Problems understanding this piece of code...

[Gary Jennejohn]

On Mon, 29 Mar 2010 00:06:49 -0400
Sergio Andr__s G__mez del Real <sergio.g.delreal at gmail.com> wrote:

> Hi.
> I can't seem to understand this piece of i386 boot code:
> What I understand is that BIOS loads MBR at 0x7c00, then jumps to the
> code there, code that relocates itself to and jumps to 0x600, but what
> does movw $main-EXEC+LOAD,%si mean? if it's the source address to copy
> from, why is it $main (0x7c00) - EXEC (0x600) + LOAD (0x7c00) above
> main? then, movw $main,%di is the address to copy to... wasn't 0x600
> the address to copy to? then jump to jmp main-LOAD+EXEC (0x600)?
> 
> I guess I am really missing something here, but I can't get to
> understand what's happening.
> 
> start:		cld				# String ops inc
> 		xorw %ax,%ax			# Zero
> 		movw %ax,%es			# Address
> 		movw %ax,%ds			#  data
> 		movw %ax,%ss			# Set up
> 		movw $LOAD,%sp			#  stack
> #
> # Relocate ourself to a lower address so that we are out of the way when
> # we load in the bootstrap from the partition to boot.
> #
> 		movw $main-EXEC+LOAD,%si	# Source
> 		movw $main,%di			# Destination
> 		movw $0x200-(main-start),%cx	# Byte count
> 		rep				# Relocate
> 		movsb				#  code
> #
> # Jump to the relocated code.
> #
> 		jmp main-LOAD+EXEC		# To relocated code
>

The boot loader was linked to run at EXEC, so main is actually EXEC plus
the offset to main from EXEC.

So main-EXEC+LOAD results in calculating the offset to main at the place
where the BIOS loaded it ==> LOAD+offset_to_main.  This must be used as
the source address for the movsb.

--
Gary Jennejohn