objcopy, ld and jumps

[Stefan Reinauer <stefan dot reinauer at coresystems dot de>]

Dear binutils developers,

we're in the progress of building a new major version of LinuxBIOS, and
I want to take the chance to get rid of some bad habits in the code.

Well, the first thing that is required is the reset vector. It's 16
bytes at 0xfffffff0 (aliased to f000:fff0 in real mode). It jumps into
the real code which is placed at 0xffffe000 (0xe000 in the current segment).

--------- 8< reset.s --------------------------
SEGMENT_SIZE = 0x10000
RVECTOR = 0x00010
 .code16
 .globl _start
_start:
 jmp SEGMENT_SIZE-(0x1f00 +0xf0 +RVECTOR)
.byte 0
.ascii "2007/02/01"
.word 0
----------------- 8< --------------------------

I build it with:
  as reset.s -o reset.o

and link it:
  ld -Ttext 0xfffffff0 reset.o -o reset.elf
  objcopy -O binary -S reset.elf reset.bin

Now I disassemble it:

ndisasm -o 0xfffffff0 reset.bin | head -1
FFFFFFF0  E9FEDF            jmp 0xdff1

Oops. It is obviously wrong. But why?

Alternatively I omit the objcopy step but I basically do the same (do I?):

  ld -Ttext 0xfffffff0 -s --oformat binary reset.o -o reset.bin

  ndisasm -o 0xfffffff0 reset.bin | head -1

FFFFFFF0  E90DE0            jmp 0xe000

Phew. It works here. But why? What's the difference?

We don't really want to use the second version, because the intermediate
elf binary is nice to dump a symbal table using nm -e reset.elf. People
are sometimes heavily relying on this:

0000000000000010 a RVECTOR
0000000000010000 a SEGMENT_SIZE
00000000fffffff0 T _start


Is this a binutils bug, or a problem between chair and keyboard?


Stefan



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