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Annotation of Net2/arch/i386/boot/readme.386bsd, revision 1.1

1.1     ! root        1: This Boot code is different from the original boot code that came with
        !             2: 386BSD in that it uses the BIOS to load the kernel and to provide all i/o
        !             3: services. The advantage ofthis is that the same boot code exactly, can run
        !             4: on any device that is supported by the BIOS. (That's most of them)
        !             5: This is important for the 'generic scsi' project because it means we can
        !             6: write drivers for new scsi adapters without having to develop an new
        !             7: set of boot blocks for each.
        !             8: 
        !             9: At this point you should read the first part of README.MACH... come back here
        !            10: when you have done that:
        !            11: 
        !            12: In normal operation, when co-existing with other operating systems, the
        !            13: following operations occur:
        !            14: 
        !            15: 1/ the BIOS loads the first block of the disk (called the Master Boot Record
        !            16: or MBR) and if it has the correct magic numbers, jumps into it:
        !            17: 
        !            18: 2/ The  MBR code, looks at the Partition table that is embedded within it,
        !            19: to detirmine which is the partition to boot from. If you are using the os-bs
        !            20: bootblocks (highly recommended) then it will give you a menu to choose from.
        !            21: 
        !            22: 3/ The MBR will load  the first record of the selected partition and
        !            23: if it has (the same) magic numbers, jumps into it. In 386bsd this is the
        !            24: first stage boot, (or boot1) it is represented in /usr/mdec by
        !            25: wdboot, asboot and sdboot. If the disk has been set up without DOS partitioning
        !            26: then this block will be at block zero, and will have been loaded directly by
        !            27: the BIOS.
        !            28: 
        !            29: 4/ Boot1 will look at block0 (which might be itself if there are no DOS
        !            30: partitions) and will find the 386bsd partition, and using the information
        !            31: regarding the start position of that partition, will load the next 13 sectors
        !            32: or so, to around 90000 (640k - 64k). and will jump into it at the appropriate
        !            33: entry point. Since boot1 and boot2 were compiled together as one file
        !            34: and then split later, boot1 knows the exact position within boot2 of the
        !            35: entry point.
        !            36: 
        !            37: Boot 1 also contains a compiled in DOS partition table
        !            38: (in case it is at block 0), which contains a 386bsd partition starting
        !            39: at 0. This ensures that the same code can work whether or not
        !            40: boot1 is at block 0.
        !            41: 
        !            42: 5/ Boot2 asks the user for a boot device, partition and filename, and then
        !            43: loads the MBR of the selected device. This may or may not be the device
        !            44: which was originally used to boot the first MBR. The partition table
        !            45: of the new MBR is searched for a 386bsd partition, and if one is found,
        !            46: that is then in turn searched for the disklabel. This could all be on the
        !            47: second disk at this point, if the user selected it.
        !            48: 
        !            49: 6/On finding the disklabel, boot2 can find the correct unix partition
        !            50: within the 386bsd partition, and using cutdown filesystem code,
        !            51: look for the file to boot (e.g. 386bsd).
        !            52: 
        !            53: 7/ Boot2 loads this file starting at the location specified by the a.out header,
        !            54: (see later) and leaps into it at the location specified in he header.
        !            55: 
        !            56: if the file does not exist or cannot be loaded, boot2 goes back to  step 5.
        !            57: 
        !            58: 386bsd is now running and will hopefully start vm etc. and get to multi-user
        !            59: mode.
        !            60: 
        !            61: ##########################################################################
        !            62: During all these steps, all i/o is performed using the BIOS. This has a number
        !            63: of side effects:
        !            64: 
        !            65: 1/ Since BIOS disk calls are specified in terms of cylinder,head and sector,
        !            66: and the BIOS read the disk information from either the CMOS or from some other
        !            67: location which is un-available to us, we must use the cyl,head,sec information
        !            68: that is given in the MBR, rather than the start address in the MBR, because
        !            69: we cannot guarentee that we can corectly calculate C,H,S from the start address.
        !            70: 
        !            71: Therefore, the C,H,S information in the MBR must be as correct for this boot
        !            72: to work as it would be for DOS to boot. For example, adaptec BIOS routines
        !            73: assume a layout of 64 heads and 32 sectors giving 1MB per ficticious cylinder.
        !            74: You must use these figures to calculate the correct values. Luckily, the DOS
        !            75: fdisk program will do all this for you if you tell it to give you a DOS
        !            76: partition, and you can change it to a 386BSD partition later. If you use 
        !            77: no DOS partitioning, then the compiled in table in Boot1 will do just fine.
        !            78: 
        !            79: If you want to do it by hand remember that BIOS counts sectors starting at 1.
        !            80: (cylinders and heads start at 0  (??))
        !            81: 
        !            82: 2/ you cannot overwrite the bottom 4k of ram until you have finished ALL
        !            83: bios calls, as BIOS uses this area as scratch memory.
        !            84: 
        !            85: 3/ Since BIOS runs in REAL mode, and Boot2 runs in protected mode,
        !            86: Boot 2 switches back to real mode just before each BIOS call and then
        !            87: back to protected mode on each return. Touch this at your peril.!
        !            88: 
        !            89: #########################################################################
        !            90: In answering the prompt from Boot2:
        !            91: you can, 
        !            92: 1/ leave it alone.. it will boot the indicated file from the first 
        !            93: partition of the first drive seen by the BIOS (C:)
        !            94: 
        !            95: 2/ enter only "-s" to boot the default to single user mode
        !            96: 
        !            97: 3/ enter only a filename (optionally with -s) to boot that kernel,
        !            98: 
        !            99: 4/ enter a whole line of the form shown in the prompt. This allows you to
        !           100: boot some other partition, possibly on the second drive, as root.
        !           101: 
        !           102: 
        !           103: ##########################################################################
        !           104: In the case you have two drives the same type (both scsi or bith IDE/ESDI),
        !           105: wd(0,a)xxx
        !           106:        will boot xxx from drive 0, a partition.
        !           107: wd(1,a)xxx
        !           108:        will boot xxx from drive 1, a partition.
        !           109: 
        !           110: similarly for sd.
        !           111: 
        !           112: if you have one wd drive and one scsi drive, then you MUST
        !           113: use device 'hd'
        !           114: 
        !           115: otherwise the following will happen:
        !           116: 
        !           117: with wd0 and sd0, you specify sd1 or wd1 to indicate the 2nd drive.
        !           118: it boots the kernel correctly, then tells the kernel to use sd1 as root.
        !           119: you however may not have an sd1, and problems arise.
        !           120: 
        !           121: hd is special in that the kernel is always told to use unit 0,
        !           122: The correct TYPE of device will be specified too, so the kernel
        !           123: will be told either sd0 or wd0.
        !           124: 
        !           125: Whether sd or wd is specified to the kernel is read from the disklabel,
        !           126: so ensure that all SCSI disks have type SCSI in their disklabel or the
        !           127: boot code will assume they are ESDI or IDE. (Remember, because it is
        !           128: working through the BIOS it has ho idea what kind of disk it is.
        !           129: 
        !           130: ##########################################################################
        !           131: Installing:
        !           132: The makefile supplied has a target install which will create the
        !           133: files wdboot,bootwd ,sdboot and bootsd in /usr/mdec.
        !           134: BEWARE these will overwrite the existing wdboot and bootwd. (so back
        !           135: them up)
        !           136: 
        !           137: there are also targets wd and sd which wil only do one of them
        !           138: 
        !           139: The commented out targets wd0 and sd0 are examples of how to 
        !           140: load the new bootblocks, however,make sure you change the 
        !           141: device type and label to suit your drive if you uncomment them.
        !           142: (see 'man disklabel')
        !           143: 
        !           144: If you already have made partitions using the old bootblocks
        !           145: these should install painlessly.
        !           146: 
        !           147: Before you do this ensure you have a booting floppy with correct
        !           148: disktab and bootblock files on it so that if it doesn't work, you can
        !           149: re-disklabel from the floppy.

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