On Thu, Jan 12, 2012 at 10:43 PM, David Laight <da...@l8s.co.uk> wrote: > On Thu, Jan 12, 2012 at 09:25:04PM +0400, Evgeniy Ivanov wrote: >> >> Magic too :-) I fall into trouble with it. In generated listing (mbr.S): >> >> 518 00f1 E9FE7B jmp BOOTADDR >> >> In obj file: >> f1: e9 fe 7b 5a 8a jmp 8a5a7cf4 <bss_end+0x8a5a76e4> >> >> In mbr (i.e. after linking): >> >> 00000f0 5a e9 0c f3 5a 8a 74 01 >> ^^^^ Address has changed. >> >> I'm porting mbr to MINIX and address isn't changed there. What does >> happen in NetBSD on linking stage? > > 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)? 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). 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. > You can get a correct disassembly with: > objdump -bbinary -D -mi8086 --adjust-vma=0x7800 mbr > The actual value for LOADADDR can be seen at the top of the output. > > Note that the bootselect code uses a big chunk (probably 0x400 bytes) > of 'bss', so 0x7800 is a bad choice of relocation address! > > David > > -- > David Laight: da...@l8s.co.uk -- Evgeniy
