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Old 09 September 2005, 03:17   #15
redblade
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Join Date: Mar 2004
Location: Middle Earth
Age: 33
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INTRODUCTION
------------

This directory contains the source to the UNIX bootstrap utilities.
These utilities can be used to produce bootstrap images with either
relocatable or fully linked kernels.

Because AmigaDOS bootstraps are limited to the first two blocks of
the boot device, for a maximum size of 1Kb, we use a two level
bootstrap. The level 1 boot simply loads the level 2 boot, verifies
it's checksum, and jumps to it. If anything goes wrong during the
load, it puts up a deadend alert.

Note that all code in the level 1 and level 2 boots must be position
independent. This is currently handled with a couple of kludges, one
to change jsr's to bsr's, which are PC relative (only needed when
trying to use GCC, which currently fails for other reasons). The other
kludge is a small assembler routine that computes the relocated value
of an address, primarily useful for getting the correct address of
strings.


BOOTSTRAP IMAGES
----------------

A bootstrap image currently has the following structure:

Boot Offset Size Function
----------- ---- -------------------------------

0 0x400 Boot level 1 bootstrap, read by
AmigaDOS and executed like any
normal AmigaDOS bootstrap.

0x400 0x200 Boot level 2 information block.
Contains size and checksum of level
2 boot.

0x600 var Level 2 bootstrap, variable length.
Loaded and jumped to by level 1
bootstrap.

var 0x200 Kernel information block. Contains
size, checksum, and prefered load
address of the ELF kernel file.

var var ELF kernel file, either relocatable
or fully linked. Loaded and jumped
to by level 2 bootstrap.


CHECKSUMS
---------

Note that the level 1 boot checksum must be a valid AmigaDOS checksum,
and the system will expect it to be valid. The level 2 boot also contains
a valid AmigaDOS checksum, but it is ignored. Instead, the SVR4 "sum"
compatible checksum, stored in the info block, is used to validate the
level 2 boot. Similarly, the checksum stored in the kernel info block
is used to validate the kernel file.

If the level 1 boot AmigaDOS checksum is incorrect, the system will not
attempt to boot from it. If the level 2 boot SVR4 checksum is incorrect,
the level 1 bootstrap will post a deadend alert. If the ELF kernel file
SVR4 checksum is incorrect, the level 2 bootstrap will post a recoverable
alert warning that the checksum does not match the expected one. Pressing
any mouse button will allow the boot to attempt to continue.


RELOCATABLE KERNELS
-------------------

If the kernel file loaded by the level 2 bootstrap is relocatable, the
level 2 bootstrap will perform the necessary relocation at boot time.
This can result in a noticable delay during boot, on the order of 15
seconds or more.

If the kernel info block contains a preferred load address, the relocation
is done to that address and an attempt is made to load and run the kernel
at the desired address. This is designed to allow testing of various
load addresses by simply changing the info block, rather than relinking
the kernel. The default load address in the info block is 0xFFFFFFFF,
which indicates to the level 2 bootstrap that it should do automatic
binding, by scanning the system's memory list and picking a suitable
relocation address (currently the largest chunk of contiguous, non-CHIP
memory). If the kernel is not relocatable, the preferred load address
is simply ignored.


MULTIVOLUME BOOT IMAGES
-----------------------

If the boot image is larger than will fit on a single floppy, the boot
image can be split among as many floppies as necessary. The level 1
bootstrap knows the expected size of the kernel file, and will prompt for
additional volumes as necessary, using a recoverable alert.


COMPRESSION
-----------

The boot code is capable of loading compressed kernels, I.E. kernels
which have been compressed by the standard public domain "compress"
utility, using LZW compression with 12 to 16 bits. The makeiblk utility
automatically recognizes that the kernel is compressed (from the compress
magic number) and sets the appropriate fields in the IBLK so the boot
code will know how much space the decompressed kernel will require. The
decompression is done as the kernel is read from the boot device, and
the decompressed data is written directly to the kernel memory buffer.
Decompression can be painfully slow on anything less than a 68030
based machine.


COMPACT RELOCATION
------------------

In order to squeeze even larger kernels on a single boot floppy, a
special form of ELF file can be generated using the elf2brel utility.
This utility takes a standard ELF relocatable file and turns it into
an ELF file with some custom relocation tables. The resulting file
is still a well-formed ELF file, it is just no longer usable by
the standard utilities as a relocatable file. This custom form is
about 40% smaller than a normal ELF relocatable, and is still fully
relocatable by the kernel boot code.


It seems that the source to the Amix boot disks are located in

/usr/sys/amiga/boot/
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