On Sun, Jun 28, 2009 at 11:44 AM, Simon
Atkinsonsimonatkin...@operamail.com wrote:
Could somebody please point me to site/article that describes the FreeDOS
boot process. And I'm interested in finding out more about what sys.com
does/how it works, when copying The FreeDOS kernel ..kernel.sys. Does
sys.com change the code in the MBR of a hard disk or only the volume boot
record on, say on the first partition of a disk?
Other than the documentation in the kernel describing usage there is
no other documentation than the source itself that I recall off-hand.
The simplist and common case of sys consists of sys copying the source
files to destination location, this is functionally equivalent to you
doing a (replace X with destination drive)
COPY KERNEL.SYS X:\KERNEL.SYS
COPY COMMAND.COM X:\COMMAND.COM
and then depending on options it will update the 1st sector of the
specified partition (note: partition not disk, so the MBR is unchanged
by sys, only FDISK and similar change the MBR) with the boot code.
The sys program contains several forms of the boot code compiled as
binary blobs and writes the one for the file system it detects in use
(FAT12/16/32, LBA/CHS, etc) and may patch small portions - it actually
reads the current boot sector then overwrites only a portion so
existing geometry information (as created by the format process) is
not modified. The boot sector may instead (or additionally) be
written as file, this allows chain loading FreeDOS from boot managers.
Currently, once sys has been run (or a equivalent action that writes
the boot code), FreeDOS kernel can be updated/changed with a simple
file copy command - there is no requirement it be located at any
location (other than in root \ directory) on the disk or have any
specific attributes (Hidden/System/Readonly).
The kernel generally doesn't care how or who loaded it (there may be
some issues with the default drive depending on how loaded, but
otherwise it will run even it it can't find any drives, just not very
useful) only that the complete KERNEL.SYS file is loaded (by default
to address 0x60:0) and jmp'd to. The boot process is identical to
every other OS for IBM compatible PCs; the BIOS determines the drive
to boot from, loads the 1st sector, passes control to it - which is
either the boot code sys put there (such as a floppy) or it does
something (like display a menu, read the MBR partition table, etc) and
loads the 1st sector of the selected parition (repeat ...) until
finally our boot code is loaded. Once the FreeDOS boot sector is
loaded, it reads from the default drive (as passed from BIOS or hard
coded, usually either 0x80 for 1st hard drive or 0x00 for 1st floppy
drive) to determine location on disk of KERNEL.SYS (name is actually
stored in boot code so can be any 8.3 name) and loads this file, then
jumps to it. The kernel does its init logic and passes control to the
shell. Where you get the friendly prompt.
The above is from memory, but should be correct. If you have specific
questions feel free to ask, I will need to look at the source but can
answer any specific questions. The current sys source has not yet
been merged into svn trunk but that should occur later this week along
with ensuring latest country, share, etc are there. The FreeDOS sys
has additional code to set and/or view the configuration options that
effect the kernel. The latest release has some additional compile
time selected funtionality as well.
Jeremy
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