Re: patch: MFSv3 support (libsa) for boot2 (i386)

[David Laight <david%l8s.co.uk@localhost>]

On Fri, Jan 13, 2012 at 06:50:19PM +0400, Evgeniy Ivanov wrote:
> > The code is linked to an address other than 0x7c00, the first thing
> > it does is copy itself to that address.
> >
> > Are you sure you are disassembling it correctly ?
> > It looks like you haven't told objdump? it is 16bit code.
> 
> Yes, I was handling that output by hands. Thanks for the proper command.
> 
> > That jmp instruction needs to goto address 7c00, the opcode contains
> > the pc-next relative value, the 7bfe value is just a parameter to
> > the relocation.
> > In the final image you have f30c+3+f1-7c00 is 0x7800 which is ok
> > if the code is expected to relocate itself to 0x7800.

> Why do you refer 0x7800, how it's related to the LOADADDR (0x8800)?

That seemed to be the required LOADADDR to get those instruction bytes.
But possibly I got the sums wrong!
(The local file I looked at is old and uses a different LOADADDR.)

> With --adjust-vma=0x8800 I get the thing I understand:
> 88f1:       e9 0c f3                jmp    0x7c00
> 0x88f1 + 3 - 3316 (0xf30c) = 0x7c00
> 
> And I still miss the meaning of relocation value 7bfe.
> In object file it is
> 131:   e9 fe 7b                jmp    0x7d32
> 7d32 = 7c00 + 0x132 (i.e. number of bytes before this command).

If you do an 'objdump -r mbr.o' you'll see there is a pc-relative
relocation applied to address 132, 'objdump -d' doesn't look at
the relocations (it would be useful if it did) - so it blindly
prints the wrong target address.

The pc-relative relocation will be defined relative to the location
of the fixup (ie 132 not 134) - so 0x7bfe is needed as a parameter
not 0x7c00. To save space this value is put into the onject code
rather than the relocation record.

> And then when link how do we get e9 0c f3? That's the thing I dream to
> know :-) Because in MINIX I get in final image
> 88f1:       e9 fe 7b                jmp    0x4f2
> 
> While preprocessed sources are the same on both systems and compiled
> with same options.

Something is going wrong with the assemble or link phase - might be
a buggy version of either the assembler or linker.

Possibly using: jmp .start - $LOADADDR + $BOOTADDR will work instead.

        David

-- 
David Laight: david%l8s.co.uk@localhost