1 files changed, 149 insertions, 0 deletions
diff --git a/sys/i386/boot/README.386BSD b/sys/i386/boot/README.386BSD
new file mode 100644
index 000000000000..ae61346800cb
--- /dev/null
+++ b/sys/i386/boot/README.386BSD
@@ -0,0 +1,149 @@
+This Boot code is different from the original boot code that came with
+386BSD in that it uses the BIOS to load the kernel and to provide all i/o
+services. The advantage ofthis is that the same boot code exactly, can run
+on any device that is supported by the BIOS. (That's most of them)
+This is important for the 'generic scsi' project because it means we can
+write drivers for new scsi adapters without having to develop an new
+set of boot blocks for each.
+
+At this point you should read the first part of README.MACH... come back here
+when you have done that:
+
+In normal operation, when co-existing with other operating systems, the
+following operations occur:
+
+1/ the BIOS loads the first block of the disk (called the Master Boot Record
+or MBR) and if it has the correct magic numbers, jumps into it:
+
+2/ The  MBR code, looks at the Partition table that is embedded within it,
+to detirmine which is the partition to boot from. If you are using the os-bs
+bootblocks (highly recommended) then it will give you a menu to choose from.
+
+3/ The MBR will load  the first record of the selected partition and
+if it has (the same) magic numbers, jumps into it. In 386bsd this is the
+first stage boot, (or boot1) it is represented in /usr/mdec by
+wdboot, asboot and sdboot. If the disk has been set up without DOS partitioning
+then this block will be at block zero, and will have been loaded directly by
+the BIOS.
+
+4/ Boot1 will look at block0 (which might be itself if there are no DOS
+partitions) and will find the 386bsd partition, and using the information
+regarding the start position of that partition, will load the next 13 sectors
+or so, to around 90000 (640k - 64k). and will jump into it at the appropriate
+entry point. Since boot1 and boot2 were compiled together as one file
+and then split later, boot1 knows the exact position within boot2 of the
+entry point.
+
+Boot 1 also contains a compiled in DOS partition table
+(in case it is at block 0), which contains a 386bsd partition starting
+at 0. This ensures that the same code can work whether or not
+boot1 is at block 0.
+
+5/ Boot2 asks the user for a boot device, partition and filename, and then
+loads the MBR of the selected device. This may or may not be the device
+which was originally used to boot the first MBR. The partition table
+of the new MBR is searched for a 386bsd partition, and if one is found,
+that is then in turn searched for the disklabel. This could all be on the
+second disk at this point, if the user selected it.
+
+6/On finding the disklabel, boot2 can find the correct unix partition
+within the 386bsd partition, and using cutdown filesystem code,
+look for the file to boot (e.g. 386bsd).
+
+7/ Boot2 loads this file starting at the location specified by the a.out header,
+(see later) and leaps into it at the location specified in he header.
+
+if the file does not exist or cannot be loaded, boot2 goes back to  step 5.
+
+386bsd is now running and will hopefully start vm etc. and get to multi-user
+mode.
+
+##########################################################################
+During all these steps, all i/o is performed using the BIOS. This has a number
+of side effects:
+
+1/ Since BIOS disk calls are specified in terms of cylinder,head and sector,
+and the BIOS read the disk information from either the CMOS or from some other
+location which is un-available to us, we must use the cyl,head,sec information
+that is given in the MBR, rather than the start address in the MBR, because
+we cannot guarentee that we can corectly calculate C,H,S from the start address.
+
+Therefore, the C,H,S information in the MBR must be as correct for this boot
+to work as it would be for DOS to boot. For example, adaptec BIOS routines
+assume a layout of 64 heads and 32 sectors giving 1MB per ficticious cylinder.
+You must use these figures to calculate the correct values. Luckily, the DOS
+fdisk program will do all this for you if you tell it to give you a DOS
+partition, and you can change it to a 386BSD partition later. If you use 
+no DOS partitioning, then the compiled in table in Boot1 will do just fine.
+
+If you want to do it by hand remember that BIOS counts sectors starting at 1.
+(cylinders and heads start at 0  (??))
+
+2/ you cannot overwrite the bottom 4k of ram until you have finished ALL
+bios calls, as BIOS uses this area as scratch memory.
+
+3/ Since BIOS runs in REAL mode, and Boot2 runs in protected mode,
+Boot 2 switches back to real mode just before each BIOS call and then
+back to protected mode on each return. Touch this at your peril.!
+
+#########################################################################
+In answering the prompt from Boot2:
+you can, 
+1/ leave it alone.. it will boot the indicated file from the first 
+partition of the first drive seen by the BIOS (C:)
+
+2/ enter only "-s" to boot the default to single user mode
+
+3/ enter only a filename (optionally with -s) to boot that kernel,
+
+4/ enter a whole line of the form shown in the prompt. This allows you to
+boot some other partition, possibly on the second drive, as root.
+
+
+##########################################################################
+In the case you have two drives the same type (both scsi or bith IDE/ESDI),
+wd(0,a)xxx
+	will boot xxx from drive 0, a partition.
+wd(1,a)xxx
+	will boot xxx from drive 1, a partition.
+
+similarly for sd.
+
+if you have one wd drive and one scsi drive, then you MUST
+use device 'hd'
+
+otherwise the following will happen:
+
+with wd0 and sd0, you specify sd1 or wd1 to indicate the 2nd drive.
+it boots the kernel correctly, then tells the kernel to use sd1 as root.
+you however may not have an sd1, and problems arise.
+
+hd is special in that the kernel is always told to use unit 0,
+The correct TYPE of device will be specified too, so the kernel
+will be told either sd0 or wd0.
+
+Whether sd or wd is specified to the kernel is read from the disklabel,
+so ensure that all SCSI disks have type SCSI in their disklabel or the
+boot code will assume they are ESDI or IDE. (Remember, because it is
+working through the BIOS it has ho idea what kind of disk it is.
+
+##########################################################################
+Installing:
+The makefile supplied has a target install which will create the
+files wdboot,bootwd ,sdboot and bootsd in /usr/mdec.
+BEWARE these will overwrite the existing wdboot and bootwd. (so back
+them up)
+
+there are also targets wd and sd which wil only do one of them
+
+The commented out targets wd0 and sd0 are examples of how to 
+load the new bootblocks, however,make sure you change the 
+device type and label to suit your drive if you uncomment them.
+(see 'man disklabel')
+
+If you already have made partitions using the old bootblocks
+these should install painlessly.
+
+Before you do this ensure you have a booting floppy with correct
+disktab and bootblock files on it so that if it doesn't work, you can
+re-disklabel from the floppy.