 0000:7C00H.  Otherwise, the
  19         boot code in that sector is bad and the boot routine stops.
  20
  21         For DOS compatibility reason, one extra stage of boot is required if
  22         the boot device is a hard disk.  The first sector of the hard disk will
  23         contain the MOS-DOS boot code and a boot record partition table.
  24         When this sector is loaded into 0000:7C00H, it will relocate itself
  25         to somewhere else and then load the first sector of the active
  26         partition into 0000:7C00H.  Both UNIX and DOS use the command "fdisk"
  27         to install this first sector into the hard disk and to manipulate
  28         the hard disk partitions.
  29
  30
  31
  32 2. The First Stage Bootstrap Loader
  33    --------------------------------
  34
  35         After startup, the first stage boot is loaded at 0000:7C00H.  This
  36         first stage boot will load itself and the second stage boot into
  37         memory at location 0000:1000H.  For floppy disks, the first cylinder
  38         is reserved as the boot cylinder, and the boot code (first and second)
  39         will be loaded from there.  Currently, only double sided, high density
  40         (15 sectors per track) floppies are supported.  For hard disks, the
  41         first 29 sectors of the active partition is reserved for boot code
  42         which will be loaded by the first stage boot.  All the disk types
  43         recognized by BIOS are supported by this bootstrap loader.
  44
  45
  46
  47 3. The Second Stage Bootstrap Loader
  48    --------------------------------
  49
  50         After the boot code is loaded, the control is passed to the second
  51         stage bootstrap loader "boot2()".  In order to be able to load the
  52         big kernel image (bigger than 512K or 640K, depends on the memory
  53         configuration), the second stage boot loader will run on the protected
  54         mode.  This bootstarp loader does not have any stand alone device
  55         drivers, all the I/O's are through the BIOS calls.  Since the first
  56         stage boot code will no longer be used at this moment, the memory
  57         location of the first stage boot code (0000:1000H to 0000:1200H) will
  58         be used as an internal buffer for BIOS calls.  Immediately after this
  59         internal buffer is the GDT table for the second stage boot loader.
  60         Since this boot loader needs to switch back and forth between protected
  61         and real mode in order to use BIOS calls, the limit of the boot code
  62         and boot data segments must not be greater than 64K.
  63
  64         The boot loader loads the kernel image at memory location above 1 MB
  65         to skip the memory hole between 521K/640K and 1MB.  After the kernel
  66         is loaded, the boot loader stores the information in the stack and
  67         then passes control to kernel.  Currently, the three information passed
  68         fromm the boot loader to the kernel are type of the boot device, size
  69         of the base memory and size of the extended memory.
  70
  71
  72 4. The UNIX Startup
  73    ----------------
  74
  75         Since the boot loader loads the kernel image at memory location above
  76         1MB, the kernel has to start as protected mode.  In addition, the
  77         link editor description file (vuifile) has to indicate that
  78         the text and data segments start above 1MB.  Also, the boot loader
  79         passes the infomation to the kernel through the stack.
  80
  81
  82 5. Disk Layout and Bad Block Handling
  83    ---------------------------------
  84
  85     The System V/386 Release 3.2 (AT) disk layout will be used as the disk
  86     layout for the MACH System on the AT platform.
  87
  88     This disk layout is as follows:
  89
  90         * Reserve the first sector of cylinder 0 for the DOS boot record which
  91           contains the master boot code (446 bytes) and the partition table.
  92           (Refer to DOS Technical Reference Manual page 9-6 to 9-10).
  93
  94         * Reserve the first 29 sectors of the UNIX partition for the first
  95           and the second stage bootstrap.
  96
  97         * Reserve the 30th sector of the UNIX partition for the pdinfo and
  98           the vtoc tables.
  99
 100         * Reserve the 31st to the 34th sectors of the UNIX partition for the
 101           bad track and the bad block mapping tables.
 102
 103         * Reserve up to 253 consecutive tracks when required, beginning with
 104           the 35th sector of the UNIX partition, for alternate tracks.
 105
 106         * Reserve up to 253 consecutive blocks, beginning with the first
 107           sector after the alternate tracks area, for alternate blocks.
 108
 109          SEC
 110           1
 111         ----------------------------------------------------
 112         | X |                                              | CYL 0, TRK 0
 113         ----------------  ..........    --------------------
 114         |                 ..........                       |
 115         ----------------  ..........    --------------------
 116         |                 ..........                       |
 117    ===============================================================
 118     ^   |               BOOTSTRAP                          | CYL N, TRK M
 119     |   ----------------------------------------------------
 120     |   |                              |30 |31 |32 |33 |34 |
 121         ----------------------------------------------------  ---
 122     U   |                 ..........                       |   ^
 123     N   ----------------  ..........   ---------------------   |
 124     I   |                 ..........                       | Alternate Tracks
 125     X   ----------------  ..........   ---------------------   |
 126         |                 ..........                       |   V
 127     P   ----------------------------------------------------  ---
 128     A   |                 ..........                       |   ^
 129     R   ----------------  ..........   ---------------------   |
 130     T   |                 ..........                       | Alternate Blocks
 131     I   ----------------  ..........   --------------------    |
 132     T   |                 ..........                       |   V
 133     I   ----------------------------------------------------  ---
 134     O   |  Unix root partition starts from here            |
 135     N   ----------------                   -----------------
 136         |                                                  |
 137         ----------------------------------------------------
 138         |                                                  |
 139         ----------------------------------------------------
 140         |                                                  |
 141     |   ---------------------------------------------------
 142     |   |                                                  |
 143     |   ----------------------------------------------------
 144     V   |                                                  |
 145    ===============================================================
 146         |                   ........                       |
 147         ---------------     ........          --------------
 148         |                   ........                       |
 149         ----------------------------------------------------
 150
 151
 152         The bad block handling mechanism is as follows:
 153
 154         * Use the alternate track in the alternate tracks area if the
 155           track containing the target sector is bad.
 156
 157         * Use the alternate block in the alternate blocks area if the
 158           target sector is bad.
 159
 160
 161
 162
 163 6. How to make:
 164    -----------
 165
 166         Since the kernel image is loaded above 1 MB, the kernel must start
 167         as protected mode.  This means that this bootstrap loader will work
 168         only when the corresponding changes on the kernel startup code are done.
 169
 170         The make command to generate this bootstrap loader is:
 171
 172         make -f boot.mk fdboot      (floppy boot loader)
 173         make -f boot.mk hdboot      (wini boot loader)