On Sun, Jan 15, 2012 at 4:40 PM, David Laight <da...@l8s.co.uk> wrote: > 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.
Useful, thanks! > 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. Aha, I see. >> 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. It works, thank you very much! And thanks for all your detailed explanations! -- Evgeniy
